Optical waveguide element, optical modulator, optical modulation module, and optical transmitter

Abstract

To materialize satisfactory operation characteristic of an optical waveguide element by effectively suppressing the occurrence of disturbance modulation in an intersection, in the optical waveguide element having the plurality of intersections between a protruded optical waveguide and a signal electrode.SOLUTION: An optical waveguide element is provided, comprising: an optical waveguide comprised of a protruded portion extending on a substrate; and a signal electrode formed on the substrate, and controlling optical waves propagating through the optical waveguide. The optical waveguide includes a Mach-Zehnder type optical waveguide equipped with two parallel waveguides having curved portions, the signal electrode is comprised of two signal lines which transmit differential signals and each of which intersects the two parallel waveguides in the curved portion, each of the two signal lines is constituted in such a manner that the intersection lengths in the two parallel waveguides are equal to each 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 transmitter. [Background technology]

[0002] In high-speed / large-capacity optical fiber communication systems, optical modulators incorporating optical modulation elements as optical waveguide elements, which consist of an optical waveguide formed on a substrate and a control electrode that controls the light waves propagating through the optical waveguide, are widely used.Among them, optical modulation elements using LiNbO3 (hereinafter also referred to as LN) substrates, which have electro-optic effects, are widely used in high-speed / large-capacity optical fiber communication systems because they can achieve low optical loss and wide-band optical modulation characteristics.

[0003] In particular, in response to the recent trend toward increased transmission capacity, modulation methods in optical fiber communication systems have become dominated by multi-level modulation and transmission formats that incorporate polarization multiplexing into multi-level modulation, such as QPSK (Quadrature Phase Shift Keying) and DP-QPSK (Dual Polarization - Quadrature Phase Shift Keying).These formats are used in backbone optical transmission networks and are also being introduced into metro networks.

[0004] Furthermore, in recent years, in order to achieve even lower voltage driving and higher speed modulation while miniaturizing the optical modulator itself, optical modulators that use rib-type optical waveguides or ridge-type optical waveguides (hereinafter collectively referred to as convex optical waveguides) that are configured by forming strip-shaped convex portions on the surface of a thin-film (or thin-plate) LN substrate (for example, a thickness of 20 μm or less) are being put into practical use in order to further strengthen the interaction between the signal electric field and the guided light in the substrate (for example, Patent Documents 1 and 2).

[0005] In addition to miniaturizing the optical modulation element itself, efforts are also being made to house an electronic circuit and an optical modulation element in a single housing and integrate them as an optical modulation module. For example, an optical modulation module has been proposed that achieves miniaturization and integration by integrating an optical modulation element and a high-frequency driver amplifier that drives the optical modulation element in a single housing and arranging optical input / output units in parallel on one surface of the housing. In the optical modulation element used in such an optical modulation module, the optical waveguide is formed on the substrate so that the optical input terminal and optical output terminal of the optical waveguide are located on one side of the substrate constituting the optical modulation element, and the optical propagation direction is folded back on the substrate (e.g., Patent Document 3). Hereinafter, an optical modulation element configured with an optical waveguide including such a folded portion of the optical propagation direction is referred to as a folded-back optical modulation element.

[0006] An optical modulator that performs QPSK modulation (QPSK optical modulator) or an optical modulator that performs DP-QPSK modulation (DP-QPSK optical modulator) includes multiple Mach-Zehnder optical waveguides in a nested structure, each of which has at least one signal electrode to which a high-frequency signal is applied. These signal electrodes are formed on the substrate and extend to the vicinity of the outer periphery of the LN substrate for connection to electrical circuits outside the substrate. Therefore, multiple optical waveguides and multiple signal electrodes intersect in a complex manner on the substrate, forming multiple intersections where the signal electrodes cross over the optical waveguides.

[0007] At such an intersection, an electric field is applied from the signal electrode crossing the optical waveguide to the portion of the optical waveguide below the signal electrode, slightly changing the phase of the light propagating through the optical waveguide and modulating the phase. The phase change or phase modulation of the light at such an intersection acts as noise in addition to the optical phase change for normal modulation generated in the optical waveguide by the signal electrode, and may disturb the optical modulation operation. Hereinafter, the phase modulation as noise generated at such an intersection will be referred to as disturbance modulation.

[0008] The degree of the noise effect of disturbance modulation on the optical modulation operation in an optical modulator increases as the electric field applied from the signal electrode to the optical waveguide at the intersection increases, and also increases due to an additive effect proportional to the number of intersections (for example, depending on the sum of the lengths of the intersections along the signal electrode (intersection length)).

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

[0010] Furthermore, in the above-described folded optical modulation element, there are more intersections between the electrodes and the optical waveguides than in a non-folded optical modulation element that is composed of an optical waveguide that does not include an optical folding portion (see, for example, Figure 1 of Patent Document 3), and noise due to turbulence modulation can also be greater. For example, in the case of the above-described DP-QPSK modulation element, the number of intersections in one electrode in a non-folded optical modulation element is about two to four, 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 in one electrode can reach more than ten, and the total intersection length can be several hundred microns to several millimeters.

[0011] Therefore, especially in a folded optical modulation element configured using a convex optical waveguide, the noise due to the turbulence modulation occurring at the intersection can be so large that it cannot be ignored with respect to normal optical modulation operation. Furthermore, especially in a Mach-Zehnder optical waveguide configured with two parallel waveguides, the turbulence 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 generates even larger noise due to optical interference when these light waves are combined within the Mach-Zehnder optical waveguide, which can significantly affect optical modulation operation.

[0012] The above-described crossing portion can be formed in various optical waveguide devices, such as optical waveguide devices using semiconductors such as InP as well as LN substrates, silicon photonics waveguide devices using Si as a substrate, etc. Furthermore, such optical waveguide devices can be not only optical modulators using Mach-Zehnder optical waveguides, but also optical modulators using optical waveguides forming directional couplers or Y-junctions, optical switches, and various other optical waveguide devices.

[0013] Furthermore, as optical waveguide elements become smaller, more multi-channel, and / or more highly integrated, the optical waveguide patterns and electrode patterns become more complex, and the number of intersections on the substrate increases, and noise due to disturbance modulation becomes a significant factor that can limit the performance of the optical waveguide elements. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-264548 [Patent Document 2] International Publication No. 2018 / 1031916 [Patent Document 3] Japanese Patent Application Publication No. 2019-152732 Summary of the Invention [Problem to be solved by the invention]

