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

Abstract

The invention provides an optical modulation element, an optical modulator, an optical modulation module, an optical transmission device and an optical transmission system. Good optical characteristics can be achieved in the optical modulation element using a convex optical waveguide and segmented electrodes as modulation electrodes. The light modulation element includes: a substrate including a multilayer portion; an optical waveguide comprising a convex portion extending on the optical waveguide layer of the multilayer portion of the substrate; and a modulation electrode formed by being divided into a plurality of segments along the direction of propagation of the light of the optical waveguide, and in the entire segment of the modulation electrode or in a segment other than a part of the modulation electrode, the interval between adjacent segments measured in the direction of extension of the optical waveguide is constant. The refractive index n1 of the first support layer in contact with the lower surface of the optical waveguide layer, the refractive index n2 of the second support layer in contact with the lower surface of the first support layer, and the refractive index n3 of the third support layer in contact with the lower surface of the second support layer satisfy the relationships n2 > n1 and n2 > n3.

Description

Technical Field

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

[0002] High-speed and high-capacity optical fiber communication systems often use optical modulators incorporating optical modulators, which are optical waveguide elements. These optical modulators consist of an optical waveguide formed on a semiconductor substrate such as InP or a substrate such as LiNbO3 (hereinafter referred to as LN), which exhibits an electro-optical effect, and a control electrode that controls the light waves propagating through the waveguide. Optical modulators using LN substrates are widely used in high-speed and high-capacity optical fiber communication systems because they offer low optical loss and wideband optical modulation characteristics.

[0003] In recent years, optical modulators using rib-type or ridge-type optical waveguides (hereinafter collectively referred to as convex waveguides) have become practical in order to achieve further low-voltage drive and high-speed modulation while miniaturizing the optical modulator itself. These rib-type or ridge-type optical waveguides are composed of strip-shaped convex portions formed on the surface of a thin-film (or plate-like) LN substrate (for example, with a thickness of 20 μm or less) to further enhance the interaction between the signal electric field and the waveguide light in the substrate.

[0004] In addition, in recent years, as a coplanar modulation electrode, a so-called segmented electrode has been proposed, which is a method of dividing the electrode into multiple segments along the light propagation direction of the optical waveguide, in order to achieve impedance matching between the modulation electrode and the driving circuit and speed integration between the high-frequency propagation speed in the modulation electrode and the light propagation speed in the optical waveguide (Patent Document 1, Patent Document 2, Patent Document 3).

[0005] [Prior art literature]

[0006] [Patent Document]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2022-148652

[0008] [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-194544

[0009] [Patent Document 3] Japanese Patent Application Laid-Open No. 2020-181173 Summary of the Invention

[0010] [Problems to be solved by the invention]

[0011] The inventors of this application discovered that convex optical waveguides equipped with segmented electrodes as modulation electrodes suffer from the following problem: even when the convex portions (i.e., ribs or ridges) constituting the optical waveguide are formed with high precision during wafer processing, variations in optical characteristics such as the modulation extinction ratio occur. The cause and solution to this problem remained unidentified for a long time.

[0012] An object of the present invention is to achieve good optical characteristics in an optical modulation element using a convex optical waveguide and segment electrodes as modulation electrodes.

[0013] [Technical means to solve the problem]

[0014] One embodiment of the present invention is an optical modulation element comprising: a substrate including a multilayer portion; an optical waveguide formed of protrusions extending on an optical waveguide layer of the multilayer portion of the substrate; and a modulation electrode for controlling light waves propagating in the optical waveguide formed on the optical waveguide layer, the modulation electrode being divided into a plurality of segments along the propagation direction of light in the optical waveguide. A spacing L between adjacent segments, measured in the direction in which the optical waveguide extends, is constant throughout or except for a portion of the modulation electrode. The multilayer portion of the substrate includes the optical waveguide layer, a first supporting layer in contact with a lower surface of the optical waveguide layer, a second supporting layer in contact with a lower surface of the first supporting layer, and a third supporting layer in contact with a lower surface of the second supporting layer. The refractive index n1 of the first supporting layer, the refractive index n2 of the second supporting layer, and the refractive index n3 of the third supporting layer satisfy the relationship n2>n1 and n2>n3.

