Optical waveguide element, optical modulator, optical modulation module, and optical transmission device

Abstract

To achieve good operating characteristics in an optical waveguide element having a plurality of intersection portions of a convex optical waveguide and a signal electrode by effectively suppressing the occurrence of disturbance modulation at the intersection portions.SOLUTION: An optical waveguide element has an optical waveguide composed of a convex portion extending on a substrate and a signal electrode for controlling light waves propagating through the optical waveguide. The optical waveguide includes a Mach-Zehnder optical waveguide having two parallel waveguides with curved portions. The signal electrode is composed of two signal lines that each intersect the two parallel waveguides at the curved portions, and transmit differential signals. In an intersection region where the two signal lines and the two parallel waveguides intersect, at least one of the two signal lines has a faster signal propagation speed than a portion other than the intersection region, or one of the two signal lines has a faster signal propagation speed than the other.SELECTED DRAWING: Figure 6

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 Art

[0002] In high-speed / large-capacity optical fiber communication systems, an optical modulator incorporating an optical modulation element as an optical waveguide element composed of an optical waveguide formed on a substrate and a control electrode for controlling light waves propagating through the optical waveguide is widely used. Among them, an optical modulation element using LiNbO 3 (hereinafter also referred to as LN) as a substrate is widely used in high-speed / large-capacity optical fiber communication systems because it can achieve low light loss and broadband optical modulation characteristics.

[0003] In particular, in recent years, in response to the increasing trend of transmission capacity, modulation formats in optical fiber communication systems, such as QPSK (Quadrature Phase Shift Keying) and DP-QPSK (Dual Polarization - Quadrature Phase Shift Keying), which are multi-value modulations and transmission formats incorporating polarization multiplexing into multi-value modulations, have become mainstream. They are used not only in backbone optical transmission networks but also being introduced into metro networks.

[0004] Also, in recent years, in order to miniaturize the optical modulator itself while achieving further low-voltage driving and high-speed modulation, a rib-type optical waveguide or a ridge-type optical waveguide (hereinafter collectively referred to as a convex optical waveguide) formed by forming strip-shaped convex portions on the surface of a thin-film (or thin-plate) LN substrate (for example, with a thickness of 20 μm or less) to enhance the interaction between the signal electric field and the guided light in the substrate is also being put into practical use (for example, Patent Documents 1 and 2).

[0005] In addition to miniaturizing the optical modulation element itself, efforts are also underway to integrate an electronic circuit and the optical modulation element in a single housing to form an optical modulation module. For example, an optical modulation module has been proposed in which an optical modulation element and a high-frequency driver amplifier for driving the optical modulation element are integrated and housed in a single housing, and the optical input / output portions are arranged in parallel on one surface of the housing to achieve miniaturization and integration. In an optical modulation element used in such an optical modulation module, an optical waveguide is formed on a substrate such that an optical input end and an optical output end of the optical waveguide are arranged on one side of the substrate constituting the optical modulation element and the optical propagation direction is folded on the substrate (for example, see Patent Document 3). Hereinafter, an optical modulation element composed of an optical waveguide including such a folded portion of the optical propagation direction is referred to as a folded optical modulation element.

[0006] By the way, an optical modulator that performs QPSK modulation (QPSK optical modulator) or an optical modulator that performs DP-QPSK modulation (DP-QPSK optical modulator) includes a plurality of Mach-Zehnder optical waveguides having a nested structure called a so-called nested type, and each of them includes at least one signal electrode to which a high-frequency signal is applied. These signal electrodes formed on the substrate are formed to extend to the vicinity of the outer periphery of the LN substrate for connection to an external electric circuit of the substrate. For this reason, on the substrate, a plurality of optical waveguides and a plurality of signal electrodes cross each other in a complicated manner, and a plurality of crossing portions are formed where the signal electrodes cross over the optical waveguides.

[0007] In such a crossing portion, an electric field is applied from the signal electrode crossing over the optical waveguide to a portion of the optical waveguide below the signal electrode, and the phase of the light propagating through the optical waveguide is slightly changed to modulate the phase. Such a phase change or phase modulation of light in such a crossing portion acts as noise with respect to the optical phase change for normal modulation generated in the optical waveguide by the signal electrode and can disturb the optical modulation operation. Hereinafter, the phase modulation as noise generated in such a crossing portion is referred to as disturbing modulation.

[0008] The degree of the noise effect of disturbing modulation on the optical modulation operation in an optical modulator becomes larger as the electric field applied from the signal electrode to the optical waveguide at the intersection is stronger, and also becomes larger due to the additive effect proportional to the number of intersections (for example, according to the total length of the intersections (intersection length) along the signal electrode).

[0009] For example, in a configuration where a conventional optical waveguide (so-called planar optical waveguide) formed by diffusing a metal such as Ti on a flat surface of an LN substrate intersects with a signal electrode formed on the substrate plane of the LN substrate, the signal electrode is formed only on the upper surface (substrate surface) of the optical waveguide, whereas in a configuration where the convex optical waveguide and the signal electrode intersect as described above, the signal electrode can also be formed on the upper surface and two side surfaces of the convex portion of the convex optical waveguide. Therefore, the electric field applied from the signal electrode to the optical waveguide at the intersection becomes stronger in the case of the convex optical waveguide than in the case of the planar waveguide, and the interaction with the more strongly confined light wave also becomes stronger. Thus, noise due to disturbing modulation can occur more greatly in the convex optical waveguide than in the case of the planar optical waveguide.

[0010] Also, in the folded optical modulation element as described above, compared with a non-folded optical modulation element composed of an optical waveguide that does not include a light folding portion, there are more intersections between the electrode and the optical waveguide (see, for example, FIG. 1 of Patent Document 3), and noise due to disturbing modulation can also become larger. For example, in the case of the above-described DP-QPSK modulation element, in a non-folded optical modulation element, the number of intersections at one electrode is about 2 to 4, and the total intersection length is several tens of microns (for example, in the range of 20 μm to 40 μm), whereas in a folded optical modulation element, the number of intersections at one electrode can reach a dozen or so, and the total intersection length can be several hundred microns to several millimeters.

[0011] Therefore, especially in a folded optical modulator configured using a convex optical waveguide, the noise caused by the disturbance modulation occurring at the intersection can be so large as to be non-negligible for normal optical modulation operations. Further, especially in a Mach-Zehnder optical waveguide configured by two parallel waveguides, the above disturbance modulation not only generates noise in each optical signal propagating through these parallel waveguides, but also generates noise in the phase difference between these two signal lights. This phase difference noise will generate even larger noise due to the optical interference effect when these light waves are combined in the Mach-Zehnder optical waveguide, and can have a great impact on the optical modulation operation.

[0012] In addition, the above-mentioned intersection can be similarly formed not only in optical waveguide elements using a semiconductor such as InP as a substrate instead of an LN substrate, but also in various optical waveguide elements such as silicon photonics waveguide devices using Si as a substrate. Further, such optical waveguide elements can be various optical waveguide elements such as not only optical modulators using Mach-Zehnder optical waveguides, but also optical modulators using optical waveguides constituting directional couplers or Y-branches, or optical switches.

[0013] And as the optical waveguide pattern and the electrode pattern become more complex with further miniaturization, multi-channelization, and / or high integration of the optical waveguide element, the number of intersections on the substrate will increase more and more, and the noise caused by the disturbance modulation may become a non-negligible factor and limit the performance of the optical waveguide element.

Prior Art Documents

Patent Documents

[0014]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0015] From the above background, in an optical waveguide element having a plurality of intersections between a convex optical waveguide and a signal electrode that propagates an electrical signal, it is required to effectively suppress the occurrence of disturbance modulation at the intersections and realize good operating characteristics.

Means for Solving the Problem

[0016] One aspect of the present invention is an optical waveguide element having an optical waveguide formed by a convex portion extending on a substrate and a signal electrode formed on the substrate for controlling light waves propagating through the optical waveguide, wherein the optical waveguide includes a Mach-Zehnder type optical waveguide including two parallel waveguides having curved portions, and the signal electrode is composed of two signal lines for transmitting differential signals that respectively intersect the two parallel waveguides at the curved portions, and in an intersection region which is a region on the substrate where the two signal lines and the two parallel waveguides intersect, The two signal lines have different crossing intervals, which are the intervals between the portions where the respective signal lines cross the two parallel waveguides. In the crossing region, at least one of the two signal lines has a signal propagation speed faster than that of a portion other than the intersection region, or the two signal lines among them, the one signal line with the longer crossing interval and the other signal line with the shorter crossing interval have a signal propagation speed faster than that of the two signal lines. According to another aspect of the present invention, an intermediate layer is provided between the substrate and the signal line on the substrate, and the thickness of the intermediate layer in the intersection region is larger than the thickness in a portion other than the intersection region. According to another aspect of the present invention, an intermediate layer is provided between the substrate and the signal line in the intersection region, and on the substrate, the intermediate layer is not provided in a portion other than the intersection region. According to another aspect of the present invention, the thickness of the intermediate layer in the intersection region is at least twice as large as the thickness in a portion other than the intersection region. According to another aspect of the present invention , be the intermediate layer in the intersection region is , be the thickness of the lower part of one of the signal line is larger than the thickness of the lower part of the other of the signal line one. According to another aspect of the present invention, the thickness of the two signal lines in the crossing region is greater than the thickness of the portions outside the crossing region. According to another aspect of the present invention, the thickness of the two signal lines in the crossing region is at least twice as large as the thickness of the portions outside the crossing region. According to another aspect of the present invention , be In the said crossing region of the one signal line the thickness is of the other signal line greater than the thickness. According to another aspect of the present invention, the width of the two signal lines in the crossing region is narrower than the width of the portion upstream of the crossing region along the propagation direction of the differential signal. According to another aspect of the present invention , be In the said crossing region the one signal line the width of the other signal line is narrower than the width of According to another aspect of the present invention, a ground electrode is provided on the substrate, and the distance between the two signals line in the crossing region and the ground electrode is wider than the distance in the portions outside the crossing region. Another aspect of the present invention is an optical modulator including an optical waveguide element according to any of the above aspects that performs optical modulation, a housing that houses the optical waveguide element, an optical fiber that inputs light to the optical waveguide element, and an optical fiber that guides the light output from the optical waveguide element to the outside of the housing. Another aspect of the present invention is an optical modulation module including an optical waveguide element according to any of the above aspects that performs optical modulation and a drive circuit that drives the optical waveguide element. Another aspect of the present invention is an optical transmission device including the above optical modulator or optical modulation module and an electronic circuit that generates an electrical signal for causing the optical waveguide element to perform a modulation operation.