[0015] In view of the above background, in an optical waveguide element having multiple intersections between a convex optical waveguide and a signal electrode that propagates an electrical signal, it is desired to effectively suppress the occurrence of disturbance modulation at the intersections and achieve 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 a light wave propagating through the optical waveguide, wherein the optical waveguide includes a Mach-Zehnder optical waveguide having two parallel waveguides each having a curved portion, and the signal electrode comprises two signal lines for transmitting differential signals, each intersecting the two parallel waveguides at the curved portion, Each of the two parallel waveguides has two crossing points with the two signal lines that transmit the differential signals. The optical waveguide element is configured so that the crossing lengths are the same. According to another aspect of the present invention, the distance between the two signal lines at the portions where the signal lines intersect with the parallel waveguides is narrower than the distance between the portions where the signal lines do not intersect with the parallel waveguides. According to another aspect of the present invention, the spacing between the two signal lines at the portion where they intersect with the parallel waveguides is narrower than the spacing between the portions upstream of the portions where they intersect with the parallel waveguides in the propagation direction of the differential signal. According to another aspect of the present invention, the two signal lines have a wider spacing at a downstream portion along the propagation direction of the differential signal than at a portion where the two signal lines intersect with the parallel waveguides. According to another aspect of the present invention, the two signal lines are formed so that the crossing angles with the two parallel waveguides are the same. According to another aspect of the present invention, the two signal lines cross each other in a region on the substrate sandwiched between the two parallel waveguides. According to another aspect of the present invention, the two signal lines have portions that intersect with the parallel waveguides and overlap each other in a plan view of the substrate. According to another aspect of the present invention, an insulating layer made of resin is provided between the parallel waveguide and the two signal lines at a portion where the two signal lines intersect with the parallel waveguide. Another aspect of the present invention is an optical modulator comprising: an optical waveguide element according to any one of the above, which is an optical modulation element that modulates light; a housing that accommodates the optical waveguide element; an optical fiber that inputs light to the optical waveguide element; and an optical fiber that guides light output by the optical waveguide element to the outside of the housing. Another aspect of the present invention is an optical modulation module including the optical waveguide element, which is an optical modulation element that modulates light, and a drive circuit that drives the optical waveguide element. Yet another aspect of the present invention is an optical transmitter comprising the optical modulator or the optical modulation module, and an electronic circuit that generates an electrical signal for causing the optical waveguide element to perform a modulation operation. [Effects of the Invention]

[0017] According to the present invention, in an optical waveguide element having multiple intersections between a convex optical waveguide and an electrode that propagates an electrical signal, it is possible to effectively suppress the occurrence of disturbance modulation at the intersections and achieve good operating characteristics. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a diagram showing a configuration of an optical modulator according to a first embodiment of the present invention. [Figure 2] 2 is a diagram showing the configuration of an optical modulation element used in the optical modulator shown in FIG. [Figure 3] 3 is a partial detailed view of a light modulation section A of the light modulation element shown in FIG. 2. FIG. [Figure 4] 3 is a partial detailed view of a light turning portion B of the optical modulation element shown in FIG. 2. FIG. [Figure 5] 4 is a cross-sectional view of the optical modulation unit A shown in FIG. 3 taken along the arrows VV. [Figure 6] 5 is a partial detailed view of an intersection region in the light turning unit B shown in FIG. 4. FIG. [Figure 7]FIG. 10 is a diagram showing a configuration of an intersection area according to a first modified example. [Figure 8] FIG. 10 is a diagram showing a configuration of an intersection area according to a second modified example. [Figure 9] 9 is a cross-sectional view of the intersection region shown in FIG. 8 taken along the line IX-IX. [Figure 10] 9 is a cross-sectional view of the intersection region shown in FIG. 8 taken along the arrow XX. [Figure 11] FIG. 10 is a diagram showing a configuration of an intersection area according to a third modified example. [Figure 12] FIG. 10 is a diagram showing a configuration of an intersection area according to a fourth modified example. [Figure 13] FIG. 13 is a diagram showing a configuration of an intersection area according to a fifth modified example. [Figure 14] 14 is a cross-sectional view of the intersection region shown in FIG. 13 taken along the line XIV-XIV. [Figure 15] FIG. 6 is a diagram showing the configuration of an optical modulation module according to a second embodiment of the present invention. [Figure 16] FIG. 10 is a diagram illustrating a configuration of an optical transmitting device according to a third embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. [1. First embodiment] First, a 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 has a housing 102, an optical modulation element 104 housed in the housing 102, and a relay substrate 106. The optical modulation element 104 has, for example, a DP-QPSK modulator configuration. A plate-shaped cover (not shown) is finally fixed to the opening of the housing 102, and the interior is hermetically sealed.

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

[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, both on the same surface of the housing 102.

[0022] Here, the input optical fiber 114 and the output optical fiber 120 are fixed to the housing 102 via supports 122 and 124, which are fixing members, respectively. Light input from the input optical fiber 114 is collimated by a lens 130 arranged in the support 122, and then input to the light modulation element 104 via a lens 134. However, this is just one example, and light can also be input to the light modulation element 104 according to conventional technology, 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 a substrate 220 (described later) of the light modulation element 104.

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

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

[0025] Fig. 2 is a diagram showing an example of the configuration of the optical modulation element 104 housed in the housing 102 of the optical modulator 100 shown in Fig. 1. Figs. 3 and 4 are partial detailed views of the optical modulation section A and the optical folding section B (described later) of the optical modulation element 104 shown in Fig. 2, respectively.

[0026] The optical modulation element 104 is configured with an optical waveguide 230 (the entire portion indicated by the thick dotted lines in the figure) formed on a 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 that has been thinned and processed to a thickness of 20 μm or less (e.g., 2 μm). The optical waveguide 230 is a convex optical waveguide (e.g., a rib-type optical waveguide or a ridge-type optical waveguide) formed on the surface of the thinned substrate 220 and consisting of a strip-like extending convex portion. Here, since the refractive index of an LN substrate can locally change due to the photoelastic effect when stress is applied, the LN substrate is generally bonded to a support plate such as a Si (silicon) substrate, glass substrate, or LN substrate to reinforce the mechanical strength of the entire substrate. In this embodiment, the substrate 220 is bonded to a support plate 500, as described below.

[0027] The substrate 220 is, for example, rectangular, and has two opposing left and right sides 280a and 280b extending in the vertical direction in the figure, and two opposing upper and lower sides 280c and 280d extending in the horizontal direction in the figure.

[0028] The optical waveguide 230 includes an input waveguide 232 that receives input light (indicated by 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 light beams having the same light intensity. The optical waveguide 230 also includes so-called nested Mach-Zehnder optical waveguides 240a and 240b, which are two modulation sections that modulate the respective light beams branched by the branching waveguide 234.

[0029] The nested Mach-Zehnder optical waveguides 240a and 240b each include two Mach-Zehnder optical waveguides 244a and 244b, and two Mach-Zehnder optical waveguides 244c and 244d, respectively, provided in two waveguide portions forming a pair of parallel waveguides. As shown in Fig. 3, the Mach-Zehnder optical waveguides 244a and 244b include parallel waveguides 246a1 and 246a2, and parallel waveguides 246b1 and 246b2, respectively. The Mach-Zehnder optical waveguides 244c and 244d include parallel waveguides 246c1 and 246c2, and parallel waveguides 246d1 and 246d2, respectively.

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

[0031] 2, the nested Mach-Zehnder optical waveguide 240 includes an optical modulation section A and an optical folding section B (each indicated by a rectangle with a two-dot chain line in the figure). The optical folding section B is a section that changes the propagation direction of light 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 sections, and the optical folding section B is made up of the curved sections of a total of eight parallel waveguides 246 that make up the two nested Mach-Zehnder optical waveguides 240.

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

[0033] Four signal electrodes 250a, 250b, 250c, and 250d are provided on the substrate 220 to cause modulation operations of the four Mach-Zehnder optical waveguides 244a, 244b, 244c, and 244d that make up the nested Mach-Zehnder optical waveguides 240a and 240b, respectively. Hereinafter, the signal electrodes 250a, 250b, 250c, and 250d will also be collectively referred to as signal electrode 250.

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

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

[0036] 3, in the optical modulation section A, the signal lines 252a1 and 252a2 constituting the signal electrode 250a have acting portions 300a and 300b (shown as diagonally hatched portions) formed above the parallel waveguides 246a1 and 246a2 of the Mach-Zehnder optical waveguide 244a, respectively, and propagate differential signals having a phase difference of 180° therethrough to cause the Mach-Zehnder optical waveguide 244a to perform a modulation operation. Furthermore, the signal lines 252b1 and 252b2 constituting the signal electrode 250b have acting portions 300c and 300d formed above the parallel waveguides 246b1 and 246b2 of the Mach-Zehnder optical waveguide 244b, respectively, and propagate differential signals having a phase difference of 180° therethrough to cause the Mach-Zehnder optical waveguide 244b to perform a modulation operation.

