Optical modulator

Abstract

The present application provides a kind of optical modulator, even in the case where signal electrode traverses light branch part or light synthesis part, the degradation of transmission characteristic is inhibited.An optical modulator is constituted by forming optical waveguide and modulation electrode for modulating optical wave propagating in the optical waveguide on substrate, and housing the substrate in the housing, characterized in that the optical waveguide at least has light branch part for branching one optical wave into two or light synthesis part for synthesizing two optical waves into one, the modulation electrode has signal electrode and ground electrode, and part of the signal electrode is configured to traverse the light branch part or the light synthesis part, the optical modulator is provided with suppression unit (for example, resin layer) for suppressing the change of intensity ratio of optical wave branched in the light branch part or intensity ratio of optical wave synthesized in the light synthesis part due to the signal electrode traversing the light branch part or the light synthesis part.

Description

Technical Field

[0001] The present invention relates to optical modulators, and more particularly to optical modulators having a structure in which an optical waveguide formed on a substrate has an optical branch or an optical combining portion and a signal electrode constituting a modulation electrode traverses the optical branch or the optical combining portion. Background Technology

[0002] In the fields of optical communication and optical measurement, optical modulators are often used, which have optical waveguides and modulation electrodes formed on substrates such as lithium niobate (LN) that have electro-optic effects.

[0003] Figure 1 This is a top view illustrating an example of an optical modulator such as a DP-QPSK (Dual Polarized Quad Phase Shift Keying) modulator, with an LN substrate containing an optical waveguide and modulation electrodes housed within a housing. Light waves input to the incident side of the LN substrate via optical fiber are guided into the optical waveguide of the LN substrate via an incident collimator or similar means. Furthermore, on the emitting side of the LN substrate, the modulated light waves are guided into the emitting optical fiber via a polarization-wave synthesis optical system, an emitting collimator, or similar means, and output as light.

[0004] Electrical signals introduced via RF connectors or the like located on the side of the housing are applied to the modulation electrodes of the LN substrate. When an electrical signal is input to the LN substrate, electrical wiring formed on the relay substrate can also be used. Furthermore, the electrodes formed on the LN substrate also include DC electrodes for applying DC bias. DC bias is supplied to the DC electrodes of the LN substrate via DC pins formed on the side of the housing.

[0005] Figure 2 This is a top view showing an example of an LN substrate. Figure 2 For simplicity, a Mach-Zehnder type optical waveguide is formed and configured in the optical waveguide formed on the LN substrate. The shape of the optical waveguide is not limited to this; nested optical waveguides, where branch waveguides constituting a main Mach-Zehnder type optical waveguide are nested with secondary Mach-Zehnder type optical waveguides, are also used. Furthermore, in... Figure 1 In DP-QPSK modulators and the like, a structure is also used in which an optical waveguide is branched into two and each waveguide of the branch is connected to a nested optical waveguide.

[0006] exist Figure 1 , Figure 2 In the optical modulator shown, the positions of the incident light (optical input) supplied from the outside to the optical waveguide and the positions of the outgoing light (optical output) emitted after being modulated by an electrical signal supplied from the outside to the modulation electrode are arranged at both ends of the substrate's length direction (the short side portion of the rectangular substrate). Figure 2An optical waveguide and a modulation electrode are formed on the LN substrate. The modulation electrode has a signal electrode and a ground electrode, and in particular, the signal electrode extends from the side of the substrate along its length (the long side portion of the rectangular substrate). Figure 2 The signal is introduced from the lower side of the LN substrate and exited from the same side or opposite side. A terminator that terminates the signal with an electrical signal (modulation signal) is connected to the end of the signal electrode.

[0007] From Figure 2 When the signal electrode is introduced into the long side of such a substrate, as shown in Patent Document 1, the signal electrode is typically arranged to traverse the straight portion of the branch waveguide and the input waveguide, avoiding the optical branching and optical combining sections of the Mach-Zehnder type optical waveguide. In this case, when the signal electrode traverses the optical waveguide, it does not affect the intensity ratio (branching ratio) of the light at the optical branching sections such as the Y branch. Furthermore, it also does not affect the intensity ratio (combination ratio) of the combined light wave at the optical combining section.