Advantages of the Invention

[0017] According to the present invention, in an optical waveguide element having a plurality of intersections between a convex optical waveguide and an electrode for propagating an electrical signal, generation of disturbance modulation at the intersections can be effectively suppressed, and good operating characteristics can be realized.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18

Figure 19

Embodiments for Carrying Out the Invention

[0019] Hereinafter, embodiments of the present invention will be described with reference to the drawings. [1. First Embodiment] First, the first embodiment will be described. FIG. 1 is a diagram showing the configuration of an optical modulator 100 using an optical modulation element, which is an optical waveguide element, according to the first embodiment of the present invention. The optical modulator 100 includes a housing 102, an optical modulation element 104 housed in the housing 102, and an intermediate substrate 106. The optical modulation element 104 has, for example, a DP-QPSK modulator configuration. Finally, a cover (not shown), which is a plate, is fixed to the opening of the housing 102, and the inside thereof is hermetically sealed.

[0020] The optical modulator 100 also has a signal pin 108 for inputting a high-frequency electrical signal used for modulating the optical modulation element 104, and a signal pin 110 for inputting an electrical signal used for adjusting the operating point of the optical modulation element 104 and the like.

[0021] Furthermore, the optical modulator 100 has an input optical fiber 114 for inputting light into the housing 102 and an output optical fiber 120 for guiding the light modulated by the optical modulation element 104 to the outside of the housing 102 on the same surface of the housing 102.

[0022] Here, the input optical fiber 114 and the output optical fiber 120 are respectively fixed to the housing 102 via supports 122 and 124 which are fixed members. The light input from the input optical fiber 114 is collimated by a lens 130 disposed within the support 122 and then input to the optical modulation element 104 via a lens 134. However, this is just an example, and the input of light to the optical modulation element 104 may also be performed, according to the prior art, for example, by introducing the input optical fiber 114 into the housing 102 via the support 122 and connecting the end face of the introduced input optical fiber 114 to the end face of the substrate 220 (described later) of the optical modulation element 104.

[0023] The optical modulator 100 also has an optical unit 116 that polarization - combines two modulated lights output from the optical modulation element 104. The light after polarization - combination output from the optical unit 116 is condensed by a lens 118 disposed within the support 124 and coupled to the output optical fiber 120.

[0024] The relay substrate 106 relays a high - frequency electrical signal input from the signal pin 108 and an electrical signal such as for operating point adjustment input from the signal pin 110 to the optical modulation element 104 through a conductor pattern (not shown) formed on the relay substrate 106. The conductor patterns on the relay substrate 106 are respectively connected to pads (described later) that constitute one end of the electrodes of the optical modulation element 104, for example, by wire bonding or the like. Also, the optical modulator 100 includes a terminator 112 having a predetermined impedance within the housing 102.

[0025] FIG. 2 is a diagram showing an example of the configuration of the optical modulation element 104 accommodated within the housing 102 of the optical modulator 100 shown in FIG. 1. FIGS. 3 and 4 are partial detailed diagrams of the optical modulation section A and the optical reflection section B (described later) of the optical modulation element 104 shown in FIG. 2, respectively.

[0026] The optical modulation element 104 is composed of an optical waveguide 230 (the entire thick dotted line in the figure) formed on the substrate 220, and performs, for example, 200G DP-QPSK modulation. The substrate 220 is, for example, a Z-cut LN substrate with an electro-optic effect, which is processed to a thickness of 20 μm or less (for example, 2 μm) and thinned. The optical waveguide 230 is a convex optical waveguide (for example, a rib-type optical waveguide or a ridge-type optical waveguide) composed of convex portions extending in a strip shape formed on the surface of the thinned substrate 220. Here, since the refractive index of the LN substrate can locally change due to the photoelastic effect when stress is applied, it is generally adhered to a support plate such as an Si (silicon) substrate, a glass substrate, or LN to reinforce the mechanical strength of the entire substrate. In this embodiment, as will be described later, the substrate 220 is adhered to the support plate 500.

[0027] The substrate 220 is, for example, rectangular, and has two sides 280a and 280b that extend in the vertical direction in the figure and face each other, and two sides 280c and 280d that extend in the horizontal direction in the figure and face each other.

[0028] The optical waveguide 230 includes an input waveguide 232 that receives input light (an arrow pointing to the right in the figure) from the input optical fiber 114 on the upper side of the left side 280a of the substrate 220 in the figure, and a branching waveguide 234 that branches the input light into two lights having the same light amount. The optical waveguide 230 also includes two modulation units, so-called nested Mach-Zehnder type optical waveguides 240a and 240b, that modulate each of the lights branched by the branching waveguide 234.

[0029] The nested Mach-Zehnder type optical waveguides 240a and 240b each include two Mach-Zehnder type optical waveguides 244a and 244b, and 244c and 244d, respectively, provided in two waveguide portions forming a pair of parallel waveguides. The Mach-Zehnder type optical waveguides 244a and 244b each have parallel waveguides 246a1 and 246a2, and parallel waveguides 246b1 and 246b2, respectively. The Mach-Zehnder type optical waveguides 244c and 244d each have parallel waveguides 246c1 and 246c2, and parallel waveguides 246d1 and 246d2, respectively.

[0030] Hereinafter, the nested Mach-Zehnder optical waveguides 240a and 240b are also collectively referred to as the nested Mach-Zehnder optical waveguide 240, and the Mach-Zehnder optical waveguides 244a, 244b, 244c, and 244d are also collectively referred to as the Mach-Zehnder optical waveguide 244. Also, the parallel waveguides 246a1 and 246a2 are also collectively referred to as the parallel waveguide 246a, and the parallel waveguides 246b1 and 246b2 are also collectively referred to as the parallel waveguide 246b. Also, the parallel waveguides 246c1 and 246c2 are also collectively referred to as the parallel waveguide 246c, and the parallel waveguides 246d1 and 246d2 are also collectively referred to as the parallel waveguide 246d. Furthermore, the parallel waveguides 246a, 246b, 246c, and 246d are also collectively referred to as the parallel waveguide 246.

[0031] As shown in FIG. 2, the nested Mach-Zehnder optical waveguide 240 includes an optical modulation section A and an optical reflection section B (each shown as a portion within the rectangle of the two-dot chain line in the figure). The optical reflection section B is a portion that changes the optical propagation direction in the two nested Mach-Zehnder optical waveguides 240. Specifically, each of the Mach-Zehnder optical waveguides 244 that make up the nested Mach-Zehnder optical waveguide 240 has two parallel waveguides 246 with curved portions, and the optical reflection section B is formed by the curved portions of a total of eight parallel waveguides 246 that make up the two nested Mach-Zehnder optical waveguides 240.

[0032] In this embodiment, for each of the input lights branched into two by the branching waveguide 234, the nested Mach-Zehnder optical waveguide 240 reflects the optical propagation direction by 180 degrees in the optical reflection section B, then performs QPSK modulation in the optical modulation section A, and outputs the modulated light (output) to the left in the figure from the respective output waveguides 248a and 248b. These two output lights are then polarization combined by the optical unit 116 arranged outside the substrate 220 and combined into one optical beam.

[0033] On the substrate 220, four signal electrodes 250a, 250b, 250c, and 250d are provided to perform modulation operations on each of the total four Mach-Zehnder optical waveguides 244a, 244b, 244c, and 244d that constitute the nested Mach-Zehnder optical waveguides 240a and 240b. Hereinafter, the signal electrodes 250a, 250b, 250c, and 250d are also collectively referred to as the signal electrode 250.

[0034] Each of the signal electrodes 250 includes two signal lines. That is, the signal electrode 250a includes the signal lines 252a1 and 252a2, the signal electrode 250b includes the signal lines 252b1 and 252b2. Also, the signal electrode 250c includes the signal lines 252c1 and 252c2, and the signal electrode 250d includes the signal lines 252d1 and 252d2. Hereinafter, the signal lines 252a1 and 252a2 are also collectively referred to as the signal line 252a, the signal lines 252b1 and 252b2 are also collectively referred to as the signal line 252b. Also, the signal lines 252c1 and 252c2 are also collectively referred to as the signal line 252c, and the signal lines 252d1 and 252d2 are also collectively referred to as the signal line 252d. Furthermore, the signal lines 252a, 252b, 252c, and 252d are also collectively referred to as the signal line 252.

[0035] A high-frequency electrical signal for performing a modulation operation is input to each of the signal electrodes 250 to the corresponding Mach-Zehnder optical waveguide 244. This high-frequency electrical signal is composed of two electrical signals having a phase difference of 180° from each other, that is, a differential signal. The two signal lines 252 that constitute each of the signal electrodes 250 transmit each of the two electrical signals that constitute the differential signal input to the signal electrode 250.