[0015] According to another embodiment of the present invention, the modulation electrode is formed by dividing into a plurality of segments of the same length, and the spacing L of the gaps between adjacent segments measured in the extension direction of the optical waveguide has a relationship of L>4×λ / n1 with respect to the wavelength λ of the light wave propagating in the optical waveguide and the refractive index n1 of the first supporting layer.

[0016] According to another embodiment of the present invention, the thickness t1 of the first supporting layer has a relationship of t1<10×λ / n1 with respect to the wavelength λ of the light wave propagating in the optical waveguide and the refractive index n1 of the first supporting layer.

[0017] According to another embodiment of the present invention, the refractive index n2 and thickness t2 of the second supporting layer are in the relationship t2 < t0 and n2 > n0 with respect to the refractive index n0 and thickness t0 of the optical waveguide layer.

[0018] According to another embodiment of the present invention, the refractive index n1 of the first supporting layer, the refractive index n2 of the second supporting layer, and the refractive index n3 of the third supporting layer have a relationship of (n2-n3)<(n2-n1).

[0019] According to another embodiment of the present invention, a light absorbing material is provided on at least a portion of the end surface of the substrate, and the light absorbing material absorbs light in a wavelength band of the light wave propagating in the optical waveguide.

[0020] According to another embodiment of the present invention, the light absorbing material is a carbon material, a black resin or a metal filler.

[0021] According to another embodiment of the present invention, the substrate is formed by stacking a plurality of plates, each of which includes one or two adjacent layers of the optical waveguide layer, the first supporting layer, the second supporting layer, and the third supporting layer.

[0022] Another embodiment of the present invention is an optical modulator, comprising: an optical modulator as described above; a frame that houses the optical modulator; an optical fiber that inputs light to the optical modulator; and an optical fiber that guides the light output by the optical modulator to the outside of the frame.

[0023] Another embodiment of the present invention is an optical modulation module, comprising: an optical modulation element as described in any one of the above items; a frame for accommodating the optical modulation element; an optical fiber for inputting light into the optical modulation element; an optical fiber for guiding the light output by the optical modulation element to the outside of the frame; and a driving circuit for driving the optical modulation element.

[0024] Another embodiment of the present invention is an optical transmission device including: the optical modulator or the optical modulation module; and an electronic circuit for generating an electrical signal for causing the optical modulation element to perform a modulation operation.

[0025] Another embodiment of the present invention is an optical transmission system, comprising: the optical transmitting device; and an optical fiber transmission path for transmitting the output light of the optical modulation element.

[0026] [Effects of the Invention]

[0027] According to the present invention, good optical characteristics can be achieved in an optical modulation element using a convex optical waveguide and segment electrodes as modulation electrodes. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a diagram showing the structure of an optical modulator using the optical modulation element according to the first embodiment of the present invention.

[0029] Figure 2 It is a plan view of the light modulation element according to the first embodiment.

[0030] Figure 3 yes Figure 2 A side view of the light modulation element is shown.

[0031] Figure 4 Yes Figure 2 The diagram shows the structure of the modulation part of the light modulation element.

[0032] Figure 5 (A) Figure 5 (B) is Figure 4 VV cross-sectional view of the modulation section shown.

[0033] Figure 6 (A) Figure 6 (B) is a cross-sectional view of the light modulation element according to the first modification of the first embodiment.

[0034] Figure 7 It is a side view of a light modulation element according to a second modification of the first embodiment.

[0035] Figure 8 It is a plan view of the light modulation element according to the second embodiment.

[0036] Figure 9 yes Figure 8 A side view of the light modulation element is shown.

[0037] Figure 10 It is a plan view of a light modulation element according to a modified example of the second embodiment.

[0038] Figure 11 It is a diagram showing the structure of an optical modulation module according to the third embodiment.

[0039] Figure 12 It is a diagram showing the configuration of an optical transmission device according to a fourth embodiment.

[0040] Figure 13 It is a diagram showing the configuration of an optical transmission system according to the fifth embodiment.

[0041] Figure 14 This is a plan view showing an example of a conventional light modulation element.

[0042] Figure 15 yes Figure 14 The figure shows a cross-sectional view taken along line XV-XV of a conventional light modulation element.