[0008] On the other hand, the higher the speed of the transmitted electrical signal (e.g., above 50 GHz), the greater the transmission loss. Therefore, it is necessary to shorten the electrical signal line from the digital signal processor (DSP) that can generate electrical signals with a specified phase and timing to the driver that amplifies the signal and uses it to drive the modulator, and then from the driver to the LN modulation element (signal electrode of the LN substrate).

[0009] To reduce transmission loss, such as Figure 3 As shown, the DSP, driver, and optical modulation element are required to be arranged in a roughly straight line, and the length and bends of the signal lines are minimized. Furthermore, integration is achieved within the housing of the optical modulator, which not only houses the LN substrate serving as the optical modulation element but also the driver element.

[0010] When performing the above procedures, with Figure 2 Unlike the optical modulator shown, the electrical signal is transmitted not from the long side of the LN substrate along its length, but rather... Figure 4 As shown in (a), the signal is input from the short side of the LN substrate, thereby shortening the signal line and enabling the signal line to be arranged linearly.

[0011] In, for example, patent document 2, Figure 4 (a) as shown from the short side portion of the LN substrate ( Figure 4 When an electrical signal is input to the short side of the right side of the LN substrate (a), the number of cases where the signal electrode is arranged to traverse the optical branch of a Mach-Zehnder type optical waveguide increases. Therefore, when the signal electrode traverses the optical waveguide, for example... Figure 4As shown in (b), applying an electric field to one of the waveguides of the two branches that constitute the optical branch, such as the Y branch, affects the branching ratio of the light. Figure 4 (b) is Figure 4 The cross-sectional view at the dashed line A-A' in (a).

[0012] Furthermore, even when the signal electrode is positioned directly above the input waveguide connected to the optical branch and traverses the center of the Y branch, as described later, modulation electric fields in different directions are applied to each branch waveguide near the point where the two branch waveguides constituting the Y branch begin to branch, resulting in different refractive index changes in the two branch waveguides. This causes the branching ratio of the light waves branching into the two branch waveguides to change.

[0013] In the case of a DP-QPSK modulator, if the branching ratio changes at the initial Y-branch of the branching towards X-polarized and Y-polarized waves, the loss difference between the X-polarized and Y-polarized waves will change. Moreover, in the case of the Y-branch of a nested optical waveguide, the change in the extinction ratio of the output signal light will, in any case, cause a deterioration in transmission characteristics.

[0014] Patent Document 1: Japanese Patent Application Publication No. 2016-194577

[0015] Patent Document 2: Japanese Patent Application Publication No. 2014-112219 Summary of the Invention

[0016] The problem that the invention aims to solve

[0017] The problem to be solved by the present invention is to provide an optical modulator that solves the problems described above and suppresses the degradation of transmission characteristics even when the signal electrode crosses the optical branch or optical synthesis section.

[0018] Solution for solving the problem

[0019] To address the aforementioned issues, the optical modulator of the present invention has the following technical features.

[0020] (1) An optical modulator comprising forming an optical waveguide and a modulation electrode for modulating an optical wave propagating in the optical waveguide on a substrate, and housing the substrate within a housing, characterized in that the optical waveguide has at least an optical branching portion that branches one optical wave into two or an optical combining portion that combines two optical waves into one, the modulation electrode having a signal electrode and a ground electrode, and a portion of the signal electrode being arranged to traverse the optical branching portion or the optical combining portion, the optical modulator comprising a suppression unit for suppressing changes in the intensity ratio of the optical waves branched in the optical branching portion or the intensity ratio of the optical waves combined in the optical combining portion due to the signal electrode traversing the optical branching portion or the optical combining portion.

[0021] (2) The optical modulator according to (1) above is characterized in that the interface portion provided in the housing and supplying an electrical signal to the modulation electrode is formed near the position where the extension line of the linear symmetry axis of the optical branch or the optical synthesis portion in the propagation direction of the light wave passing through the optical branch or the optical synthesis portion intersects the side surface of the housing.

[0022] (3) The optical modulator according to (1) or (2) above, characterized in that the optical branch or the optical combining part is composed of a Y branch, the Y branch having: an input / output waveguide having a waveguide width W corresponding to a single mode; a conversion waveguide connected to the input / output waveguide and formed with a waveguide width wider than W; and two branch waveguides connected to the conversion waveguide, the position where the signal electrode crosses the optical branch or the optical combining part is the conversion waveguide, or either of the two branch waveguides within a range from the conversion waveguide to the position where the distance between the two branch waveguides is three times the mode field diameter of the light wave propagating in the branch waveguide.