[0036] For easy understanding, as an example, hereinafter, it is described assuming that the differential signal is a pulse signal, but the differential signal is not limited to a pulse signal. The differential signal can have an arbitrary waveform in which the mutual phase information of the electrical signals in the two signal lines affects the modulation.

[0037] An intermediate layer 260 is formed between the substrate 220 and the signal line 252 on the substrate 220. The intermediate layer 260 is, for example, a buffer layer and can be composed of silicon dioxide (SiO 2 ) or resin or the like. In the present embodiment, the intermediate layer 260 is resin. Specifically, the intermediate layer 260 is a photoresist, includes a coupling agent (crosslinking agent), and is a so-called photosensitive permanent film that undergoes a crosslinking reaction by heat and cures. Hereinafter, this photosensitive permanent film will also be referred to as a permanent resist. In the present embodiment, the intermediate layer 260, which is a permanent resist, is formed on the entire surface of the substrate 220. However, this is an example, and the intermediate layer 260 may be partially formed on the substrate 220 so as to be disposed between the substrate 220 and the signal line 252.

[0038] As shown in FIG. 3, in the optical modulation unit A, the signal lines 252a1 and 252a2 that constitute the signal electrode 250a respectively have acting portions 300a and 300b (hatched portions in the figure) formed on the upper portions of the parallel waveguides 246a1 and 246a2 of the Mach-Zehnder optical waveguide 244a, and propagate differential signals having a phase difference of 180° from each other to cause a modulation operation in the Mach-Zehnder optical waveguide 244a. Further, the signal lines 252b1 and 252b2 that constitute the signal electrode 250b respectively have acting portions 300c and 300d formed on the upper portions of the parallel waveguides 246b1 and 246b2 of the Mach-Zehnder optical waveguide 244b, and propagate differential signals having a phase difference of 180° from each other to cause a modulation operation in the Mach-Zehnder optical waveguide 244b.

[0039] Similarly, the signal lines 252c1 and 252c2 that constitute the signal electrode 250c each have an acting part 300e and 300f formed on the upper parts of the parallel waveguides 246c1 and 246c2 of the Mach-Zehnder optical waveguide 244c, and propagate differential signals having a phase difference of 180° from each other to cause the Mach-Zehnder optical waveguide 244c to perform a modulation operation. Also, the signal lines 252d1 and 252d2 that constitute the signal electrode 250d each have an acting part 300g and 300h formed on the upper parts of the parallel waveguides 246d1 and 246d2 of the Mach-Zehnder optical waveguide 244d, and propagate differential signals having a phase difference of 180° from each other to cause the Mach-Zehnder optical waveguide 244d to perform a modulation operation.

[0040] Hereinafter, the acting parts 300a, 300b, 300c, 300d, 300e, 300f, 300g, and 300h are collectively referred to as the acting part 300.

[0041] Referring to FIG. 2, the signal lines 252a1, 252a2, 252b1, 252b2, 252c1, 252c2, 252d1, and 252d2 each extend rightward in the drawing of the substrate 220, cross over the eight parallel waveguides 246 in the optical turning part B, and then extend to the side 280b and are connected to the pads 254a, 254b, 254c, 254d, 254e, 254f, 254g, and 254h.

[0042] Hereinafter, the pads 254a, 254b, 254c, 254d, 254e, 254f, 254g, and 254h are also collectively referred to as the pad 254. Each of the pads 254 is connected to the relay substrate 106 of the optical modulator 100 shown in FIG. 1 by wire bonding or the like.

[0043] To the left of the illustration of signal lines 252a1, 252a2, 252b1, 252b2, 252c1, 252c2, 252d1, and 252d2, they bend downward in the illustration and extend to side 280d of substrate 220, and are connected to pads 258a, 258b, 258c, 258d, 258e, 258f, 258g, and 258h. Hereinafter, pads 258a, 258b, 258c, 258d, 258e, 258f, 258g, and 258h are also collectively referred to as pad 258. Each of pads 258 is connected to each of terminating resistors (not shown) that constitute terminator 112 by wire bonding or the like. The terminating resistors are configured by eight when inserted between each of the eight signal lines 252 and a ground electrode (not shown in FIG. 2), and can be configured by four when inserted between each pair of two signal lines 252a, 252b, 252c, and 252d that propagate differential signals.

[0044] In FIG. 2, each of signal lines 252, in accordance with the prior art, together with a ground electrode (not shown) formed on the surface of substrate 220 so as to sandwich these signal lines 252 at positions separated by a certain distance, constitutes a distributed constant line having a predetermined impedance. In the present embodiment, this distributed constant line is a coplanar transmission line.

[0045] Thereby, high-frequency electrical signals input from signal pins 108 to respective pads 254 via relay substrate 106 become traveling waves and propagate through respective signal lines 252, and at each of acting portions 300, light waves propagating through corresponding Mach-Zehnder optical waveguides 244 are respectively modulated.

[0046] The optical modulation element 104 is also provided with bias electrodes 270a, 270b, and 270c for compensating for fluctuations in the bias point due to so-called DC drift and adjusting the operating point. Bias electrode 270a is used for compensating for fluctuations in the bias points of nested Mach-Zehnder optical waveguides 240a and 240b. Also, bias electrodes 270b and 270c are respectively used for compensating for fluctuations in the bias points of Mach-Zehnder optical waveguides 244a, 244b, and 244c, 244d.

[0047] These bias electrodes 270a, 270b, and 270c each extend to the upper side edge 280c of the substrate 220 in the drawing and are connected to any one of the signal pins 110 via the relay substrate 106. The corresponding signal pin 110 is connected to a bias control circuit provided outside the housing 102. Thereby, the bias electrodes 270a, 270b, and 270c are driven by the bias control circuit, and the operating point is adjusted to compensate for the bias point variation with respect to each corresponding Mach-Zehnder optical waveguide. Hereinafter, the bias electrodes 270a, 270b, and 270c are also collectively referred to as the bias electrode 270.

[0048] As described above, in the optical modulation element 104 of the present embodiment, in the optical reflection part B, each of the Mach-Zehnder optical waveguides 244 has two parallel waveguides 246 having a curved part. For this reason, there are a plurality of intersections of the parallel waveguides 246 and the signal electrodes 250 on the substrate 220. Specifically, on the substrate 220, in the right side in the drawing of the optical modulation part A and in the optical reflection part B, each of the two signal lines 252 constituting the signal electrode 250 intersects with each of the two parallel waveguides 246 constituting the four Mach-Zehnder optical waveguides 244. Here, on the surface of the substrate 220, a portion where the two signal lines 252 propagating the differential signal and the two parallel waveguides 246 constituting one Mach-Zehnder optical waveguide 244 intersect is also referred to as an intersection region.

[0049] In FIG. 2, to the right of the light modulation section A in the drawing, there is an intersection region 400c1 where two signal lines 252c (i.e., signal lines 252c1 and 252c2) for propagating differential signals and two parallel waveguides 246c (i.e., parallel waveguides 246c1 and 246c2) constituting the Mach-Zehnder optical waveguide 244c intersect. Also, there are intersection regions 400b1 and 400b2 where two signal lines 252c and 252d respectively intersect with two parallel waveguides 246b. Further, there are intersection regions 400a1, 400a2, and 400a3 where two signal lines 252b, 252c, and 252d respectively intersect with two parallel waveguides 246a.

[0050] Similarly, in the optical reflection section B, as shown in FIG. 4, there are intersection regions 400a4, 400a5, 400a6, and 400a7 where two parallel waveguides 246a intersect with two signal lines 252d, 252c, 252b, and 252a respectively for propagating differential signals. Also, there are intersection regions 400b3, 400b4, 400b5, and 400b6 where the parallel waveguide 246b intersects with the signal lines 252d, 252c, 252b, and 252a.

[0051] Also, there are intersection regions 400c2, 400b4, 400b5, and 400b6 formed by the parallel waveguide 246c intersecting with the signal lines 252d, 252c, 252b, and 252a. Further, there are intersection regions 400d1, 400d2, 400d3, and 400d4 where the parallel waveguide 246d intersects with the signal lines 252d, 252c, 252b, and 252a.

[0052] Hereinafter, the crossing regions 400a1, 400a2, 400a3, 400a4, 400a5, 400a6 are collectively referred to as the crossing region 400a, and the crossing regions 400b1, 400b2, 400b3, 400b4, 400b5, 400b6 are collectively referred to as the crossing region 400b. Also, the crossing regions 400c1, 400c2, 400c3, 400c4, 400c5 are collectively referred to as the crossing region 400c, and the crossing regions 400d1, 400d2, 400d3, 400d4 are collectively referred to as the crossing region 400d. Furthermore, the crossing regions 400a, 400b, 400c, 400d are collectively referred to as the crossing region 400.

[0053] In the optical modulation element 104, in these crossing regions 400, disturbance modulation caused by the crossing of the parallel waveguide 246 and the signal line 252 may occur. As described above, in the Mach-Zehnder type optical waveguide 244 composed of two parallel waveguides 246, the disturbance modulation not only generates noise in each optical signal propagating through these two parallel waveguides 246, but also generates noise in the phase difference between these two signal lights, so the influence on the modulation operation is great.

[0054] In the crossing region 400, since differential signals are propagated on the two signal lines 252, the disturbance modulation generated by one signal line 252 in the two parallel waveguides 246 can be canceled by the disturbance modulation generated by the other signal line 252 in the two parallel waveguides 246.