[0037] Similarly, the signal lines 252c1 and 252c2 constituting the signal electrode 250c have acting portions 300e and 300f formed above the parallel waveguides 246c1 and 246c2 of the Mach-Zehnder optical waveguide 244c, respectively, and propagate differential signals having a phase difference of 180° therethrough, causing the Mach-Zehnder optical waveguide 244c to perform a modulation operation. Furthermore, the signal lines 252d1 and 252d2 constituting the signal electrode 250d have acting portions 300g and 300h formed above the parallel waveguides 246d1 and 246d2 of the Mach-Zehnder optical waveguide 244d, respectively, and propagate differential signals having a phase difference of 180° therethrough, causing the Mach-Zehnder optical waveguide 244d to perform a modulation operation.

[0038] Hereinafter, the action portions 300a, 300b, 300c, 300d, 300e, 300f, 300g, and 300h will also be collectively referred to as action portion 300.

[0039] 4, in the optical folding section 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 at the curved portions of those parallel waveguides 246. Here, on the surface of the substrate 220, a portion where the two signal lines 252 propagating differential signals intersect with the two parallel waveguides 246 constituting one Mach-Zehnder optical waveguide 244 is also referred to as an intersecting region.

[0040] 4, two signal lines 252a (i.e., signal lines 252a1 and 252a2) that propagate differential signals intersect with two parallel waveguides 246a (i.e., parallel waveguides 246a1 and 246a2) that constitute the Mach-Zehnder optical waveguide 244a in an intersecting region 400a1. Also, the signal line 252a intersects with two parallel waveguides 246b, 246c, and 246d that constitute the Mach-Zehnder optical waveguides 244b, 244c, and 244d in intersecting regions 400a2, 400a3, and 400a4, respectively.

[0041] Similarly, the two signal lines 252b intersect with the two parallel waveguides 246a, 246b, 246c, and 246d that constitute the Mach-Zehnder optical waveguides 244a, 244b, 244c, and 244d, respectively, at intersection regions 400b1, 400b2, 400b3, and 400b4. The two signal lines 252c intersect with the two parallel waveguides 246a, 246b, 246c, and 246d that constitute the Mach-Zehnder optical waveguides 244a, 244b, 244c, and 244d, respectively, at intersection regions 400c1, 400c2, 400c3, and 400c4. Furthermore, the two signal lines 252d intersect with the two parallel waveguides 246a, 246b, 246c, and 246d that constitute the Mach-Zehnder optical waveguides 244a, 244b, 244c, and 244d, respectively, in intersecting regions 400d1, 400d2, 400d3, and 400d4.

[0042] Hereinafter, intersection regions 400a1, 400a2, 400a3, and 400a4 will be collectively referred to as intersection regions 400a, and intersection regions 400b1, 400b2, 400b3, and 400b4 will be collectively referred to as intersection regions 400b. Furthermore, intersection regions 400c1, 400c2, 400c3, and 400c4 will be collectively referred to as intersection regions 400c, and intersection regions 400d1, 400d2, 400d3, and 400d4 will be collectively referred to as intersection regions 400d. Furthermore, intersection regions 400a, 400b, 400c, and 400d will be collectively referred to as intersection regions 400.

[0043] Referring to FIG. 2, signal lines 252a1, 252a2, 252b1, 252b2, 252c1, 252c2, 252d1, and 252d2 extend to the right of the substrate 220 as shown in the figure, cross over eight parallel waveguides 246 at the optical folding section B, and then extend to side 280b and are connected to pads 254a, 254b, 254c, 254d, 254e, 254f, 254g, and 254h, respectively.

[0044] Hereinafter, the pads 254a, 254b, 254c, 254d, 254e, 254f, 254g, and 254h will also be collectively referred to as pads 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.

[0045] The left side of signal lines 252a1, 252a2, 252b1, 252b2, 252c1, 252c2, 252d1, and 252d2 bend downward in the figure, 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 will be collectively referred to as pads 258. Each of pads 258 is connected to eight termination resistors (not shown) that constitute terminator 112 by wire bonding or the like.

[0046] In FIG. 2, each of the signal lines 252, in accordance with conventional technology, forms, for example, a coplanar transmission line having a predetermined impedance together with ground electrodes (not shown) formed on the surface of the substrate 220 so as to sandwich each of the signal lines 252.

[0047] As a result, a high-frequency electrical signal input from the signal pin 108 to each of the pads 254 via the relay substrate 106 propagates through each of the signal lines 252 as a traveling wave, and modulates the light wave propagating through the corresponding Mach-Zehnder optical waveguide 244 in each of the action sections 300.

[0048] Fig. 5 is a VV cross-sectional view of the optical modulation unit A shown in Fig. 3, illustrating the cross-sectional structure of the signal lines 252c1 and 252c2 (specifically, the action portions 300e and 300f) that constitute the signal electrode 250c. Note that, in the optical modulation unit A, each of the other signal lines 252 is configured in the same manner as the signal lines 252c1 and 252c2 shown in Fig. 5.

[0049] 5, substrate 220 is adhesively fixed to support plate 500 such as glass for reinforcement. Convex portions 504c1 and 504c2 that constitute parallel waveguides 246c1 and 246c2 of Mach-Zehnder optical waveguide 244c, which is a convex optical waveguide, are formed on substrate 220. Here, the dashed circle shown in FIG. 5 schematically indicates the field diameter of the light waves propagating through parallel waveguides 246c1 and 246c2.

[0050] An intermediate layer 502 is formed on each of the protrusions 504c1 and 504c2, and signal lines 252c1 and 252c2 (specifically, acting portions 300e and 300f) are formed thereon, respectively. The intermediate layer 502 is made of, for example, SiO2 (silicon dioxide). However, this is just one example, and the intermediate layer 502 may also be made of a resin such as a photosensitive permanent film, which will be described later.

[0051] Furthermore, three ground electrodes 272 are formed on substrate 220 at positions on the surface of substrate 220 that sandwich signal lines 252c1 and 252c2. The distance between each of ground electrodes 272 and signal lines 252c1 and 252c2 is determined according to conventional technology based on various design conditions, such as the impedance required for the coplanar transmission line formed by these electrodes and the widths of protruding portions 504c1 and 504c2. Hereinafter, the protruding portions on substrate 220 that form optical waveguide 230, including protruding portions 504c1 and 504c2, will be collectively referred to as protruding portion 504.

[0052] The substrate 220 is formed to a thickness of 20 μm or less, preferably 10 μm or less, so as to enhance the interaction between the electric field formed at the convex portion 504 by the signal line 252 and the guided light propagating through the Mach-Zehnder optical waveguide 244 and thereby enable high-speed modulation operation at a lower voltage. In this embodiment, for example, the thickness of the substrate 220 is 1.2 μm, and the height of the convex portion 504 constituting the optical waveguide 230 is 0.8 μm.

[0053] 5, the intermediate layer 502 is provided only on the convex portions 504 that constitute the optical waveguide 230, but the intermediate layer 502 may be formed over the entire surface of the substrate 220. In addition to this, the intermediate layer 502 may be formed on the side surfaces (left and right side surfaces in the figure) of the convex portions 504. For example, when the intermediate layer 502 is made of a resin such as a photosensitive permanent film, the intermediate layer 502 can be easily formed on the side surfaces of the convex portions 504 as well.