[0023] (4) The light modulator according to (3) above, characterized in that the suppression unit is a resin layer disposed between the light branching part or the light synthesis part and the modulation electrode.

[0024] (5) The optical modulator according to (4) above is characterized in that the length of the portion of the modulation electrode overlapping the resin layer is set to less than one-tenth of the wavelength of the modulation signal.

[0025] (6) The optical modulator according to any one of (3) to (5) above, characterized in that the suppression unit is arranged along the linear axis of symmetry of the optical branch or the optical synthesis part in the direction of propagation of the light wave passing through the optical branch or the optical synthesis part, and has a symmetrical shape with respect to the linear axis of symmetry, and at the position where the signal electrode crosses the optical branch or the optical synthesis part, the width of the signal electrode is more than three times the mode field diameter of the light wave propagating in the branch waveguide.

[0026] Invention Effects

[0027] This invention relates to an optical modulator comprising forming an optical waveguide and a modulation electrode for modulating light waves propagating in the optical waveguide on a substrate, and housing the substrate within a housing. The optical waveguide is characterized by having at least one optical branching section that branches one light wave into two or one optical combining section that combines two light waves into one. The modulation electrode has a signal electrode and a ground electrode, and a portion of the signal electrode is arranged to traverse the optical branching section or the optical combining section. The optical modulator includes a suppression unit for suppressing changes in the intensity ratio of the light waves branched in the optical branching section or the intensity ratio of the light waves combined in the optical combining section due to the signal electrode traversing the optical branching section or the optical combining section. Therefore, an optical modulator that suppresses degradation of transmission characteristics can be provided. Attached Figure Description

[0028] Figure 1 This is a top view showing a general outline of a conventional optical modulator.

[0029] Figure 2 This is a top view showing a general outline of a conventional optical modulation element.

[0030] Figure 3 This diagram illustrates a scenario where the DSP, driver elements, and optical modulation elements are roughly arranged in a straight line.

[0031] Figure 4 The figure shows an example of an optical modulation element in which the signal electrode crosses the optical branch of the optical waveguide. (a) is a top view and (b) is a cross-sectional view at the dashed line A-A' in (a).

[0032] Figure 5 This is a diagram illustrating the shape of the Y-branch used in optical branching.

[0033] Figure 6 The figure shows an example (1) of the optical modulation element used in the optical modulator of the present invention, (a) is a top view, and (b) is a cross-sectional view at the dashed line A-A' of (a).

[0034] Figure 7 The figure shows an example (2) of the optical modulation element used in the optical modulator of the present invention, (a) is a top view and (b) is a cross-sectional view at the dashed line A-A' of (a).

[0035] Figure 8 This is a top view illustrating the interior of the housing of the optical modulator of the present invention.

[0036] Figure 9 This is a figure illustrating an example (3) of the optical modulation element used in the optical modulator of the present invention.

[0037] Figure 10This is a top view illustrating another aspect of the housing of the optical modulator of the present invention.

[0038] Figure 11 This is a figure illustrating an example (4) of the optical modulation element used in the optical modulator of the present invention.

[0039] Figure 12 This is a figure illustrating an example (5) of the optical modulation element used in the optical modulator of the present invention. Detailed Implementation

[0040] Hereinafter, the optical modulator of the present invention will be described in detail using preferred embodiments.

[0041] The optical modulator of the present invention uses Figure 6 , 7 The target is an optical modulator constructed in the manner shown in 9, 11 or 12, in which the signal electrode crosses the optical waveguide optical branch or optical synthesis section.

[0042] Specifically, an optical modulator, through, as Figure 6 An optical waveguide and a modulation electrode for modulating light waves propagating in the optical waveguide are formed on a substrate as shown in the figure. Figure 8 The optical waveguide is configured such that the substrate is housed within a housing as shown in diagram 10. The waveguide is characterized by having at least one optical branch that branches one optical wave into two or one optical combining portion that combines two optical waves into one. The modulation electrode has a signal electrode and a ground electrode, and a portion of the signal electrode is arranged to traverse the optical branch or the optical combining portion. The optical modulator includes a suppression unit for suppressing changes in the intensity ratio of the optical waves branched in the optical branch or the intensity ratio of the optical waves combined in the optical combining portion due to the signal electrode traversing the optical branch or the optical combining portion.