[0055] However, such a canceling effect is ideally achieved when the differential signals propagating on the two signal lines 252 reach the two parallel waveguides 246 simultaneously, that is, when there is no phase difference between the differential signals reaching the parallel waveguide 246. To realize such an ideal state, in the crossing region 400, it is desirable that the length (crossing interval) between the crossing portions of the two parallel waveguides 246 in one signal line 252 and the length (crossing interval) between the crossing portions of the two parallel waveguides 246 in the other signal line 252 are equal to each other.

[0056] However, when a plurality of Mach-Zehnder optical waveguides 244 having curved parallel waveguides 246 as in this embodiment and a plurality of sets of signal lines 252 each composed of two signal lines 252 for propagating differential signals intersect, due to the pattern arrangement of the signal lines 252 and the parallel waveguides 246, the intersection intervals of the two signal lines 252 often do not necessarily become equal to each other.

[0057] FIG. 19 is a diagram for explaining the propagation of an electrical signal when two signal lines and two parallel waveguides intersect. In the illustrated example, two signal lines 910a and 910b for propagating differential signals intersect with each of two parallel waveguides 920a and 920b constituting a Mach-Zehnder optical waveguide. Hereinafter, the signal lines 910a and 910b are also collectively referred to as the signal line 910, and the parallel waveguides 920a and 920b are also collectively referred to as the parallel waveguide 920.

[0058] In FIG. 19, the intersection interval L2, which is the length between the intersections of one signal line 910b with the two parallel waveguides 920a and 920b, is longer than the intersection interval L1 of the other signal line 910a with the two parallel waveguides 920a and 920b.

[0059] For this reason, for example, even if a non-inverted pulse 930a propagating through the signal line 910a and an inverted pulse 930b propagating through the signal line 910b as differential signal pulses simultaneously pass through one parallel waveguide 920a and the disturbing modulation in the parallel waveguide 920a is completely canceled out, when the inverted pulse 930b reaches the parallel waveguide 920b, the non-inverted pulse 930a will be at a position that has passed through the parallel waveguide 920b. As a result, in the parallel waveguide 920b, the canceling effect of the disturbing modulation by these differential signal pulses is not exerted.

[0060] Viewing this from the time aspect, the phenomenon of disappearance or reduction of the canceling effect of the disturbing modulation as described above can be explained as follows using the propagation delay time between the differential signal pulses from when they pass through the parallel waveguide 920a until they reach the 920b.

[0061] For a non-inverting pulse 930a propagating in a section with a crossing interval L1 in one signal line 910a, the propagation delay time ΔT of an inverting pulse 930b propagating in a section with a crossing interval L2 (>L1) in the other signal line 910b is given by the following equation, where V is the signal propagation speed of the electrical signal in these signal lines 910. ΔT = (L2 - L1) / V (1)

[0062] That is, even if the differential signal propagating through the two signal lines 910 is in a state where their phases are exactly in an inverted phase state (hereinafter referred to as the ideal inversion state) at the time of arrival at one parallel waveguide 920a, at the time of arrival at the other parallel waveguide 920b, it will deviate from the ideal inversion state by a phase corresponding to the propagation delay time ΔT. As a result, in the above-mentioned other parallel waveguide 920b, the cancellation effect of the disturbance modulation by the two signal lines 910 propagating the differential signal will disappear or decrease.

[0063] In order to reduce the deviation from the ideal inversion state at the time of arrival at the other parallel waveguide 920b, it is conceivable to provide an offset phase between the differential signals at the time of arrival at one parallel waveguide 920a. However, in this case, disturbance modulation will remain in both of the two parallel waveguides 920. As a result, these disturbance modulations may affect the modulation operation of the Mach-Zehnder optical waveguide formed by the parallel waveguides 920 additively.

[0064] In order to suppress the reduction or disappearance of the cancellation effect of such disturbance modulation and improve the cancellation effect of the disturbance modulation, in the present embodiment and the modified examples described later, in the crossing region 400, the two signal lines 252 are configured to have a signal propagation speed higher than that of the portion outside the crossing region 400, or one of the two signal lines 252 is configured to have a signal propagation speed higher than that of the other.

[0065] Accordingly, when the two signal lines 252 have a faster signal propagation speed in the crossing region 400 than in the portions outside the crossing region 400, V in Equation (1) becomes larger in the crossing region 400 than in the portions outside the crossing region 400, so that the propagation delay time ΔT in the crossing region 400 is reduced. Further, when one of the two signal lines 252 has a faster signal propagation speed than the other, the propagation time difference of the differential signal propagating through each crossing interval can be reduced between the two signal lines, so that the propagation delay time ΔT in the crossing region 400 is reduced.

[0066] As a result, the deviation of the differential signal propagating through the two signal lines 252 from the ideal inversion state when crossing the two parallel waveguides 246 is reduced, and the cancellation effect of the disturbance modulation is improved as compared with the conventional optical modulation element in which the intermediate layer is formed with the same thickness over the entire substrate.

[0067] As an example, the configuration of the crossing region 400a3 in FIG. 2 will be described. FIG. 5 is a partial detailed view of the crossing region 400a3 in FIG. 2. In FIG. 5, the illustration of the intermediate layer 260 is omitted. Further, in FIG. 5, ground electrodes 290a, 290b, and 290c not shown in FIG. 2 are shown. As described above, ground electrodes are formed on the substrate 220 so as to sandwich each of the signal lines 252 from positions separated by a certain interval (separation distance), and the signal lines 252, together with these ground electrodes, constitute a distributed constant line (for example, a coplanar line) having a predetermined impedance. The ground electrodes 290a, 290b, and 290c are part of such ground electrodes.

[0068] In the example of FIG. 5, the ground electrodes 290a and 290b are formed so as to sandwich the signal line 252d1 from positions separated by a constant separation distance g10, and the ground electrodes 290b and 290c are formed so as to sandwich the signal line 252d2 from positions separated by a constant separation distance g10.

[0069] FIG. 6 is a cross-sectional view taken along the line VI-VI in FIG. 5, showing the cross-sectional structure of the substrate 220 along the signal line 252d2. Note that the cross-sectional structure of the substrate 220 along the signal line 252d1 is configured in the same manner as the cross-sectional structure along the signal line 252d2 shown in FIG. 6. As shown in FIG. 6, the substrate 220 is fixed to the support plate 500.

[0070] In the intersection region 400a3, two signal lines 252d1 and 252d2 that propagate differential signals cross the parallel waveguides 246a1 and 246a2 that constitute the Mach-Zehnder optical waveguide 244a, respectively, with different crossing angles. As a result, the crossing interval L20, which is the distance between the crossing portions of the signal line 252d2 with the parallel waveguides 246a1 and 246a2, is larger than the crossing interval L10, which is the distance between the crossing portions of the signal line 252d1 with the parallel waveguides 246a1 and 246a2.

[0071] Also, in the present embodiment, in particular, unlike the conventional optical modulator in which an intermediate layer such as a buffer layer is formed with a uniform thickness on the substrate, in the intersection region 400a3, the thickness t20 of the intermediate layer 260 between the substrate 220 and the signal line 252 is larger than the thickness t10 in the portion other than the intersection region 400a3. Here, the thickness t10 can be a thickness (for example, 1 μm or less) such that substantially no optical absorption loss due to the metal constituting the signal line 252 occurs in the parallel waveguide 246, similar to the conventional optical modulator.

[0072] The relative permittivity of SiO 2 or permanent resist, etc., used for the intermediate layer 260 such as a buffer layer is about one digit smaller than the relative permittivity (several tens) of a ferroelectric material such as LN used for the substrate 220. Therefore, when the intermediate layer below the signal line 252 is thickened, the electric signal (high-frequency electric signal in the microwave band) propagating through the signal line 252 propagates faster because the effective refractive index in the signal line 252 is reduced (that is, the signal propagation speed becomes faster).

[0073] In this embodiment, the thickness of the intermediate layer between the signal lines 252d1 and 252d2 and the substrate 220 is such that the thickness t20 in the crossing region 400a3 is greater than the thickness t10 of the portions other than the crossing region 400a3. For this reason, the signal propagation speed V10 of the two signal lines 252d1 and 252d2 in the crossing region 400a3 becomes faster than the signal propagation speed V0 of the portions other than the crossing region 400a3 (that is, V10 > V0). As a result, since the signal propagation speed V in Equation (1) increases from V0 to V10, the propagation delay time ΔT decreases.

[0074] As a result, in the crossing region 400a3, the deviation of the differential signal propagating through the two signal lines 252d1 and 252d2 from the ideal inversion state when crossing each of the two parallel waveguides 246 is reduced, and the cancellation effect of the disturbance modulation is improved compared to the conventional optical modulation element in which the intermediate layer is formed with the same thickness (for example, t10) over the entire substrate.

[0075] Note that from the viewpoint of obtaining an effective cancellation effect for the disturbance modulation, it is desirable that the propagation delay time ΔT is 1 / 2 or less of the signal pulse width Δt in the differential signal (that is, ΔT ≤ Δt / 2). This is because when ΔT exceeds Δt / 2, the phase difference between the two differential signal pulses that are phase-inverted from each other exceeds half of the pulse width, and the cancellation effect of the disturbance modulation when passing through the parallel waveguide 246 is reduced to less than half.

[0076] Also, in order to obtain an effective improvement effect for the cancellation effect of the disturbance modulation, it is desirable that the thickness t20 of the intermediate layer 260 in the crossing region 400a3 is 2 times or more the thickness t10 of the portions other than the crossing region 400a3 (that is, t20 ≥ 2 × t10). In this embodiment, for example, t10 is 0.5 μm and t20 is 1 μm.