[0054] 2, the optical modulator 104 is also provided with bias electrodes 270a, 270b, and 270c for adjusting the operating point by compensating for bias point fluctuations due to so-called DC drift. The bias electrode 270a is used to compensate for bias point fluctuations of the nested Mach-Zehnder optical waveguides 240a and 240b. The bias electrodes 270b and 270c are used to compensate for bias point fluctuations of the Mach-Zehnder optical waveguides 244a, 244b, and 244c and 244d, respectively.

[0055] These bias electrodes 270a, 270b, and 270c each extend to an upper side 280c of the substrate 220 in the figure, and are connected to 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. As a result, the bias electrodes 270a, 270b, and 270c are driven by the bias control circuit, and the operating point of each corresponding Mach-Zehnder optical waveguide is adjusted to compensate for bias point fluctuation. Hereinafter, the bias electrodes 270a, 270b, and 270c will also be collectively referred to as bias electrodes 270.

[0056] The bias electrode 270 is an electrode to which a DC or low-frequency electrical signal is applied, and is formed to a thickness in the range of 0.3 μm to 5 μm, for example, when the thickness of the substrate 220 is 20 μm. In contrast, the signal line 252 constituting the signal electrode 250 is formed to a thickness in the range of 20 μm to 40 μm, for example, to reduce conductor loss of the applied high-frequency electrical signal. The thickness of the signal line 252 is determined according to the thickness of the substrate 220 in order to set the impedance and microwave effective refractive index to desired values; the signal line 252 can be determined to be thicker when the thickness of the substrate 220 is thick, and thinner when the thickness of the substrate 220 is thin.

[0057] As described above, in the optical modulation element 104, each of the signal lines 252 constituting the signal electrode 250 crosses over the eight parallel waveguides 246 constituting the optical folding section B at the respective curved portions of these parallel waveguides 246. Therefore, if the above-mentioned disturbance modulation occurs due to the intersections between the eight parallel waveguides 246 and the signal line 252, the modulation operation of the optical modulation element 104 may be degraded. (The disturbance modulation occurring in the two parallel waveguides of the Mach-Zehnder has a significant effect on the modulation operation.)

[0058] In order to reduce such disturbance modulation, in the optical modulation element 104 of this embodiment, particularly in each of the intersection regions 400, each of the two signal lines 252 propagating a differential signal is configured so that the intersection lengths in the parallel waveguides 246 at two intersections with each of the two parallel waveguides 246 of the corresponding Mach-Zehnder optical waveguide 244 are at approximately the same angle and / or approximately the same length. Here, the intersection lengths can be defined as lengths measured along the parallel waveguides 246.

[0059] As a result, at two intersections of the parallel waveguide 246 with the two signal lines 252 that propagate differential signals in opposite phases, disturbance modulations of opposite phases occur with the same intensity, and these disturbance modulations cancel each other out. As a result, the total disturbance modulation can be effectively reduced in each of the parallel waveguides 246.

[0060] Here, the terms "the same length" and "substantially the same length" regarding the intersection lengths at two intersections of the parallel waveguides 246 mean that the intersections are the same lengths with an error to the extent that the above-mentioned disturbance modulation can be canceled out to a predetermined extent (for example, 1 / 10). Also, the terms "the same angle" and "substantially the same angle" regarding the intersection angles at two intersections of the parallel waveguides 246 mean that the intersection lengths at those intersections are the same angles with an error to the extent that they are "the same length" or "substantially the same length" in the above sense.

[0061] Specifically, in this embodiment, the two signal lines 252 constituting the signal electrode 250 are formed so that the distance between them at their intersections with the same parallel waveguide 246 is narrower than the distance between them at their non-intersecting portions with the parallel waveguide 246. As a result, the distance between the two intersections of one parallel waveguide 246 and the two signal lines 252 is close to each other, so that the intersection angles between the parallel waveguide 246 and the signal lines 252 at the two intersections are substantially the same. As a result, if the two signal lines have the same width, the intersection lengths at these two intersections are substantially the same.

[0062] More specifically, in this embodiment, the distance between the two signal lines 252 at the portions where they intersect with the same parallel waveguide 246 is narrower than the distance between the portions upstream of the intersecting portion along the propagation direction of the differential signal. As a result, for example, the distance between the two signal lines at the portions where they intersect with the same parallel waveguide 246 can be narrower than the distance between the pads 254 (for example, the distance between pads 254a and 254b), thereby making the distance between the intersecting portions closer to each other.

[0063] Fig. 6 is a partial detailed view of the intersection region 400a4 in the light turning unit B shown in Fig. 4. The other intersection regions 400 other than the intersection region 400a4 are configured in the same manner as the intersection region 400a4 shown in Fig. 6.

[0064] In FIG. 6, the two signal lines 252a are formed with the same width We with respect to each other and propagate differential signals from the upper right to the lower left in the figure. Further, the parallel waveguide 246d propagates light waves from the lower right to the upper left in the figure. And, for the two signal lines 252a, the interval d2 of the downstream portion intersecting the parallel waveguide 246d is formed to be narrower than the interval d1 of the upstream portion along the propagation direction of the differential signal (that is, d2 < d1). Here, the interval d1 is equal to, for example, the interval between the pads 254a and 254b.

[0065] In FIG. 6, the two signal lines 252a1 and 252a2 intersect the parallel waveguide 246d1 and constitute intersection portions 600a and 600b (hatched portions in the figure). Further, the two signal lines 252a1 and 252a2 intersect the parallel waveguide 246d2 and constitute intersection portions 602a and 602b (hatched portions in the figure). In FIG. 6, for reference, virtual intersection portions 604a and 604b when the two signal lines 252a are formed at a constant interval d1 along the one-dot chain line shown in the figure and intersect the parallel waveguide 246d1 are also shown.

[0066] Here, let the intersection angles of the parallel waveguide 246d with respect to the signal line 252a at the intersection portions 600a, 600b, 602a, and 602b be θ1, θ2, θ3, and θ4, respectively. Also, let the intersection lengths along the parallel waveguide 246d at the intersection portions 600a, 600b, 602a, and 602b be d3, d4, d5, and d6, respectively. Similarly, let the intersection angles at the virtual intersection portions 604a and 604b be θ5 and θ6, respectively, and the intersection lengths be d7 and d8, respectively. Here, the intersection angle of the parallel waveguide 246d with respect to the signal line 252a is, for example, the angle formed by the propagation direction of the differential signal propagating through the signal line 252a and the propagation direction of the light propagating through the parallel waveguide 246d at the corresponding intersection portion.

[0067] As will be apparent to those skilled in the art, from general geometric considerations, in the parallel waveguide 246d1, the crossing angles increase in the order of θ6, θ2, θ1, and θ5. The crossing lengths d3, d4, d5, d6, d7, and d8 and the crossing angles θ1, θ2, θ3, θ4, θ5, and θ6 have the following relationship: d3=|We / sinθ1| d4=|We / sinθ2| d5=|We / sinθ3| d6=|We / sinθ4| d7=|We / sinθ5| d8=|We / sinθ6|

[0068] As described above, the two signal lines 252a are formed such that the distance d2 between the downstream portions intersecting with the parallel waveguide 246d is narrower than the distance d1 between the upstream portions along the propagation direction of the differential signal. That is, the distance d2 between the two signal lines 252a at the portions intersecting with the parallel waveguides 246d1 and 246d2 is narrower than the distance d1 between the portions not intersecting with these parallel waveguides 246d.