[0043] As the substrate used in the optical modulator of the present invention, substrates with electro-optic effects such as lithium niobate (LN) or semiconductor substrates can be used. Regarding the optical waveguide, for example, known techniques such as forming a waveguide by growing an LN crystal, processing an LN substrate, or forming an optical waveguide by thermally diffusing a metal such as Ti on an LN substrate can be used. Furthermore, regarding the electrodes, for example, known techniques such as forming electrodes by gold plating can be used. In the description of the present invention, the substrate (chip) on which the optical waveguide and electrodes are formed is sometimes also referred to as an "optical modulation element".

[0044] In the optical modulator of the present invention, such as Figure 3 , 8 Or as shown in 10, in order to supply an electrical signal to the modulation electrode of the optical modulation element disposed within the housing, an interface portion of the housing is provided ( Figure 8The RF connector (especially the RF connector of type 10) is formed near the location where the extension of the linear axis of symmetry of the optical branch or optical synthesizer intersects the side of the housing in the direction of propagation of the light wave through the optical branch or optical synthesizer (the left-right direction in each figure). As a result, the DSP can be arranged near the outside of the interface, the driver element can be arranged near the inside of the interface within the housing, and the optical modulation element can be arranged on the side of the driver element opposite to the interface. Furthermore, the connection from the DSP to the optical modulation element (modulation electrode) can be made in a relatively straight line with shorter wiring, minimizing the transmission loss of electrical signals.

[0045] exist Figure 8 In option 10, the RF / DC connector, serving as the interface, is located on the short side of the housing, while the optical input / output section is located on the opposite short side. The optical input section may also be located on the long side of the housing (the lower long side in the attached diagram). Figure 8 In this case, the light input portion of the light modulation element itself is positioned on the long side of the substrate, and the light wave introduced from the light input portion of the housing is directed to the substrate constituting the light modulation element via a spatial optical system. Furthermore, it can also be done as follows: Figure 10 In this way, the light input section and the light output section are arranged on the short side of the same substrate, as described later. Figure 11 Or, as in 12, it is composed of an optical waveguide forming a folded optical path within the substrate, from the optical input section to the optical modulation section (the part of the optical waveguide that receives the modulation effect of the optical wave through the modulation signal).

[0046] In the light modulation section within the substrate, light waves originate from... Figure 8 The light propagates from right to left, receives light modulation, and is emitted from the end of the substrate. It is then polarized by a spatial optical system containing a polarization synthesizer and emitted from the light output section. The electrical signal also originates from the modulation electrodes (signal electrodes) within the substrate. Figure 8 Or 10 propagates from right to left. A relay board containing a driver element is arranged near the interface section inside the housing, and the modulation signal is transmitted from the interface section to the relay board, and from the relay board to the optical modulation element (board) via the shortest path.

[0047] To suppress transmission losses in the signal electrodes that transmit electrical signals, such as Figure 4 As shown, the distance from the input terminal of the signal electrode to the interaction section between the optical waveguide and the modulation electrode needs to be approximately a straight line. As a result, the signal electrode traverses the optical branch of the optical waveguide.

[0048] Figure 5This is an enlarged view illustrating an example of an optical branching section utilizing a Y-branch. The optical branching section is constructed via an input waveguide to the right of dashed line C1, a conversion waveguide in the interval from dashed line C1 to C2, and a branching waveguide to the left of dashed line C2. The optical combining section can also employ a Y-branching structure, in which case the two branching waveguides are connected to the conversion waveguide, and then connected to an output waveguide via the conversion waveguide. For example, this could be used to... Figure 5 The light propagation direction of the Y-branch is reversed (from left to right in the figure). The following explanation focuses on the light branch; however, even when the signal electrode traverses a portion of the light combining section, the application of this invention can suppress changes in the intensity ratio of the combined light waves. It should be noted that... Figure 5 The Y-branch is shown in the figure, but the present invention can also be applied to other photosynthetic branching units, such as MMIs (multimode interferometer waveguides), couplers, etc.