[0077] Also, in this embodiment, the intermediate layer 260 is configured as a single layer, but it may be a multilayer. For example, the intermediate layer 260 is SiO formed on the substrate 220 2It can be configured by a first intermediate layer composed of , and a second intermediate layer which is a permanent resist formed on the first intermediate layer in the intersection region 400. In this case, the first intermediate layer is formed with a thickness t10, and the second intermediate layer can be formed such that the total thickness of the intermediate layer 260 including the first intermediate layer becomes t20.

[0078] In addition, in FIG. 2, the intersection regions 400 other than the intersection region 400a3 are drawn such that the intersection intervals between the intersections of the two parallel waveguides 246 of the two signal lines 252 that propagate differential signals are equal to each other. However, this is just an example, and depending on the pattern arrangement of the optical waveguide 230 and the signal lines 252 on the substrate 220, for the intersection regions 400 other than the intersection region 400a3, similar to the intersection region 400a3, the intersection intervals in the two signal lines 252 can be different from each other. In this case, for any intersection region 400 where the intersection intervals in such two signal lines 252 are different, by using the configuration of the above-described intersection region 400a3 and the configurations of the following modified examples, the cancellation effect of the disturbing modulation in those intersection regions 400 can be improved.

[0079] Next, a modified example of the configuration of the intersection region 400 will be described. For ease of understanding, the following modified examples will be described as modified examples of the intersection region 400a3. However, as described above, these modified examples can also be applied to any intersection region 400 where the intersection intervals in the two signal lines 252 are different. [1.1 First Modified Example] FIG. 7 is a diagram showing the configuration of an intersection region 400a3-1 according to a first modification of the intersection region 400a3 shown in FIGS. 5 and 6. The configuration of the intersection region 400a3-1 shown in FIG. 7 can be used in place of the configuration of the intersection region 400a3 shown in FIGS. 5 and 6 in the optical modulation element 104. FIG. 7 is a diagram corresponding to a VI-VI cross-sectional view of the intersection region 400a3 shown in FIG. 6, and shows a cross-sectional structure of the substrate 220 along the signal line 252d2. Note that the cross-sectional structure of the substrate 220 along the signal line 252d1 is the same as the cross-sectional structure along the signal line 252d2 shown in FIG. 7. In FIG. 7, the same components as those in FIG. 6 are denoted by the same reference numerals as those in FIG. 6, and the description of FIG. 6 described above is incorporated herein.

[0080] The intersection region 400a3-1 has a configuration similar to that of the intersection region 400a3, except that it has an intermediate layer 260-1 instead of the intermediate layer 260. The intermediate layer 260-1 has a configuration similar to that of the intermediate layer 260, but while the intermediate layer 260 shown in FIG. 6 extends to the periphery of the intersection region 400a3, the intermediate layer 260-1 is formed only in the portion of the intersection region 400a3-1. That is, the intermediate layer 260-1 is not provided in the portion other than the intersection region 400a3-1.

[0081] The configuration of the intersection region 400a3-1 is useful, for example, when the optical absorption loss generated in the parallel waveguide 246 due to the metal constituting the signal line 252 can be suppressed to a level that is not a practical problem even without a buffer layer such as the intermediate layer 260 or 260-1 on the substrate.

[0082] [1.2 Second Modification Example] Figures 8 and 9 are diagrams showing the configuration of the intersection region 400a3-2, which pertains to a second modification of the intersection region 400a3 shown in FIGS. 5 and 6. The configuration of the intersection region 400a3-2 shown in FIGS. 8 and 9 can be used in place of the configuration of the intersection region 400a3 shown in FIGS. 5 and 6 in the optical modulation element 104. FIG. 8 is a plan view of the intersection region 400a3-2 corresponding to FIG. 5. Although the illustration of the ground electrodes is omitted in FIG. 8, it is assumed that the ground electrodes 290a, 290b, and 290c are arranged at the same positions as those shown in FIG. 5.

[0083] FIG. 9 is a cross-sectional view taken along the arrow IX-IX in FIG. 8, and is a diagram showing the cross-sectional structure of the substrate 220 along the parallel waveguide 246a2. The cross-sectional structure of the substrate 220 along the parallel waveguide 246a1 is the same as the cross-sectional structure of the substrate 220 along the parallel waveguide 246a2 shown in FIG. 9. In FIGS. 8 and 9, the same components as those in FIGS. 5 and 6 are denoted by the same reference numerals as those in FIGS. 5 and 6, and the descriptions of FIGS. 5 and 6 described above are incorporated herein by reference.

[0084] In the above-described intersection region 400a3 and its modification, the intersection region 400a3-1, both of the two signal lines 252 are configured to have a signal propagation speed faster than that of the portions other than these intersection regions. On the other hand, in the intersection region 400a3-2 according to the second modification, of the two signal lines 252, the signal line 252 with a longer intersection interval has a faster signal propagation speed than the other signal line with a shorter intersection interval.

[0085] Specifically, the intersection region 400a3-2 has the same configuration as the intersection region 400a3, except that it has an intermediate layer 260-2 instead of the intermediate layer 260. The intermediate layer 260-2 has the same configuration as the intermediate layer 260, but while the intermediate layer 260 extends to the periphery of the intersection region 400a3, the intermediate layer 260-2 is formed only in the portion of the intersection region 400a3-1.

[0086] In particular, the intermediate layer 260-2 has a greater thickness t21 at the lower part of the signal line 252d2 having the intersection interval L20 that is longer than the intersection interval L10, compared to the thickness t11 at the lower part of the signal line 252d1 having the intersection interval L10 (i.e., t21 > t11). As a result, the signal line 252d2 having the long intersection interval L20 has a faster signal propagation speed than the signal line 252d1 having the short intersection interval L10.

[0087] Therefore, in the intersection region 400a3-2, in the signal lines 252d1 and 252d2, the propagation time difference, i.e., the propagation delay time ΔT, between the electrical signals propagating through their respective intersection intervals is reduced or eliminated. As a result, in the intersection region 400a3-2, the cancellation effect of the disturbance modulation can be improved.

[0088] [1.3 Third Modified Example] FIGS. 10 and 11 are diagrams showing the configuration of the intersection region 400a3-3 according to a third modified example of the intersection region 400a3 shown in FIGS. 5 and 6. The configuration of the intersection region 400a3-3 shown in FIGS. 10 and 11 can be used in place of the configuration of the intersection region 400a3 shown in FIGS. 5 and 6 in the optical modulation element 104. FIG. 10 is a plan view of the intersection region 400a3-3 corresponding to FIG. 5. Although the illustration of the ground electrode is omitted in FIG. 10, it is assumed that on the substrate 220, ground electrodes that sandwich these signal lines are arranged at positions spaced a certain distance from the signal lines 252d1-1 and 252d2-1, similar to the ground electrodes 290a, 290b, and 290c shown in FIG. 5.

[0089] FIG. 11 is a cross-sectional view taken along the line XI-XI in FIG. 10, showing the cross-sectional structure of the substrate 220 along the signal line 252d2-1. The cross-sectional structure of the substrate 220 along the signal line 252d1-1 is also the same as that in FIG. 11. In FIGS. 10 and 11, the same components as those in FIGS. 5 and 6 are denoted by the same reference numerals as in FIGS. 5 and 6, and the descriptions of FIGS. 5 and 6 described above are incorporated herein by reference.

[0090] The intersection region 400a3-3 has the same configuration as the intersection region 400a3, except that it has an intermediate layer 260-3 instead of the intermediate layer 260. The intermediate layer 260-3 has the same configuration as the intermediate layer 260, but while the intermediate layer 260 extends to the periphery of the intersection region 400a3, it is formed only in the portion of the intersection region 400a3-3.

[0091] Also, the intersection region 400a3-3 has signal lines 252d1-1 and 252d2-1 instead of the signal lines 252d1 and 252d2. The signal lines 252d1-1 and 252d2-1 have the same configuration as the signal lines 252d1 and 252d2, but their thicknesses are different from those of the signal lines 252d1 and 252d2.

[0092] The signal lines 252d1-1 and 252d2-1 each have the same configuration as the signal lines 252d1 and 252d2, but their thicknesses are not uniform, and the thickness t40 in the intersection region 400a3-3 is greater than the thickness t30 in the portion outside the intersection region 400a3-3.

[0093] As the thickness of the signal line 252 increases, the contact area with an environment having a relative permittivity smaller than that of the substrate 220 (for example, the gas inside the housing 102) increases, and the effective refractive index for the electrical signal decreases, so the signal propagation speed in that portion becomes faster.

[0094] Since the thickness t40 of the signal lines 252d1-1 and 252d2-1 in the intersection region 400a3-3 is greater than the thickness t30 in the portion outside the intersection region 400a3-3, the signal propagation speed in the intersection region 400a3-3 becomes faster. This, combined with the fact that the intermediate layer 260-3 is provided in the intersection region 400a3-3, further improves the cancellation effect of the disturbance modulation in the intersection region 400a3-3.

[0095] In order to obtain an effective improvement effect regarding the cancellation effect of the scrambling modulation, it is desirable that the thickness t40 in the intersection region 400a3-3 of the signal lines 252d1-1 and 252d2-1 is twice or more the thickness t30 of the portion other than the intersection region 400a3-3 (that is, t40 ≧ 2 × t30). In this modification example, for example, t30 is 5 μm and t40 is 10 μm. Such a thick signal line 252 can be formed, for example, by electrolytic plating of gold (Au).