[0043] Figure 16 yes Figure 14 The XVI-XVI cross-sectional view of the conventional light modulation element is shown.

[0044] [Explanation of Reference Numerals]

[0045] 1a, 1b, 1c, 90: Light modulation element

[0046] 2: Optical modulator

[0047] 3: Frame

[0048] 4: Relay substrate

[0049] 5a, 5b: Signal pins

[0050] 6a: Input fiber

[0051] 6b: Output fiber

[0052] 7a, 7b: Support parts

[0053] 8a, 8b, 8c: Lenses

[0054] 9: Optical unit

[0055] 10: Terminal

[0056] 20: Substrate

[0057] 21a, 21b, 21c, 21d: Edge

[0058] 22: Optical waveguide layer

[0059] 23: Support layer

[0060] 231: First support layer

[0061] 232: Second support layer

[0062] 233: The third support layer

[0063] 24, 91: Optical substrate

[0064] 25, 94: Support substrate

[0065] 26: Optical waveguide

[0066] 27: Input waveguide

[0067] 28: Branched waveguide

[0068] 29a, 29b: Nested Mach-Zehnder waveguides

[0069] 30: Turnaround area

[0070] 31a, 31b: Output waveguide

[0071] 32, 32a, 32b, 32c, 32d, 92: Mach-Zehnder optical waveguide

[0072] 33a, 33b, 33c: Bias electrodes

[0073] 34, 34a, 34b, 34c, 34d: Modulation unit

[0074] 35: Wire bonding

[0075] 36, 36a1, 36a2, 92a, 92b: Arm waveguide

[0076] 40, 40a, 40b, 93a, 93b: Modulation electrodes

[0077] 40a1, 40b1, 93a1, 93b1: Thermoplastics

[0078] 40a2, 40b2, 93a2, 93b2: Ground electrodes

[0079] 41a, 96a: Heat transfer path

[0080] 41b, 41c, 96b, 96c: Ground transmission paths

[0081] 43: Light-absorbing material

[0082] 50: Optical modulation module

[0083] 51: Circuit board

[0084] 52: Driving circuit

[0085] 55: Optical transmitter

[0086] 56: Light Source

[0087] 57: Modulator driver

[0088] 58: Modulation signal generation unit

[0089] 60: Optical transmission system

[0090] 61: Fiber optic transmission path

[0091] 62: Optical receiving device DETAILED DESCRIPTION

[0092] The inventors of the present invention have diligently studied the deviation in the optical properties of a convex optical waveguide provided with a segmented electrode as a modulation electrode, and have come to the following conclusion: the factor causing this deviation is the interference of leakage light generated from the convex optical waveguide at the positions of the gap portions between the segments (parts of the electrode divided at certain intervals) constituting the segmented electrode.

[0093] Figure 14 、 Figure 15 、 Figure 16 This is an explanatory diagram for explaining factors causing the above-mentioned variation in optical characteristics in a conventional light modulator. Figure 14This is a plan view of an optical modulation element comprising a convex optical waveguide with segmented electrodes as modulation electrodes. Figure 15 yes Figure 14 The XV-XV cross-sectional arrow view of the light modulation element is shown. Figure 16 yes Figure 14 The XVI-XVI cross-sectional arrow view of the light modulation element is shown.

[0094] Reference Figure 14 、 Figure 15 、 Figure 16 As an example, a conventional optical modulation element 90 includes: a Mach-Zehnder optical waveguide 92 formed on one main surface (upper surface) of an optical substrate 91, which is an LN substrate having a thickness of several micrometers to several tens of micrometers, and is composed of a convex optical waveguide; and modulation electrodes 93a and 93b for controlling light waves propagating in each of the two arm waveguides 92a and 92b of the Mach-Zehnder optical waveguide 92. The other main surface (lower surface) of the optical substrate 91 is bonded to a support substrate 94 (see FIG. 1 ). Figure 15 、 Figure 16 The supporting substrate 94 is generally a glass plate having a lower refractive index than the optical substrate 91 .

[0095] The modulation electrode 93a includes a hot electrode 93a1 and a ground electrode 93a2 facing each other across the arm waveguide 92a in the main surface of the optical substrate 91. Similarly, the modulation electrode 93b includes a hot electrode 93b1 and a ground electrode 93b2 facing each other across the arm waveguide 92b in the main surface of the optical substrate 91.