[0069] As a result, the distance between the two intersections 600a and 600b between the two signal lines 252a and the parallel waveguide 246d1 is narrower than the distance between the intersections 604a and 604b when the two signal lines 252a are formed at a constant distance d1, and the two intersections 600a and 600b are formed close to each other.

[0070] Therefore, the intersection angles θ1 and θ2 between the signal line 252a and the parallel waveguide 246d at the intersections 600a and 600b are approximately the same (i.e., θ1≒θ2). As a result, the intersection lengths d3 (=|We / sinθ1|) and d4 (=|We / sinθ2|) at the intersections 600a and 600b are approximately the same (i.e., d3≒d4).

[0071] Similarly, at the two intersections 602a and 602b between the two signal lines 252a and the parallel waveguide 246d2, the intersection angles θ5 and θ6 are substantially the same as each other (i.e., θ5≒θ6). As a result, the intersection lengths d7 (=|We / sinθ5|) and d8 (=|We / sinθ6|) are substantially the same as each other (i.e., d7≒d8).

[0072] Therefore, in the configuration shown in FIG. 6, since the two intersections 600a and 600b between the two signal lines 252a that propagate differential signals with opposite phases and the parallel waveguide 246d1 have the same intersection length as each other (i.e., d3≒d4), the disturbing modulations generated at these two intersections 600a and 600b cancel each other out, and the disturbing modulation generated in the entire parallel waveguide 246d1 can be effectively reduced. For the same reason, the disturbing modulations generated at the two intersections 602a and 602b between the two signal lines 252a and the parallel waveguide 246d2 cancel each other out, and the disturbing modulation generated in the entire parallel waveguide 246d2 can be reduced.

[0073] As a result, the noise of each optical signal propagating through these two parallel waveguides 246d is reduced, and the phase difference noise between these two signal lights is also reduced, effectively reducing the generation of noise in the optical modulation operation of the Mach-Zehnder type optical waveguide 244d constituted by the parallel waveguides 246d.

[0074] In consideration of the general interval between parallel waveguides of a Mach-Zehnder type optical waveguide and the curvature of the curved portions provided in the parallel waveguides as used in the optical modulation element 104, as a condition for making the intersection angles and intersection lengths between the intersections 600a and 600b and between the intersections 602a and 602b substantially the same, the interval d2 (<d1) between the signal lines 252a at the portion where the parallel waveguide 246d intersects is preferably d2≦w with respect to the interval w (see FIG. 6) between the two parallel waveguides 246d at the intersecting portion.

[0075] 2, the signal lines 252a1 and 252a2 and the signal lines 252b1 and 252b2 each intersect with the eight parallel waveguides 246 while maintaining a constant distance between them, and extend to the optical modulation unit A. On the other hand, the signal lines 252c1 and 252c2 and the signal lines 252d1 and 252d2 each intersect with the eight parallel waveguides 246 while maintaining a constant distance between them, and then extend to the optical modulation unit A with the distance between them widened. The configuration of the signal lines 252c and 252d is advantageous from the viewpoint of reducing crosstalk between the signal lines that propagate differential signals.

[0076] However, the above configuration is just one example, and for example, if crosstalk does not have a significant effect, one or both of the signal lines 252c and 252d can be formed, like the signal lines 252a and 252b, so that the signal lines that propagate differential signals are extended to the optical modulation unit A while maintaining a constant interval between them. Alternatively, if the effect of crosstalk cannot be ignored, one or both of the signal lines 252a and 252b can be formed, like the signal lines 252c and 252d, so that the signal lines that propagate differential signals are extended to the optical modulation unit A with a wider interval between them.

[0077] Next, a modified example of the configuration of the crossing region 400 of the light modulation element 104 will be described. [1.1 First Modification] 7 is a diagram showing the configuration of an intersection region 400a4-1 according to a first modification of the intersection region 400a4 shown in FIG. 6. The configuration of the intersection region 400a4-1 shown in FIG. 7 can be used in place of the intersection region 400a4 shown in FIG. 6 in the light modulation element 104. As will be apparent to those skilled in the art, a configuration similar to that of the intersection region 400a4-1 can also be used in intersection regions 400 other than the intersection region 400a4 in FIG. 4. Note that 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 above is incorporated herein.

[0078] In the intersection region 400a4-1, signal lines 252a1-1 and 252a2-1 are used instead of the signal lines 252a1 and 252a2. The signal lines 252a1-1 and 252a2-1 have the same configuration as the signal lines 252a1 and 252a2, but differ from the signal lines 252a1 and 252a2 in the manner in which they intersect with the parallel waveguide 246d. Hereinafter, the signal lines 252a1-1 and 252a2-1 will also be collectively referred to as the signal line 252a-1.

[0079] 6, signal lines 252a1 and 252a2 are formed parallel to and close to each other at the intersection with parallel waveguide 246d, whereas signal line 252a-1 shown in Fig. 7 is formed such that one signal line 252a1-2 is inclined with respect to the other signal line a2-1 so that two signal lines 252a1-1 and 252a2-1 intersect with parallel waveguide 246d at the same intersection angle θ10. As a result, intersections 610a and 610b have the same intersection length d10 (=|We / sin θ10|).

[0080] Similarly, two intersections 612a and 612b between the parallel waveguide 246d2 and the signal lines 252a1-1 and 252a2-1 intersect at the same intersection angle θ12 as the two signal lines 252a-1, so that the intersections 612a and 612b have the same intersection length d12 (=|We / sinθ12|).

[0081] Therefore, in the configuration of the intersection region 400a4-1, as in the configuration of the intersection region 400a4 shown in Fig. 6, the disturbance modulations occurring at the two intersections 610a and 610b in the parallel waveguide 246d1 cancel each other out. Similarly, in the parallel waveguide 246d2, the disturbance modulations occurring at the two intersections 612a and 612b cancel each other out. As a result, the generation of noise due to the disturbance modulation in the Mach-Zehnder optical waveguide 244d is effectively reduced.

[0082] [1.2 Second Modification] 8 is a diagram showing the configuration of an intersection region 400a4-2 according to a second modification of the intersection region 400a4 shown in FIG. 6. The configuration of the intersection region 400a4-2 shown in FIG. 8 can be used in place of the intersection region 400a4 shown in FIG. 6 in the light modulation element 104. As will be apparent to those skilled in the art, a configuration similar to that of the intersection region 400a4-2 can also be used in intersection regions 400 other than the intersection region 400a4 in FIG. 4. Note that in FIG. 8, 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 above is incorporated herein.

[0083] In the intersection region 400a4-2, signal lines 252a1-2 and 252a2-2 are used instead of the signal lines 252a1 and 252a2. The signal lines 252a1-2 and 252a2-2 have the same configuration as the signal lines 252a1 and 252a2, but differ from the signal lines 252a1 and 252a2 in the manner in which they intersect with the parallel waveguide 246d. Hereinafter, the signal lines 252a1-2 and 252a2-2 will also be collectively referred to as the signal line 252a-2.

[0084] 6, signal lines 252a1 and 252a2 are formed close to each other and parallel to each other at the portion where the signal line 252a intersects with the two parallel waveguides 246d, whereas signal lines 252a1-2 and 252a2-2 intersect with each other in a region on the substrate 220 that is sandwiched between the two parallel waveguides 246d. As a result, the intersection lengths of two intersections 614a and 614b between the two signal lines 252a-2 and the parallel waveguide 246d1 are made equal to each other, and the intersection lengths of two intersections 616a and 616b between the two signal lines 252a-2 and the parallel waveguide 246d2 are also made equal to each other.