[0096] [1.4 Fourth Modification Example] FIGS. 12 and 13 are diagrams showing the configuration of the intersection region 400a3-4 according to a fourth modification example of the intersection region 400a3 shown in FIGS. 5 and 6. The configuration of the intersection region 400a3-4 shown in FIGS. 12 and 13 can be used in place of the configuration of the intersection region 400a3 shown in FIGS. 5 and 6 in the optical modulation element 104. FIG. 12 is a plan view of the intersection region 400a3-4 corresponding to FIG. 5. Note that, in FIG. 12, the illustration of the ground electrode is omitted, but on the substrate 220, ground electrodes that sandwich these signal lines are arranged at positions separated from the signal lines 252d1-2 and 252d2-2 by a certain interval distance, similar to the ground electrodes 290a, 290b, and 290c shown in FIG. 5.

[0097] FIG. 13 is a cross-sectional view taken along the arrow XIII-XIII in FIG. 12 and shows a cross-sectional structure along the parallel waveguide 246a2. Note that the cross-sectional structure along the parallel waveguide 246a1 is also the same as that in FIG. 13. In FIGS. 12 and 13, the same components as those in FIGS. 5 and 6 are denoted by the same reference numerals as those in FIGS. 5 and 6, and the description of FIGS. 5 and 6 described above is incorporated herein.

[0098] The intersection region 400a3-4 has a configuration similar to that of the intersection region 400a3, except that it has an intermediate layer 260-4 instead of the intermediate layer 260. The intermediate layer 260-4 has a configuration similar to that of the intermediate layer 260, but while the intermediate layer 260 extends to the periphery of the intersection region 400a3, it is formed only in the portion of the intersection region 400a3-4.

[0099] Also, the intersection region 400a3-4 has signal lines 252d1-2 and 252d2-2 instead of signal lines 252d1 and 252d2. Signal lines 252d1-1 and 252d2-1 have the same configuration as signal lines 252d1 and 252d2, but their thicknesses are different from each other.

[0100] Specifically, in the intersection region 400a3-4, the thickness t41 of the signal line 252d2-2 having a long intersection interval L20 is greater than the thickness t31 of the signal line 252d1-2 having an intersection interval L10 shorter than the intersection interval L20. As a result, in the intersection region 400a3-4, among the two signal lines 252d1-2 and 252d2-2 that propagate differential signals, the signal line 252d2-2 with the longer intersection interval has a faster signal propagation speed than the other signal line 252d1-2 with the shorter intersection interval.

[0101] Thereby, in the intersection region 400a3-4, in the signal lines 252d1-2 and 252d2-2, the propagation time difference, that is, the propagation delay time ΔT, between the electrical signals propagating through their respective intersection intervals is reduced or eliminated. As a result, in the intersection region 400a3-4, the cancellation effect of the disturbance modulation can be further improved.

[0102] Note that in order to obtain an effective improvement effect on the cancellation effect of the disturbance modulation, in the intersection region 400a3-4, it is desirable that the thickness t41 of the signal line 252d2-2 is twice or more the thickness t31 of the signal line 252d1-2 (that is, t41≧2×t31).

[0103] [1.5 Fifth Modification Example] FIG. 14 is a diagram showing the configuration of an intersection region 400a3-5 according to a fifth modification of the intersection region 400a3 shown in FIGS. 5 and 6. The configuration of the intersection region 400a3-5 shown in FIG. 14 can be used instead of the configuration of the intersection region 400a3 shown in FIGS. 5 and 6 in the optical modulation element 104. FIG. 14 is a plan view of the intersection region 400a3-5 corresponding to FIG. 5. In FIG. 14, the same components as those in FIG. 5 are denoted by the same reference numerals as in FIG. 5, and the description of FIG. 5 described above is incorporated herein.

[0104] Note that, in FIG. 14, the illustration of the ground electrode is omitted, but on the substrate 220, similar to the ground electrodes 290a, 290b, and 290c shown in FIG. 5, ground electrodes that sandwich these signal lines are arranged at positions separated from the signal lines 252d1-3 and 252d2-3 by a certain distance. Also, in the configuration of the intersection region 400a3-5 shown in FIG. 14, an intermediate layer such as a buffer layer may or may not be present between the substrate 220 and the signal line 252.

[0105] The intersection region 400a3-5 has a configuration similar to that of the intersection region 400a3, but has signal lines 252d1-3 and 252d2-3 instead of the signal lines 252d1 and 252d2. The signal lines 252d1-3 and 252d2-4 have a configuration similar to that of the signal lines 252d1 and 252d2, but their widths are different from those of the signal lines 252d1 and 252d2.

[0106] Specifically, in the intersection region 400a3-5, the signal lines 252d1-3 and 252d2-3 that propagate differential signals have widths w20 that are narrower than the width w10 in the portion upstream of the intersection region 400a3-5 along the propagation direction of the differential signals.

[0107] When the width of the signal line 252 is narrowed, the signal propagation speed increases. For this reason, in the signal lines 252d1-3 and 252d2-3, the signal propagation speed in the intersection region 400a3-5 is faster than the signal propagation speed in the portion upstream of the intersection region 400a3-5. Therefore, in the intersection region 400a3-5, the propagation delay time ΔT is reduced, so the cancellation effect of the disturbance modulation is improved.

[0108] In addition, in order to obtain an effective improvement effect on the cancellation effect of the disturbance modulation, in the signal lines 252d1-3 and 252d2-3, it is desirable that the width w20 in the intersection region 400a3-5 is 1 / 2 or less of the width w10 upstream of the intersection region 400a3-5 (that is, w20 ≦ 1 / 2 × w10). In this modification example, for example, w10 is 20 μm and w10 is 10 μm.

[0109] Also, in the configuration shown in FIG. 14, the signal lines 252d1-3 and 252d2-3 may be configured to have the same width as the width w10 in the portion upstream of the intersection region 400a3-5 in the portion downstream of the intersection region 400a3-5 along the propagation direction of the differential signal.

[0110] [1.6 Sixth Modification Example] FIG. 15 is a diagram showing the configuration of the intersection region 400a3-6 according to a sixth modification example of the intersection region 400a3 shown in FIGS. 5 and 6. The configuration of the intersection region 400a3-6 shown in FIG. 15 can be used in place of the configuration of the intersection region 400a3 shown in FIGS. 5 and 6 in the optical modulation element 104. FIG. 15 is a plan view of the intersection region 400a3-6 corresponding to FIG. 5. In FIG. 15, the same components as those in FIG. 5 are denoted by the same reference numerals as those in FIG. 5, and the description of FIG. 5 described above is incorporated herein.

[0111] In FIG. 15, although the illustration of the ground electrode is omitted, on the substrate 220, similar to the ground electrodes 290a, 290b, and 290c shown in FIG. 5, ground electrodes that sandwich these signal lines are arranged at positions separated from the signal lines 252d1-4 and 252d2-4 by a certain interval distance. Also, in the configuration of the intersection region 400a3-6 shown in FIG. 15, an intermediate layer such as a buffer layer may or may not be present between the substrate 220 and the signal line 252.

[0112] The intersection region 400a3-6 has the same configuration as the intersection region 400a3, but has signal lines 252d1-4 and 252d2-4 instead of the signal lines 252d1 and 252d2. The signal lines 252d1-4 and 252d2-4 have the same configuration as the signal lines 252d1 and 252d2, but the relationship between their line widths is different from that of the signal lines 252d1 and 252d2.

[0113] Specifically, in the intersection region 400a3-6, the width w21 of one signal line 252d2-4 having a long intersection interval L20 is narrower than the width w11 of the other signal line 252d1-4 having a short intersection interval L10.

[0114] As a result, in the intersection region 400a3-6, among the two signal lines 252d1-4 and 252d2-4 that propagate differential signals, the signal line 252d2-4 with the longer intersection interval is configured to have a faster signal propagation speed than the other signal line 252d1-4 with the shorter intersection interval.

[0115] Therefore, in the intersection region 400a3-6, in the signal lines 252d1-4 and 252d2-4, the propagation time difference, that is, the propagation delay time ΔT, between the electrical signals propagating through their respective intersection intervals is reduced or eliminated. As a result, in the intersection region 400a3-6, the cancellation effect of disturbance modulation is improved.

[0116] In order to obtain an effective improvement effect regarding the cancellation effect of the disturbing modulation, in the intersection region 400a3-6, it is desirable that the width w21 of the signal line 252d2-4 is 1 / 2 or less of the width w11 of the signal line 252d1-4 (that is, w21 ≤ 1 / 2 × w11).

[0117] [1.7 Seventh Modified Example] FIG. 16 is a diagram showing the configuration of the intersection region 400a3-7 according to the seventh modified example of the intersection region 400a3 shown in FIGS. 5 and 6. The configuration of the intersection region 400a3-7 shown in FIG. 15 can be used in place of the configuration of the intersection region 400a3 shown in FIGS. 5 and 6 in the optical modulation element 104. FIG. 16 is a plan view of the intersection region 400a3-7 corresponding to FIG. 5. In FIG. 16, the same components as those in FIG. 5 are denoted by the same reference numerals as those in FIG. 5, and the description of FIG. 5 described above is incorporated herein. In the configuration of the intersection region 400a3-7 shown in FIG. 16, an intermediate layer such as a buffer layer may or may not be provided between the substrate 220 and the signal line 252.

[0118] The intersection region 400a3-7 has the same configuration as the intersection region 400a3, but has ground electrodes 290a-1, 290b-1, and 290c-1 instead of the ground electrodes 290a, 290b, and 290c. Hereinafter, the ground electrodes 290a, 290b, and 290c are also collectively referred to as the ground electrode 290, and the ground electrodes 290a-1, 290b-1, and 290c-1 are also collectively referred to as the ground electrode 290-1.