[0096] The modulation electrode 93a and the modulation electrode 93b are each configured as a segmented electrode divided into multiple sections along the light propagation direction of the arm waveguides 92a and 92b. Specifically, the hot electrode 93a1 and the ground electrode 93a2 constituting the modulation electrode 93a are each divided into multiple sections (segments) of equal length along the light propagation direction of the arm waveguide 92a. Furthermore, the hot electrode 93b1 and the ground electrode 93b2 constituting the modulation electrode 93b are each divided into multiple sections of equal length along the light propagation direction of the arm waveguide 92b, with gaps between the segments arranged at regular intervals.

[0097] Furthermore, the segments of hot electrodes 93a1 and 93b1 are electrically connected to each other via heat transfer path 96a. Furthermore, the segments of ground electrode 93a2 are electrically connected to each other via ground transfer path 96b, and the segments of ground electrode 93b2 are electrically connected to each other via ground transfer path 96c. Thus, hot electrodes 93a1 and 93b1, which are connected to each other via heat transfer path 96a, ground electrode 93a2 connected to ground transfer path 96b, and ground electrode 93b2 connected to ground transfer path 96c, collectively constitute coplanar electrodes.

[0098] Moreover, if Figure 16 As shown in the XVI-XVI cross-sectional view, in the arm waveguide 92a, when a high-frequency signal is transmitted to the modulation electrode 93a, for example, in the portion where the segment of the hot electrode 93a1 and the segment of the ground electrode 93a2 face each other, by applying an electric field to the arm waveguide 92a, the refractive index changes (for example, increases) by Δn from the refractive index na of the optical substrate 91 (substrate refractive index na), and in the gap portion where the segments are not facing each other, the electric field is not applied to the arm waveguide 92a, so the refractive index remains the substrate refractive index na.

[0099] The unchanged portions of the refractive index generated at the gap portions of the segmented non-facing modulation electrodes 93a arranged along the arm waveguide 92a are refractive index discontinuities (disorder in the refractive index change) along the light propagation direction of the arm waveguide 92a, and become factors causing light leakage from the arm waveguide 92a.

[0100] Moreover, the leakage light generated from each of the unchanged parts of the refractive index arranged along the arm waveguide 92a leaks out to the supporting substrate 94 having a lower refractive index nb than the optical substrate 91, and is repeatedly reflected between the main surfaces of the supporting substrate 94, and propagates in the left and right directions shown in the figure while interfering with each other and reinforcing each other within the supporting substrate 94.

[0101] In particular, the segment electrodes serving as the modulation electrodes 93a are typically divided into hundreds to thousands of segments, and thus the number of gaps between the segments also ranges from hundreds to thousands. Consequently, the amount of light leakage generated from each of the equally spaced gaps between the segments in the arm waveguide 92a also ranges from hundreds to thousands. These leaked lights interfere with each other within the support substrate 94, reinforcing each other and generating a significant amount of light leakage within the support substrate 94.

[0102] The above phenomenon also occurs in the arm waveguide 92b where the modulation electrode 93b is formed. The leakage light from the arm waveguide 92b interferes with each other in the support substrate 94 and reinforces each other. Therefore, the leakage light of non-negligible intensity further increases in the support substrate 94.

[0103] Furthermore, a portion of this high-intensity leakage light generated by the interference may enter the arm waveguide 92a, the arm waveguide 92b, and other portions of the Mach-Zehnder optical waveguide 92, and couple with the signal light (or modulated light) propagating within the Mach-Zehnder optical waveguide 92. This leakage light coupled with the signal light (or modulated light) propagating within the Mach-Zehnder optical waveguide 92 is noise light, which degrades optical characteristics such as the extinction ratio of the optical modulation operation in the Mach-Zehnder optical waveguide 92, causing variations in the optical characteristics.

[0104] The present invention is based on the above-mentioned understanding of the factors causing optical characteristic deviation, and in particular suppresses the interference between leaked lights in a supporting substrate and the increase in the leakage light intensity caused by the interference, thereby reducing the deviation of optical characteristics in light modulation operation.