[0085] Here, from the viewpoint of making the intersection lengths between the intersections 614a and 614b and between the intersections 616a and 616b the same, it is desirable that the signal lines 252a1-2 and 252a2-2 be formed symmetrically with respect to a line segment CL (the two-dot chain line shown in the figure) that is perpendicular to the two parallel waveguides 246d.

[0086] At the portion where the two signal lines 252a-2 intersect with the parallel waveguide 246d, an electrically insulating resin layer 800 is formed to prevent these signal lines 252a-2 from contacting each other. The resin constituting the resin layer 800 may be, for example, a photoresist, which contains a coupling agent (crosslinking agent) and is a so-called photosensitive permanent film that hardens as a crosslinking reaction progresses when heated.

[0087] Fig. 9 is a cross-sectional view taken along line IX-IX of the intersection region 400a4-2 shown in Fig. 8, and Fig. 10 is a cross-sectional view taken along line XX of the intersection region 400a4-2 shown in Fig. 8. As shown in Fig. 9 and Fig. 10, in this embodiment, the resin layer 800 is formed between the parallel waveguide 246d and the signal line 252a2-2, and between the signal lines 252a2-2 and 252a1-2.

[0088] [1.3 Third Modification] 11 is a diagram showing the configuration of an intersection region 400a4-3 according to a third modification of the intersection region 400a4 shown in FIG. 6. The configuration of the intersection region 400a4-3 shown in FIG. 11 can be used in place of the configuration of the intersection region 400a4 shown in FIG. 6 in the light modulation element 104. As will be apparent to those skilled in the art, a configuration similar to the intersection region 400a4-3 can also be used in intersection regions 400 other than the intersection region 400a4 in FIG. 4. Note that in FIG. 11, 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 above is incorporated herein by reference.

[0089] In the crossing area 400a4-3, instead of the signal lines 252a1 and 252a2, signal lines 252a1-3 and 252a2-3 are used. The signal lines 252a1-3 and 252a2-3 have the same configuration as the signal lines 252a1 and 252a2. Further, with respect to the interval d2 between the portions each intersecting with the parallel waveguide 246d, an interval d3 of a portion downstream of these intersecting portions along the propagation direction of the differential signal is formed wider. Here, the interval d3 can be selected, for example, within the range of d2 < d3 ≦ d1. Hereinafter, the signal lines 252a1-3 and 252a2-3 are also collectively referred to as the signal line 252a-3.

[0090] The configuration of the crossing area 400a4 shown in FIG. 6 is a means to most simply suppress the disturbing modulation generated at the intersection 600a or the like of the two curved parallel waveguides 246d, and is particularly effective when there are a plurality of such intersections in each of the two signal lines 252a. However, on the other hand, when the two signal lines 252a are brought close to each other, a new problem may occur that crosstalk occurs between the differential signals that are high-frequency signals propagating through these signal lines 252a.

[0091] On the contrary, in the configuration of the crossing area 400a4-3 shown in FIG. 11, since the interval d1 and the interval d_{3} upstream and downstream of the intersecting portions along the propagation direction of the differential signal are widely configured with respect to the interval d2 of the portions where the two signal lines 252a-3 intersect with the parallel waveguide 246d, the length of the portion where the two signal lines 252a are provided close to each other at the interval d2 can be reduced, and the amount of occurrence of the above crosstalk can be reduced.

[0092] 11, the length L1 of the section where the two signal lines 252a are provided adjacent to each other with a distance d2 between them (or, if there are multiple sections of the two signal lines 252a provided adjacent to each other with a distance d2 between them, the sum of the lengths L1 of these multiple sections) is preferably shorter than the wavelength at the center frequency of the differential high-frequency signals propagating through the two signal lines 252a (the wavelength of the central electrical signal), from the perspective of reducing crosstalk. For example, the wavelength is approximately 6 mm when the center frequency is 50 GHz, 3 mm when it is 100 GHz, 1.5 mm when it is 200 GHz, and 750 μm when it is 400 GHz.

[0093] [1.4 Fourth Modification] 12 is a diagram showing the configuration of an intersection region 400a4-4 according to a fourth modification of the intersection region 400a4 shown in FIG. 6. The configuration of the intersection region 400a4-4 shown in FIG. 12 can be used in place of the configuration of the intersection region 400a4 shown in FIG. 6 in the light modulation element 104. As will be apparent to those skilled in the art, a configuration similar to that of the intersection region 400a4-4 can also be used in intersection regions 400 other than the intersection region 400a4 in FIG. 4. In FIG. 12, the same components as those in FIGS. 6 and 11 are denoted by the same reference numerals as those in FIGS. 6 and 11, and the explanations for FIGS. 6 and 11 are incorporated herein by reference.

[0094] Intersection region 400a4-4 has a similar configuration to intersection region 400a4-3, but a resin layer 802 is formed between parallel waveguides 246d1 and 246d2 and signal lines 252a1-3 and 252a2-3. The resin constituting resin layer 802 may be, for example, the above-mentioned photosensitive permanent film.

[0095] In the configuration of the intersection region 400a4-4, the resin layer 802 weakens the electric fields applied from the two signal lines 252a1-3 and 252a2-3 to the parallel waveguides 246d1 and 246d2, and therefore, the disturbance modulation occurring in the parallel waveguides 246d1 and 246d2 can be further reduced compared to the configuration shown in the intersection region 400a3-4 in FIG. 11.

[0096] [1.5 Fifth Modification] 13 is a diagram showing the configuration of intersection region 400a4-5 according to a fifth modified example of intersection region 400a4 shown in FIG. 6. The configuration of intersection region 400a4-5 shown in FIG. 13 can be used in place of the configuration of intersection region 400a4 shown in FIG. 6 in light modulation element 104. As will be apparent to those skilled in the art, a configuration similar to intersection region 400a4-5 can also be used in intersection regions 400 other than intersection region 400a4 in FIG. 4. In FIG. 13, the same components as those in FIGS. 6 and 11 are denoted by the same reference numerals as those in FIGS. 6 and 12, and the explanations for FIGS. 6 and 12 above are incorporated herein by reference.

[0097] Intersection region 400a4-5 has a similar configuration to intersection region 400a4, but uses signal lines 252a1-5 and 252a2-5 instead of signal lines 252a1 and 252a2. Hereinafter, signal lines 252a1-5 and 252a2-5 will also be collectively referred to as signal line 252a-5.

[0098] The signal lines 252a1-5 and 252a2-5 have a configuration similar to that of the signal lines 252a1-3 and 252a2-3 in the intersection region 400a-4 shown in FIG. 12, but differ in that the portions intersecting with the parallel waveguide 246d have overlapping portions 810 (hatched portions shown) that overlap each other in a planar view of the substrate 220.

[0099] As a result, in the layered portion 810, the intersection between the signal line 252a1-5 and the parallel waveguide 246d1 and the intersection between the signal line 252a1-5 and the parallel waveguide 246d1 form a single intersection portion 618a and have the same intersection length, and the intersection between the signal line 252a2-5 and the parallel waveguide 246d2 and the intersection between the signal line 252a2-5 and the parallel waveguide 246d2 form a single intersection portion 618b and have the same intersection length.