[0049] The optical branch includes: an input waveguide having a waveguide width W corresponding to a single mode; a switching waveguide connected to the input waveguide and formed between an end having a waveguide width equal to W and an end having a waveguide width wider than W; and two branch waveguides connected to the switching waveguide. In this invention, it was found that when the signal electrode traverses the optical branch at a position on the switching waveguide, or at any one of the two branch waveguides within a range from the switching waveguide to a position described below, the branching ratio of the optical wave in the optical branch varies from a desired set value, for example, 1:1, thereby completing the optical modulator of this invention. This position is where the two branch waveguides are separated to a distance D between them that is three times the mode field diameter of the optical wave propagating in the branch waveguides.

[0050] exist Figure 5 As shown, within the range from dashed line C1 to dashed line C3 (distance D is three times the mode field diameter of the single-mode light wave propagating in the branching waveguide), when the signal electrode crosses the optical waveguide, the branching ratio of the light wave at the optical branching section is easily varied from a set value (e.g., 1:1). It should be noted that, in this invention, "signal electrode crossing the optical branching section or optical combining section" means that... Figure 5 The diagram shows the configuration of signal electrodes within the range from dashed line C1 to dashed line C3 of the Y-branch.

[0051] To suppress changes in the branching ratio, such as Figure 6As shown, a resin layer serves as a suppression unit between the optical waveguide constituting the optical branch and the modulation electrode (signal electrode and ground electrode (GND)). The resin layer can be a thermoplastic resin or a thermosetting resin; examples include polyamide resins, melamine resins, phenolic resins, amino resins, and epoxy resins. Furthermore, the resin layer can use, for example, a permanent photoresist, or a photoresist material based on a thermosetting resin. In the manufacturing process of the optical modulation element, the resin layer is spin-coated onto a substrate, patterned using a conventional photolithography process, and then thermoset, thereby allowing the resin layer to be disposed between the modulation electrode and the substrate. Additionally, the refractive index of the resin layer needs to be selected to be lower than the refractive index of the core of the optical waveguide.

[0052] Furthermore, by utilizing a permanent resist, there are advantages such as ease of patterning and the ability to form relatively thick films with a thickness of 3 to 5 μm. In the structure of the present invention, a buffer layer (SiO2 film) can be added to the substrate surface, but the buffer layer is only about 1 μm thick, and the effect of the present invention cannot be obtained.

[0053] Figure 6 (b) indicates Figure 6 The sectional view at the dashed line A-A' in (a). Figure 6 As shown in (b), by configuring a resin layer as a suppression unit, the branching of the electric field formed between the signal electrode and the ground electrode to one side can be suppressed using a waveguide. Figure 6 The situation is as follows (b) on the right side of the optical waveguide. Therefore, no extra electric field is applied to the branching waveguide constituting the optical branch, and the branching ratio of the branched optical waves can be suppressed. It should be noted that the resin layer can be locally disposed at the location where the signal electrode, or where the signal electrode and ground electrode cross the optical waveguide, and can also be widely formed on the substrate surface, except for the areas where the electric field formed by electrodes such as the modulation electrode and DC electrode interacts with the optical waveguide.

[0054] like Figure 6 When a resin layer is placed midway through the transmission path of a modulation electrode containing a signal electrode, the characteristic impedance of the modulation electrode changes between the portions where the resin layer is present and those where it is absent. If the length of the overlap between the modulation electrode and the resin layer is less than one-tenth of the wavelength of the modulation signal, the change in characteristic impedance has minimal impact. However, if the overlap is longer, it is necessary to suppress the change in characteristic impedance. Specifically, one could consider making the width of the signal electrode on the resin layer wider than other portions, or making the spacing between the signal electrode and the ground electrode on the resin layer narrower than other portions. Furthermore, when varying the width of the signal electrode and the spacing, the varying portions can be continuously adjusted in a conical shape.

[0055] Figure 7 Shown to be with Figure 6 The input of a Mach-Zehnder type optical waveguide has signal electrodes arranged in a waveguide overlap configuration. Furthermore, Figure 7 (b) shows Figure 7 The sectional view at the dashed line A-A' in (a), especially Figure 7 (b) shows the situation after the branching waveguide constituting the optical branching section has just branched. Even in such a close arrangement of branching waveguides, by configuring the resin layer as a suppression unit, it is possible to suppress the situation where different electric fields (including electric fields acting in different directions) act on the two branching waveguides respectively, and to prevent different refractive index changes in the two branching waveguides. As a result, it is possible to suppress changes in the branching ratio of the light waves branching into the two branching waveguides.