[0105] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0106] [1. First embodiment]

[0107] First, a first embodiment of the present invention will be described. Figure 1 This diagram shows the structure of an optical modulator 2 using the optical modulator 1a according to the first embodiment of the present invention. The optical modulator 2 includes the optical modulator 1a and a relay substrate 4 within a housing 3. The optical modulator 1a employs, for example, a dual polarization quadrature phase-shift keying (DP-QPSK) modulator. A cover (not shown) serving as a plate is secured to the opening of the housing 3, hermetically sealing the interior.

[0108] The optical modulator 2 also includes a signal pin 5a for inputting a high-frequency electrical signal for modulating the optical modulator 1a and a signal pin 5b for inputting an electrical signal for adjusting the operating point of the optical modulator 1a.

[0109] Furthermore, the optical modulator 2 includes an input optical fiber 6 a for inputting light into the housing 3 and an output optical fiber 6 b for guiding the light modulated by the optical modulation element 1 a to the outside of the housing 3 on the same surface of the housing 3 .

[0110] Here, the input optical fiber 6a and the output optical fiber 6b are fixed to the housing 3 via supports 7a and 7b, respectively, serving as fixing members. Light input from the input optical fiber 6a is collimated by a lens 8a disposed within the support 7a, and then input to the optical modulator 1a via lens 8b. However, this is merely an example, and light input to the optical modulator 1a can also be performed in accordance with conventional techniques, for example, by introducing the input optical fiber 6a into the housing 3 via the support 7a and connecting the end face of the introduced input optical fiber 6a to the end face of a substrate 20 (described later) of the optical modulator 1a.

[0111] The optical modulator 2 also includes an optical unit 9 that combines the polarizations of the two modulated lights output from the optical modulation element 1a. The combined polarization light output from the optical unit 9 is focused by a lens 8c disposed within the support 7b and coupled to the output optical fiber 6b.

[0112] Relay substrate 4 transmits high-frequency electrical signals input from signal pin 5a and electrical signals for operating point adjustment, etc., input from signal pin 5b, to optical modulator 1a via a conductive pattern (not shown) formed on relay substrate 4. The conductive pattern on relay substrate 4 is connected to the electrodes of optical modulator 1a, for example, by wire bonding. Furthermore, optical modulator 2 includes a terminator 10 having a predetermined impedance within housing 3.

[0113] Figure 2 1 is a plan view showing an example of the structure of the light modulator 1a. The light modulator 1a has a substrate 20 formed into multiple layers. The substrate 20 is, for example, rectangular in plan view and has Figure 2 The two sides 21a and 21b on the left and right sides of the figure extend in the vertical direction of the figure and face each other, and the upper and lower sides 21c and 21d on the left and right sides of the figure extend in the horizontal direction of the figure and face each other.

[0114] Figure 3 yes Figure 2The side view of the optical modulator 1a shown in FIG. The substrate 20 includes an optical waveguide layer 22 and a support layer 23. Furthermore, in this embodiment, the support layer 23 includes a first support layer 231 and a second support layer 232. In this embodiment, as an example, the substrate 20 is formed by stacking a plurality of plates. Specifically, the substrate 20 is formed by stacking an optical substrate 24 and a support substrate 25. The optical substrate 24 includes the optical waveguide layer 22, and the support substrate 25 includes the support layer 23 composed of a first support layer 231 and a second support layer 232. The optical substrate 24 is, for example, an X-cut LN substrate having an electro-optical effect that has been thinned to a thickness of less than 20 μm (e.g., 2 μm). Furthermore, the support substrate 25 is, for example, a glass substrate including a first support layer 231 and a second support layer 232 made of glass having different raw materials or compositions.

[0115] Furthermore, the substrate 20 does not necessarily need to be composed of multiple plates as described above. The substrate 20 may also be composed of a film formed in a layered form on a suitable substrate. For example, the substrate 20 may include the first support layer 231 and the optical waveguide layer 22 formed in a layered form by a film formation process such as sputtering, evaporation, and / or crystal growth on a suitable plate constituting the second support layer 232.

[0116] The light modulation element 1a includes an optical waveguide 26 ( Figure 2 The optical waveguide 26 is a convex optical waveguide (e.g., a rib-type optical waveguide or a ridge-type optical waveguide) composed of convex portions extending on the optical waveguide layer 22, and performs coherent multi-level modulation at, for example, over 100 GBaud.