[0119] The signal line 252d1 and the signal line 252d2 constitute a distributed constant line (for example, a coplanar line) having a predetermined impedance together with the ground electrode 290-1. The ground electrode 290-1 has the same configuration as the ground electrode 290, but the separation distance from the signal line 252 is not constant like the ground electrode 290 shown in FIG. 5, and the separation distance g20 in the intersection region 400a3-7 is wider than the separation distance g10 in the portion other than the intersection region 400a3-7 (that is, g20 > g10).

[0120] When the distance between the signal line 252 and the ground electrode is increased, the signal propagation speed in that portion increases. Therefore, in the signal lines 252d1 and 252d2, the signal propagation speed in the intersection region 400a3-7 is faster than the signal propagation speed in portions other than the intersection region 400a3-7. Thus, in the intersection region 400a3-7, the propagation delay time ΔT is reduced, so the cancellation effect of the disturbance modulation is improved.

[0121] Note that the separation distances g10 and g20 between the signal line 252 and the ground electrode 290-1 need to be carefully set in consideration of the thickness of an intermediate layer such as a buffer layer that can be formed on the substrate 220, the width of the signal line 252, etc., in order to suppress the impedance change of the signal line 252 accompanying the change in the separation distance. In this modification, for example, the separation distance g10 is 5 μm and the separation distance g20 is 7 μm.

[0122] [2. Second Embodiment] Next, a second embodiment of the present invention will be described. This embodiment is an optical modulation module 1000 that uses the optical modulation element 104 included in the optical modulator 100 according to the first embodiment. FIG. 17 is a diagram showing the configuration of the optical modulation module 1000 according to this embodiment. In FIG. 17, the same components as those of the optical modulator 100 according to the first embodiment shown in FIG. 1 are denoted by the same reference numerals as those shown in FIG. 1, and the description of FIG. 1 described above is incorporated herein.

[0123] The optical modulation module 1000 has the same configuration as the optical modulator 100 shown in FIG. 1, but is different from the optical modulator 100 in that it includes a circuit board 1006 instead of the relay board 106. The circuit board 1006 includes a drive circuit 1008. The drive circuit 1008 generates a differential signal, which is a high-frequency electrical signal for driving the optical modulation element 104, based on, for example, a modulation signal supplied from the outside via the signal pin 108, and outputs the generated differential signal to the optical modulation element 104.

[0124] The optical modulation module 1000 having the above configuration includes an optical modulation element 104, similar to the optical modulator 100 according to the first embodiment described above. Therefore, similar to the optical modulator 100, it is possible to effectively reduce the disturbance modulation generated in the intersection region 400 and realize a good modulation operation.

[0125] In this embodiment, the optical modulation module 1000 is assumed to include an optical modulation element 104 as an example, but it may be configured to include an optical modulation element having an intersection region according to the modification examples shown in FIGS. 7 to 16.

[0126] [3. Third Embodiment] Next, a third embodiment of the present invention will be described. This embodiment is an optical transmission device 1100 equipped with the optical modulator 100 according to the first embodiment. FIG. 18 is a diagram showing the configuration of the optical transmission device 1100 according to this embodiment. This optical transmission device 1100 includes an optical modulator 100, a light source 1104 that emits light to the optical modulator 100, a modulator driving unit 1106, and a modulation signal generation unit 1108. Note that instead of the optical modulator 100 and the modulator driving unit 1106, the optical modulation module 1000 according to the second embodiment can also be used.

[0127] The modulation signal generation unit 1108 is an electronic circuit that generates an electrical signal for causing the optical modulator 100 to perform a modulation operation. Based on transmission data given from the outside, it generates a modulation signal, which is a high-frequency signal for causing the optical modulator 100 to perform an optical modulation operation according to the modulation data, and outputs it to the modulator driving unit 1106.

[0128] The modulator driving unit 1106 amplifies the modulation signal input from the modulation signal generation unit 1108 and outputs a differential signal, which is a set of four high-frequency electrical signals for driving the four signal electrodes 250 of the optical modulation element 104 included in the optical modulator 100. As described above, instead of the optical modulator 100 and the modulator driving unit 1106, for example, the optical modulation module 1000 provided with a driving circuit 1008 including a circuit corresponding to the modulator driving unit 1106 inside the housing 102 can also be used.

[0129] When the four sets of differential signals are input to the signal pin 108 of the optical modulator 100, they propagate through the four sets of signal lines 252 (i.e., signal lines 252a, 252b, 252c, and 252d) of the optical modulation element 104 to drive the optical modulation element 104. As a result, the light output from the light source 1104 is modulated by the optical modulator 100, for example, by DP-QPSK modulation, and becomes modulated light and is output from the optical transmission device 1100.

[0130] In particular, in the optical transmission device 1100, since the optical modulator 100 or the optical modulation module 1000 according to the above-described first embodiment is used, similar to the optical modulator 100 or the optical modulation module 1000, good modulation characteristics can be realized and good optical transmission can be performed.

[0131] Note that the present invention is not limited to the configurations of the above-described embodiments and their alternative configurations, and can be implemented in various aspects without departing from the gist thereof.

[0132] For example, in the above-described embodiments and their modifications, the resin constituting the intermediate layers 260, 260-1, 260-2, 260-3, 260-4 is, as an example, a permanent resist, but the resin constituting these intermediate layers is not limited to a permanent resist. For example, as the resin constituting these intermediate layers, a thermosetting or thermoplastic resin other than a permanent resist may be used.

[0133] Also, in the above-described embodiments and their modifications, the corners of the intermediate layers 260, 260-1, 260-3 are drawn at right angles (FIGS. 6, 7, 11), but from the viewpoint of the signal transmission characteristics (particularly, the radiation loss of the electrical signal) of the signal lines 252 formed on their upper portions, it is desirable that the corners of these intermediate layers 260, etc. are not right angles but curves. Such an intermediate layer having curved corners can be easily realized by heat treatment or the like when a resin such as a permanent resist is used as its material.

[0134] Also, as is apparent from the above-described embodiments and their modifications, if the two signal lines 252 that propagate the differential signal have a faster signal propagation speed in the intersection region 400 than in the portions outside the intersection region 400, or if one of the two signal lines 252 has a faster signal propagation speed than the other in the intersection region 400, the cancellation effect of the disturbance modulation in the intersection region 400 can be improved.

[0135] Therefore, for example, the seventh modification shown in FIG. 16 may be modified such that the signal propagation speed of the signal line 252d2 is faster than that of the signal line 252d1 in the intersection region 400a3-7. Such a configuration can be realized, for example, by configuring the separation distances between the signal line 252d1 and the ground electrodes 290a-1 and 290b-1 to be smaller than the separation distance g20 between the signal line 252d2 and the ground electrodes 290b-1 and 290c-1 in the intersection region 400a3-7.

[0136] Also, the configurations of the intersection regions 400a3, 400a3-1, 400a3-2, 400a3-3, 400a3-5, 400a3-6, and 400a3-7 shown in the above-described first embodiment and its modifications can be applied to any other intersection region 400 other than the intersection region 400a3, and these configurations may be mixed in one optical modulation element 104.

[0137] Also, as described above, the differential signal propagating through the signal line 252 is not limited to a pulse signal, and can have any waveform in which the mutual phase information of the electrical signals in the two signal lines affects the modulation. That is, the optical modulation element 104 can be a modulation element having any configuration with a signal electrode capable of operating.

[0138] In the above-described embodiment, as an example of the optical waveguide element, the optical modulation element 104 formed by the substrate 220 made of LN (LiNbO3) is shown. However, the optical waveguide element is not limited to this. The optical waveguide element can be an element having an arbitrary function (such as an optical switch, an optical directional coupler, etc., in addition to optical modulation) and is composed of a substrate of an arbitrary material (in addition to LN, InP, Si, etc.). Such an element can be, for example, a so-called silicon photonics waveguide device.

[0139] [4. Configuration Supported by the Above Embodiment] The above embodiment and modification examples support the following configurations.