[0100] Furthermore, in intersection region 400a4-5, a resin layer 804 similar to resin layer 802 shown in FIG. 13 is provided to prevent contact between signal lines 252a1-5 and 252a2-5 in overlapping portion 810. FIG. 14 is a cross-sectional view taken along the line XIV-XIV in intersection region 400a4-5 shown in FIG. 13. In this embodiment, resin layer 804 is provided to prevent contact between parallel waveguides 246d1 and 246d2 and signal lines 252a2-5 and 252a1-5. Like resin layer 802, resin layer 804 may be, for example, a photosensitive permanent film.

[0101] As a result, in the layered portion 810, the electric fields from the two signal lines 252a1-5 and 252a2-5 that transmit differential signals cancel each other out at the positions of the two parallel waveguides 246d.

[0102] That is, in the configuration of the intersection region 400a4-5, the electric field cancellation effect between the two signal lines 252a-5 is also added, and therefore, the disturbance modulation occurring at the intersections 618a and 618b can be further suppressed compared to the configuration of the intersection region 400a4 in which the two signal lines 252a are arranged close to each other as shown in FIG. 6 or the configuration in which the two signal lines 252a-2 cross each other as shown in FIG. 8.

[0103] The configuration of the intersection region 400a4-5 is particularly suitable when it is desired to increase the effect of suppressing disturbance modulation or when it is desired to narrow the region where the two signal lines 252a-5 intersect with the parallel waveguide 246d.

[0104] [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. 15 is a diagram showing the configuration of the optical modulation module 1000 according to this embodiment. In Fig. 15, the same components as those in the optical modulator 100 according to the first embodiment shown in Fig. 1 are indicated by the same reference numerals as those shown in Fig. 1, and the description of Fig. 1 above is used.

[0105] 1, the optical modulation module 1000 differs 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 that drives 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.

[0106] 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, and therefore, similar to the optical modulator 100, can reduce disturbance modulation occurring at the intersection 600a, etc., thereby achieving good modulation operation.

[0107] In this embodiment, the optical modulation module 1000 includes the optical modulation element 104 as an example, but may also include an optical modulation element having an intersection region according to the modified examples shown in Figures 7 to 14.

[0108] 3. Third Embodiment Next, a third embodiment of the present invention will be described. This embodiment is an optical transmitting device 1100 equipped with the optical modulator 100 according to the first embodiment. Fig. 16 is a diagram showing the configuration of the optical transmitting device 1100 according to this embodiment. This optical transmitting device 1100 has the optical modulator 100, a light source 1104 that inputs light to the optical modulator 100, a modulator driving unit 1106, and a modulation signal generating unit 1108. Note that the optical modulation module 1000 according to the second embodiment can also be used instead of the optical modulator 100 and the modulator driving unit 1106.

[0109] The modulation signal generating unit 1108 is an electronic circuit that generates an electrical signal to cause the optical modulator 100 to perform modulation operations. Based on transmission data provided from the outside, the modulation signal generating unit 1108 generates a modulation signal, which is a high-frequency signal to cause the optical modulator 100 to perform optical modulation operations in accordance with the modulation data, and outputs the modulation signal to the modulator driving unit 1106.

[0110] The modulator driver 1106 amplifies the modulation signal input from the modulation signal generator 1108 and outputs differential signals, which are four pairs of 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 driver 1106, it is also possible to use an optical modulation module 1000 that includes, inside the housing 102, a driver circuit 1008 that includes a circuit equivalent to the modulator driver 1106.

[0111] The four pairs of differential signals are input to the signal pins 108 of the optical modulator 100, propagate through the four pairs of signal lines 252 (i.e., signal lines 252a, 252b, 252c, and 252d) of the optical modulation element 104, and drive the optical modulation element 104. As a result, the light output from the light source 1104 is modulated, for example, by DP-QPSK by the optical modulator 100, and is output from the optical transmitting device 1100 as modulated light.

[0112] In particular, the optical transmitting device 1100 uses the optical modulator 100 or the optical modulation module 1000 according to the first embodiment described above, and thus can achieve good modulation characteristics and perform good optical transmission, similar to the optical modulator 100 or the optical modulation module 1000.

[0113] The present invention is not limited to the configurations of the above-described embodiments and their alternative configurations, and can be implemented in various forms without departing from the spirit of the present invention.

[0114] For example, in the above-described embodiment, photosensitive permanent films are used as the resin layers 800 and 802, but the material constituting the resin layers 800 and 802 is not limited to this. Any material can be used for the resin layers 800 and 802 as long as it satisfies the required conditions for electrical properties such as insulation resistance and mechanical properties such as thermal expansion coefficient. Such materials may include thermosetting or thermoplastic resins other than photosensitive permanent films.

[0115] Furthermore, when the signal line 252 is formed in contact with the optical waveguide 230, loss (so-called optical absorption loss) may occur in the light wave propagating through the optical waveguide 230 due to the metal constituting the signal line 252. Therefore, at the portion where the signal line 252 intersects with the optical waveguide 230 (including the parallel waveguide 246), a resin layer similar to the resin layers 800, 802, and / or 804 may be provided between the signal line 252 and the optical waveguide 230. For example, in intersection region 400a4 shown in FIG. 6, intersection region 400a4-1 shown in FIG. 7, intersection region 400a4-3 shown in FIG. 11, and other intersection regions 400 configured similarly to these intersection regions, a resin layer similar to resin layer 800 in intersection region 400a4-2 shown in FIG. 8, resin layer 802 in intersection region 400a4-4 shown in FIG. 12, and / or resin layer 804 in intersection region 400a-5 shown in FIG. 13 may be provided between the corresponding signal line and parallel waveguide.

[0116] Furthermore, in the first embodiment and its modified examples described above, the 16 intersection regions 400 shown in Fig. 4 have the same configuration, but a plurality of configurations may be mixed among these intersection regions 400. That is, the 16 intersection regions 400 may be configured by mixing a plurality of configurations among the configurations shown in intersection regions 400a4, 400a4-1, 400a4-2, 400a-3, 400a4-4, and 400a4-5 shown in Figs. 6, 7, 8, 11, 12, and 13.

[0117] In the above-described embodiment, the optical modulator 104 formed on the substrate 220 made of LN (LiNbO3) is shown as an example of an optical waveguide element, but the optical waveguide element is not limited to this. The optical waveguide element may be an element made of a substrate made of any material (such as LN, InP, or Si) and having any function (such as optical modulation, optical switch, or optical directional coupler). Such an element may be, for example, a so-called silicon photonics waveguide device.

[0118] The above embodiment supports the following configurations.