[0056] Figure 9 This is another example of an optical modulation element that can be used in the optical modulator of the present invention. It should be noted that, in Figure 9 The input portion of the optical waveguide formed on the substrate is located on the long side of the substrate, which can be well used in applications such as... Figure 8 Such a light modulator has an input section for light waves on the long side of the substrate.

[0057] Figure 9 The main feature of an optical modulation element is the shape of the signal electrodes that are arranged overlapping with the optical branch. Figure 9 An example is shown using signal electrodes arranged along the linear symmetry axis E of the optical branch and having a symmetrical shape with respect to this linear symmetry axis E. Because the signal electrodes have a symmetrical shape with respect to the aforementioned linear symmetry axis, the effect on the branching ratio of the optical branch is approximately the same across the linear symmetry axis, thus suppressing variations in the branching ratio from, for example, 1:1. It should be noted that... Figure 9 The figure shows a case where a driver element (DRV) is arranged on a relay substrate.

[0058] exist Figure 9 In this process, the width of the signal electrode varies, and the active portion of the optical waveguide is narrower than that of the input waveguide. A particularly important aspect of the signal electrode's shape is its width at the location where it traverses the optical branch. Figure 5 The mode field diameter of the light wave propagating in the branching waveguide shown is more than three times (symbol D). By using such a wide signal electrode to traverse the optical branch (Y branch), the electric field applied to the waveguide is dispersed, thus enabling it to function as a suppression unit to suppress changes in the branching ratio, etc.

[0059] Alternatively, the signal electrodes can be configured such that they split into two midway and then rejoin, with the signal electrodes arranged in the split sections by means of a waveguide or similar device that bypasses the optical branch (Y-branch). Of course, the spacing between the signal electrodes in the branches that traverse the optical branch is preferably [ratio missing]. Figure 5 The symbol D is wide.

[0060] Even when using Figure 9 In the case of such signal electrodes, they can also be configured at the overlapping area of ​​the optical waveguide and the modulation electrode. Figure 6 Or a resin layer as shown in Figure 7. This further suppresses the influence on the branching ratio of the optical branch.

[0061] exist Figure 7 or Figure 9 In this design, the signal electrode and ground electrode are symmetrically arranged relative to a portion of the input waveguide and its continuous branch waveguide, particularly within the Mach-Zehnder type optical waveguide. This symmetrical arrangement of the electrodes (including the DC bias electrode) relative to the optical waveguide homogenizes or symmetrs the internal stress applied to the waveguide, thereby suppressing DC drift caused by temperature variations in the optical modulation element.

[0062] like Figure 10 As shown, the situation of inputting and outputting light waves from the short side of one of the substrates constituting the light modulation element is explained in more detail. Figure 11 and 12 yes Figure 10 A specific example of an optical modulation element uses two nested optical waveguides arranged in parallel with four Mach-Zehnder type optical waveguides as optical waveguides. Furthermore, the light waves modulated by each nested optical waveguide are polarized and output through a polarization wave combining section outside the substrate.

[0063] exist Figure 11 and 12 In this design, input and output sections for light waves are arranged on the same short side of the substrate, thus forming an optical waveguide for refracting light waves input into the substrate. Furthermore, in Figure 11 and 12 In this process, multiple electrical signals (modulated signals) are input from the other short side of the substrate. Furthermore, a terminator, including a terminating resistor, is connected to the terminal portion of the signal electrodes to suppress signal reflection. The terminators are arranged separately on the two long sides of the substrate, but they can also be concentrated on one long side. Alternatively, they can be disposed on the substrate using flip-chip bonding or similar methods.