[0140] (Configuration 1) An optical waveguide element having an optical waveguide formed by a convex portion extending on a substrate and a signal electrode formed on the substrate for controlling light waves propagating through the optical waveguide, wherein the optical waveguide includes a Mach-Zehnder type optical waveguide having two parallel waveguides with a curved portion, and the signal electrode is composed of two signal lines for transmitting differential signals that respectively intersect the two parallel waveguides at the curved portion, and in an intersection region which is a region on the substrate where the two signal lines and the two parallel waveguides intersect, at least one of the two signal lines has a signal propagation speed faster than a portion other than the intersection region, or one of the two signal lines has a signal propagation speed faster than the other, the optical waveguide element. According to the optical waveguide element of Configuration 1, in an optical waveguide element having a plurality of intersection regions between a convex optical waveguide and an electrode for propagating an electrical signal, the generation of disturbance modulation in the intersection region can be effectively suppressed, and good operating characteristics can be realized. (Configuration 2) An intermediate layer is provided between the substrate and the signal line on the substrate, and the thickness of the intermediate layer in the intersection region is larger than the thickness in a portion other than the intersection region, the optical waveguide element according to Configuration 1. According to the optical waveguide element of Configuration 2, the signal propagation speed in the signal line can be adjusted by the thickness of the intermediate layer, and the generation of disturbance modulation in the intersection region can be effectively suppressed. (Configuration 3) In the intersection region, an intermediate layer is provided between the substrate and the signal line, and on the substrate, the intermediate layer is not provided in a portion other than the intersection region, the optical waveguide element according to Configuration 1. According to the optical waveguide element of Configuration 3, even when an intermediate layer such as a buffer layer is not provided on the substrate, the occurrence of disturbance modulation in the intersection region can be effectively suppressed. (Configuration 4) The thickness of the intermediate layer in the intersection region is at least twice as large as the thickness in a portion other than the intersection region, the optical waveguide element according to Configuration 2. According to the optical waveguide element of Configuration 4, the occurrence of disturbance modulation in the intersection region can be more effectively suppressed. (Configuration 5) In the intersection region, one of the two signal lines has a longer intersection interval, which is the interval between the intersection positions with the two parallel waveguides, compared to the other, and in the intermediate layer in the intersection region, the thickness at the lower part of the one of the two signal lines is larger than the thickness at the lower part of the other, the optical waveguide element according to any one of Configurations 2 to 4. According to the optical waveguide element of Configuration 5, for the signal line with a shorter intersection interval, the signal propagation speed of the signal line with a longer intersection interval can be increased, and the occurrence of disturbance modulation in the intersection region can be effectively suppressed. (Configuration 6) The two signal lines have a thickness in the intersection region that is larger than the thickness in a portion other than the intersection region, the optical waveguide element according to Configuration 1. According to the optical waveguide element of Configuration 6, the signal propagation speed can be adjusted by the thickness of the signal line, and the occurrence of disturbance modulation in the intersection region can be effectively suppressed. (Configuration 7) The two signal lines have a thickness in the intersection region that is at least twice as large as the thickness in a portion other than the intersection region, the optical waveguide element according to Configuration 6. According to the optical waveguide element of Configuration 7, the occurrence of disturbance modulation in the intersection region can be more effectively suppressed. (Configuration 8) In the intersection region, one of the two signal lines has a longer intersection interval, which is the interval between the intersection positions with the two parallel waveguides, compared to the other. In the intersection region, the thickness of one of the two signal lines is greater than the thickness of the other of the two signal lines. The optical waveguide element according to Configuration 6 or 7. According to the optical waveguide element of Configuration 8, with respect to the signal line with the shorter intersection interval, the signal propagation speed of the signal line with the longer intersection interval can be increased, and the occurrence of disturbance modulation in the intersection region can be effectively suppressed. (Configuration 9) The widths of the two signal lines in the intersection region are narrower than the widths in the portion upstream of the intersection region along the propagation direction of the differential signal. The optical waveguide element according to Configuration 1. According to the optical waveguide element of Configuration 9, the signal propagation speed can be adjusted by the line width of the signal line, and the occurrence of disturbance modulation in the intersection region can be effectively suppressed. (Configuration 10) In the intersection region, one of the two signal lines has a longer intersection interval, which is the interval between the intersection positions with the two parallel waveguides, compared to the other. In the intersection region, the width of one of the two signal lines is narrower than the width of the other of the two signal lines. The optical waveguide element according to Configuration 1. According to the optical waveguide element of Configuration 10, with respect to the signal line with the shorter intersection interval, the signal propagation speed of the signal line with the longer intersection interval can be increased, and the occurrence of disturbance modulation in the intersection region can be effectively suppressed. (Configuration 11) A ground electrode is provided on the substrate, and the intervals between the two signal electrodes and the ground electrode in the intersection region are wider than the intervals in the portions other than the intersection region. The optical waveguide element according to Configuration 1. According to the optical waveguide element of Configuration 11, the signal propagation speed can be adjusted by the separation distance between the signal line and the ground electrode, and the occurrence of disturbance modulation in the intersection region can be effectively suppressed. (Configuration 12) An optical modulator including the optical waveguide element according to any one of Configurations 1 to 11, which is an optical modulation element for performing optical modulation, a housing for housing the optical waveguide element, an optical fiber for inputting light to the optical waveguide element, and an optical fiber for guiding the light output from the optical waveguide element to the outside of the housing. According to the optical modulator of Configuration 12, it is possible to reduce the occurrence of disturbance modulation and realize good optical modulation characteristics. (Configuration 13) An optical modulation module including the optical waveguide element according to any one of Configurations 1 to 11, which is an optical modulation element that modulates light, and a drive circuit that drives the optical waveguide element. According to the optical modulation module of Configuration 13, it is possible to reduce the occurrence of disturbance modulation and realize good optical modulation characteristics. (Configuration 14) An optical transmission device including the optical modulator according to Configuration 12 or the optical modulation module according to Configuration 13, and an electronic circuit that generates an electrical signal for causing the optical waveguide element to perform a modulation operation. According to the optical transmission device of Configuration 14, good optical transmission characteristics can be realized.

Explanation of Signs

[0141] 100, … optical modulator, 102 … housing, 104 … optical modulation element, 106 … relay substrate, 108, 110 … signal pins, 112 … terminator, 114 … input optical fiber, 116 … optical unit, 118, 130, 134 … lenses, 120 … output optical fiber, 122, 124 … supports, 220 … substrate, 230 … optical waveguide, 232 … input waveguide, 234 … branching waveguide, 240, 240a, 240b … nested Mach-Zehnder optical waveguide, 244, 244a, 244b, 244c, 244d … Mach-Zehnder optical waveguide, 246, 246a, 246a1, 246a2, 246b, 246b1, 246b2, 246c, 246c1, 246c2, 246d, 246d1, 246d2, 920a, 920b … parallel waveguides, 248a, 248b … output waveguides, 250a, 250b, 250c, 250d … signal electrodes, 252, 252a, 252a1, 252a2, 252b, 252b1, 252b2, 252c, 252c1, 252c2, 252d, 252d1, 252d1-1, 252d1-2, 252d1-3, 252d1-4, 252d2, 252d2-1, 252d2-2, 252d2-3, 252d2-4, 910a, 910b … signal lines, 254a, 254b, 254c, 254d, 254e, 254f, 254g, 254h, 258a, 258b, 258c, 258d, 258e, 258f, 258g, 258h … pads, 270a, 270b, 270c … bias electrodes, 290, 290a, 290b, 290c, 290-1, 290a-1, 290b-1, 290c-1 … ground electrodes, 280a, 280b, 280c, 280d … sides, 300, 300a, 300b, 300c, 300d … active part, 400, 400a, 400a1, 400a2, 400a3, 400a3-1, 400a3-2, 400a3-3, 400a3-4, 400a3-5, 400a3-6, 400a3-7, 400a4, 400a5, 400a6, 400a7, 400b, 400b1, 400b2, 400b3, 400b4, 400b5, 400b6, 400c, 400c1, 400c2, 400c3, 400c4, 400c5, 400d, 400d1, 400d2, 400d3, 400d4 … crossing region, 500 … support plate, 260, 260-1, 260-2, 260-3, 260-4 … intermediate layer, 1000 … optical modulation module, 1006 … circuit board1008... drive circuit, 1100... optical transmission device, 1104... light source, 1106... modulator drive unit, 1108... modulation signal generation unit.

Claims

1. An optical waveguide device comprising an optical waveguide formed by a convex portion extending on a substrate, and a signal electrode formed on the substrate for controlling light waves propagating through the optical waveguide, wherein the optical waveguide includes a Mach-Zehnder type optical waveguide having two parallel waveguides with a curved portion, the signal electrode is composed of two signal lines for transmitting differential signals, each of which intersects the two parallel waveguides at the curved portion, in an intersection region on the substrate where the two signal lines intersect the two parallel waveguides, the intersection intervals, which are the intervals between the portions where the respective signal lines intersect the two parallel waveguides, are different from each other, in the intersection region, at least one of the two signal lines has a signal propagation speed higher than that of a portion outside the intersection region, or one of the two signal lines having a longer intersection interval has a signal propagation speed higher than that of the other signal line having a shorter intersection interval, an optical waveguide device.

2. an intermediate layer is provided between the substrate and the signal lines on the substrate, and the thickness of the intermediate layer in the intersection region is larger than that in a portion outside the intersection region, The optical waveguide device according to claim 1.

3. an intermediate layer is provided between the substrate and the signal lines in the intersection region, and on the substrate, the intermediate layer is not provided in a portion outside the intersection region, The optical waveguide device according to claim 1.

4. the thickness of the intermediate layer in the intersection region is at least twice as large as that in a portion outside the intersection region, The optical waveguide device according to claim 2.

5. In the intersection region, the thickness of the intermediate layer under one of the signal lines is larger than that under the other signal line, The optical waveguide device according to any one of claims 2 to 4.

6. the two signal lines have a thickness in the intersection region that is larger than that in a portion outside the intersection region, The optical waveguide device according to claim 1.

7. the two signal lines have a thickness in the intersection region that is at least twice as large as that in a portion outside the intersection region, The optical waveguide device according to claim 6.

8. In the intersection region, the thickness of one of the signal lines is larger than that of the other signal line, The optical waveguide device according to claim 6 or 7.

9. The widths of the two signal lines in the intersection region are narrower than the widths in the portion upstream of the intersection region along the propagation direction of the differential signal. The optical waveguide element according to claim 1.

10. In the intersection region, the width of one of the signal lines is narrower than the width of the other signal line. The optical waveguide element according to claim 1 or 9.

11. A ground electrode is provided on the substrate. The distances between the two signal lines and the ground electrode in the intersection region are wider than the distances in the portions other than the intersection region. The optical waveguide element according to claim 1.

12. An optical waveguide element according to any one of claims 1 to 11, which is an optical modulation element for performing optical modulation, A housing that houses the optical waveguide element, An optical fiber that inputs light to the optical waveguide element, An optical fiber that guides the light output from the optical waveguide element to the outside of the housing, An optical modulator comprising:

13. An optical modulation module comprising an optical waveguide element according to any one of claims 1 to 11, which is an optical modulation element for performing optical modulation, and a drive circuit that drives the optical waveguide element.

14. An optical modulator according to claim 12 or an optical modulation module according to claim 13, An electronic circuit that generates an electrical signal for causing the optical waveguide element to perform a modulation operation, An optical transmission device comprising:

Images