[0119] (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 optical waveguide having two parallel waveguides with curved portions, and the signal electrode is made of two signal lines for transmitting differential signals that intersect with the two parallel waveguides at the curved portions, and each of the two signal lines is configured so that the crossing lengths of the two parallel waveguides are the same. According to the optical waveguide element of configuration 1, in an optical waveguide element having multiple intersections between a convex optical waveguide and a signal line for propagating an electrical signal, it is possible to effectively suppress the occurrence of disturbance modulation at the intersections and achieve good operating characteristics. (Configuration 2) The optical waveguide element according to configuration 1, wherein the distance between the two signal lines at the portions where the signal lines intersect with the parallel waveguides is narrower than the distance between the portions where the signal lines do not intersect with the parallel waveguides. According to the optical waveguide element of Configuration 2, in the curved portions of the two parallel waveguides, the intersection lengths of the two intersections between each of the two parallel waveguides and the two signal lines can be easily configured to be the same. (Configuration 3) The optical waveguide element according to Configuration 2, wherein the spacing between the two signal lines at the portion where they intersect with the parallel waveguides is narrower than the spacing between the portions upstream of the portion where they intersect with the parallel waveguides along the propagation direction of the differential signal. According to the optical waveguide element of Configuration 3, the two signal lines intersecting the parallel waveguides can be easily placed close to each other, and the intersecting lengths of the two intersecting portions of the parallel waveguides can be made the same. (Configuration 4) The optical waveguide element according to Configuration 3, wherein the two signal lines have a wider spacing at a downstream portion along the propagation direction of the differential signal than at a portion where the two signal lines intersect with the parallel waveguides. According to the optical waveguide element of Configuration 4, it is possible to reduce crosstalk between two signal lines formed close to each other at the intersection with the parallel waveguide. (Configuration 5) The optical waveguide element according to configuration 1, wherein the two signal lines are formed so that the crossing angles with the two parallel waveguides are the same. According to the optical waveguide element of Configuration 5, in the curved portions of the two parallel waveguides, the intersection lengths of the two intersections between each of the two parallel waveguides and the two signal lines can be easily configured to be the same. (Configuration 6) The optical waveguide element according to configuration 5, wherein the two signal lines intersect with each other in a region on the substrate sandwiched between the two parallel waveguides. According to the optical waveguide element of Configuration 6, in the curved portions of the two parallel waveguides, the intersection lengths of the two intersections between each of the two parallel waveguides and the two signal lines can be easily configured to be the same. (Configuration 7) The optical waveguide element according to configuration 1, wherein the two signal lines overlap each other at portions where they intersect with the parallel waveguides when viewed from above the substrate. According to the optical waveguide element of Configuration 7, in the curved portions of the two parallel waveguides, the intersection lengths of the two intersections between the two parallel waveguides and the two signal lines can be easily configured to be the same, and disturbance modulation can be more effectively reduced. (Configuration 8) The optical waveguide element according to any one of Configurations 2 to 7, wherein an insulating layer made of resin is provided between the parallel waveguide and the two signal lines at a portion where the two signal lines intersect with the parallel waveguide. According to the optical waveguide element of Configuration 8, it is possible to reduce the optical absorption loss that occurs in the parallel waveguide due to the metal of the signal line that intersects with the parallel waveguide. (Configuration 9) An optical modulator comprising: an optical waveguide element of any one of Configurations 1 to 7, which is an optical modulation element that modulates light; a housing that accommodates the optical waveguide element; an optical fiber that inputs light to the optical waveguide element; and an optical fiber that guides light output by the optical waveguide element to the outside of the housing. According to the optical modulator of configuration 9, it is possible to reduce the occurrence of disturbance modulation and achieve good optical modulation characteristics. (Configuration 10) An optical modulation module comprising: an optical waveguide element of any one of configurations 1 to 7, 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 10, it is possible to reduce the occurrence of disturbance modulation and achieve good optical modulation characteristics. (Configuration 11) An optical transmitter comprising the optical modulator according to configuration 9 or the optical modulation module according to configuration 10, and an electronic circuit that generates an electrical signal for causing the optical waveguide element to perform a modulation operation. According to the optical transmitter of configuration 11, good optical transmission characteristics can be achieved. [Explanation of symbols]

[0120] 100...optical modulator, 102...housing, 104...optical modulation element, 106...relay board, 108, 110...signal pin, 112...terminator, 114...input optical fiber, 116...optical unit, 118, 130, 134...lens, 120...output optical fiber, 122, 124...support, 220...substrate, 230...optical waveguide, 232...input waveguide, 234...branching waveguide, 240, 240a, 240b...nested Mach-Zehnder type optical waveguide, 244, 244a, 244b, 244c, 244d...Mach-Zehnder type optical waveguide, 246, 246a, 246a1, 246a2, 24 6b, 246b1, 246b2, 246c, 246c1, 246c2, 246d, 246d1, 246d2...Parallel waveguide, 248a, 24 8b...Output waveguide, 250a, 250b, 250c, 250d...Signal electrode, 252, 252a, 252a1, 252a2, 252a-1, 252a1-1, 252a2-1, 252a-2, 252a1-2, 252a2-2, 252a-3, 252a1-3, 252a2-3, 252a-4, 252a1-4, 252a2-4, 252a-5, 252a1-5, 252a2-5, 252a1-5, 252a2-5, 2 52b, 252b1, 252b2, 252c, 252c1, 252c2, 252d, 252d1, 252d2...signal lines, 254a, 254b, 254c, 254d, 254e, 254f, 254g, 254h, 258a, 258b, 258c, 258d, 258e, 258f, 258g, 258h...pads, 270a, 270b, 270c...bias electrodes, 272...ground electrodes, 280a, 280b, 280c, 280d...sides, 300, 300a, 300b, 300c, 300d...acting portions, 400, 400a, 400a1, 400a2, 400a3 , 400a4, 400a4-1, 400a4-2, 400a4-3, 400a4-4, 400a4-5, 400b, 400b1, 400b 2, 400b3, 400b4, 400c, 400c1, 400c2, 400c3, 400c4, 400d, 400d1, 400d2, 40 0d3, 400d4...intersection area, 500...support plate, 502...intermediate layer, 504, 504c1, 504c2...protrusions, 600a, 600b, 602a, 602b, 604a, 604b, 610a, 610b, 612a, 612b, 614a, 614b, 616a, 616b, 618a,618b... intersection portion, 800, 802, 804... resin layer, 810... overlapping portion, 1000... optical modulation module, 1006... circuit board, 1008... drive circuit, 1100... optical transmitter, 1104... light source, 1106... modulator drive unit, 1108... modulation signal generation unit.

Claims

1. an optical waveguide formed by a protrusion extending on a substrate; a signal electrode formed on the substrate for controlling a light wave propagating through the optical waveguide; An optical waveguide element having the optical waveguide includes a Mach-Zehnder optical waveguide having two parallel waveguides each having a curved portion; the signal electrode includes two signal lines for transmitting differential signals, each intersecting the two parallel waveguides at the curved portion; each of the two parallel waveguides is configured so that the lengths of two intersections with the two signal lines transmitting the differential signals are the same; Optical waveguide element.

2. In the two signal lines, the interval between the portions intersecting with the parallel waveguide is narrower than the interval between the portions not intersecting with the parallel waveguide. The optical waveguide element according to claim 1 .

3. the two signal lines have a narrower spacing between the portions intersecting with the parallel waveguides than between the portions upstream of the portions intersecting with the parallel waveguides along the propagation direction of the differential signals; The optical waveguide element according to claim 2 .

4. the two signal lines have a wider interval at a downstream portion along a propagation direction of the differential signal than at a portion where the two signal lines intersect with the parallel waveguides; The optical waveguide element according to claim 3 .

5. the two signal lines are formed so that the intersection angles with the two parallel waveguides are the same; The optical waveguide element according to claim 1 .

6. the two signal lines intersect with each other in a region on the substrate sandwiched between the two parallel waveguides; The optical waveguide element according to claim 5 .

7. the two signal lines are arranged such that portions thereof that intersect with the parallel waveguides overlap each other in a plan view of the substrate; The optical waveguide element according to claim 1 .

8. an insulating layer made of resin is provided between the parallel waveguide and the two signal lines at a portion where the two signal lines intersect with the parallel waveguide; 8. The optical waveguide element according to claim 2.

9. The optical waveguide element according to any one of claims 1 to 7, which is an optical modulation element for modulating light; a housing that houses the optical waveguide element; an optical fiber for inputting light into 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:

10. 8. An optical modulation module comprising: the optical waveguide element according to claim 1, which is an optical modulation element for modulating light; and a drive circuit for driving the optical waveguide element.

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

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