[0064] exist Figure 11 In optical modulation elements, with Figure 6 Similarly, the case where the input waveguide of the optical branch overlaps with the signal electrode (RF signal electrode) and the signal electrode crosses the optical branch is shown for four Mach-Zehnder type optical waveguides. Furthermore, in Figure 12 In optical modulation elements, with Figure 7 Similarly, for four Mach-Zehnder type optical waveguides, the case where the input waveguide of the optical branch is arranged overlapping the signal electrode (RF signal electrode) and the signal electrode traverses the optical branch is shown. Figure 11 and 12 In this case, a resin layer is also placed at the location where the signal electrode (RF signal electrode) crosses the optical branch to suppress changes in the branching ratio. It should be noted that resin layers can also be placed at other locations where the RF signal electrode and DC signal electrode cross the optical waveguide to suppress the application of the electric field of the signal electrode to the optical waveguide in non-useful locations and the absorption of light waves propagating in the optical waveguide by the signal electrode.

[0065] Industrial applicability

[0066] As described above, according to the present invention, it is possible to provide an optical modulator that suppresses the degradation of transmission characteristics even when the signal electrode crosses the optical branch or optical synthesis section.

Claims

1. An optical modulator, comprising forming an optical waveguide and a modulation electrode for modulating a light wave propagating in the optical waveguide on a substrate, and housing the substrate within a housing, characterized in that, The optical waveguide has at least one optical branching section that splits a light wave into two or two light waves that combine into one. The modulation electrode has a signal electrode and a ground electrode, and a portion of the signal electrode is configured to traverse the optical branch or the optical combining section. The optical modulator includes a suppression unit for suppressing changes in the intensity ratio of the light waves branched at the optical branch or synthesized at the optical synthesis section due to the signal electrode traversing the optical branch or the optical synthesis section. The suppression unit is a resin layer disposed between the optical branch or the optical synthesis part and the modulation electrode, and the resin layer is not formed in the part where the electric field generated by the modulation electrode acts on the optical waveguide.

2. The optical modulator according to claim 1, characterized in that, An interface portion disposed on the housing and supplying electrical signals to the modulation electrode is formed near the location where the extension line of the linear symmetry axis of the light branch or the light synthesis portion in the propagation direction of the light wave passing through the light branch or the light synthesis portion intersects with the side surface of the housing.

3. The optical modulator according to claim 1 or 2, characterized in that, The optical branch or the optical synthesis section is composed of a Y-branch. The Y-branch has: an input / output waveguide having a waveguide width W corresponding to a single mode; a conversion waveguide connected to the input / output waveguide and formed with a waveguide width wider than W; and two branch waveguides connected to the conversion waveguide. The position where the signal electrode crosses the optical branch or the optical synthesis section is the switching waveguide, or either of the two branch waveguides within the range from the switching waveguide to the position where the distance between the two branch waveguides is three times the mode field diameter of the light wave propagating in the branch waveguide.

4. The optical modulator according to claim 3, characterized in that, The length of the portion where the modulation electrode overlaps with the resin layer is set to be less than one-tenth of the wavelength of the modulation signal.

5. An optical modulator, comprising forming an optical waveguide and a modulation electrode for modulating a light wave propagating in the optical waveguide on a substrate, and housing the substrate within a housing, characterized in that, The optical waveguide has at least one optical branching section that splits a light wave into two or two light waves that combine into one. The modulation electrode has a signal electrode and a ground electrode, and a portion of the signal electrode is configured to traverse the optical branch or the optical combining section. The optical modulator includes a suppression unit for suppressing changes in the intensity ratio of the light waves branched at the optical branch or synthesized at the optical synthesis section due to the signal electrode traversing the optical branch or the optical synthesis section. The optical branch or the optical synthesis section is composed of a Y-branch. The Y-branch has: an input / output waveguide having a waveguide width W corresponding to a single mode; a conversion waveguide connected to the input / output waveguide and formed with a waveguide width wider than W; and two branch waveguides connected to the conversion waveguide. The signal electrode traverses the optical branch or the optical combining section at the location of the switching waveguide, or either of the two branch waveguides within a range extending from the switching waveguide to a location where the distance between the two branch waveguides is three times the mode field diameter of the light wave propagating in the branch waveguide. The suppression unit is arranged along the linear axis of symmetry of the optical branch or the optical synthesis section in the direction of propagation of the light wave passing through the optical branch or the optical synthesis section, and has a symmetrical shape with respect to the linear axis of symmetry. At the position where the signal electrode crosses the optical branch or the optical synthesis section, the width of the signal electrode is more than three times the mode field diameter of the light wave propagating in the branch waveguide.

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