[0117] Reference Figure 2 The optical waveguide 26 includes an input waveguide 27, which receives input light from the input optical fiber 6a (arrows pointing to the right in the figure) on the upper side of the left-hand side 21a of the optical waveguide layer 22, and a branching waveguide 28, which branches the input light into two beams of equal intensity. Furthermore, the optical waveguide 26 includes two modulation sections, so-called nested Mach-Zehnder optical waveguides 29a and 29b, which modulate the respective beams branched from the branching waveguide 28.

[0118] The nested Mach-Zehnder optical waveguides 29a and 29b cause the propagation direction of light to bend 180 degrees in the turning region 30 on the right side of the optical waveguide layer 22 as shown, and output the light from the edge 21a of the optical waveguide layer 22 to the left side as shown through the output waveguides 31a and 31b.

[0119] The nested Mach-Zehnder optical waveguides 29a and 29b each include two Mach-Zehnder optical waveguides 32a and 32b, and 32c and 32d, respectively, provided in two waveguide portions forming a pair of arm waveguides. Hereinafter, the Mach-Zehnder optical waveguides 32a, 32b, 32c, and 32d will be collectively referred to as Mach-Zehnder optical waveguides 32. Each Mach-Zehnder optical waveguide 32 includes two arm waveguides.

[0120] In the upper portion of the optical waveguide layer 22 shown in the figure, upstream of the return region 30 along the propagation direction of the light wave in the optical waveguide 26, a bias electrode 33a is formed for adjusting the operating points of the nested Mach-Zehnder optical waveguides 29a and 29b. Furthermore, bias electrodes 33b and 33c are provided on the Mach-Zehnder optical waveguides 32a, 32b, 32c, and 32d, respectively, for adjusting their respective operating points.

[0121] Furthermore, modulation sections 34a, 34b, 34c, and 34d, shown in the lower portion of the nested Mach-Zehnder optical waveguides 29a and 29b folded in the folded region 30, form modulation electrodes for performing modulation operations on each of the four Mach-Zehnder optical waveguides 32a, 32b, 32c, and 32d. Hereinafter, modulation sections 34a, 34b, 34c, and 34d will also be collectively referred to as modulation section 34.

[0122] The high-frequency electrical signals for performing modulation operations on each of the Mach-Zehnder optical waveguides 32 are input from the relay substrate 4 via the wire bonding 35 shown on the right side of the figure. These high-frequency electrical signals propagate through the modulation electrodes formed in each modulation section 34 and are terminated by a terminal resistor (not shown) provided in the terminator 10 shown below the figure.

[0123] To avoid making the diagram complex and easier to understand, Figure 2 The details of the electrodes formed in the modulation section 34a, modulation section 34b, modulation section 34c, and modulation section 34d are not described in detail. Figure 14 As in the conventional art shown, a segmented electrode formed by dividing the optical waveguide into a plurality of segments along the propagation direction of light serves as a modulation electrode.

[0124] As an example, in Figure 4 The structure of the modulation electrodes in the modulation unit 34a is shown in FIG. The modulation electrodes in the other modulation units 34b, 34c, and 34d are also shown in FIG. Figure 4 The same structure.

[0125] exist Figure 4 In FIG. 3 , the modulation electrode 40 a and the modulation electrode 40 b control light waves propagating through the arm waveguide 36 a 1 and the arm waveguide 36 a 2 of the Mach-Zehnder optical waveguide 32 a , respectively.

[0126] The modulation electrode 40a includes a hot electrode 40a1 and a ground electrode 40a2 facing each other across one arm waveguide 36a1 within the surface of the optical waveguide layer 22. Similarly, the modulation electrode 40b includes a hot electrode 40b1 and a ground electrode 40b2 facing each other across the other arm waveguide 36a2 within the surface of the optical waveguide layer 22.

[0127] The modulation electrode 40a and the modulation electrode 40b are each configured as segmented electrodes, divided into multiple sections along the light propagation direction of the arm waveguides 36a1 and 36a2. Specifically, the hot electrode 40a1 and the ground electrode 40a2 constituting the modulation electrode 40a are each divided into multiple sections (segments) of equal length along the light propagation direction of the arm waveguide 36a1, with the gaps between the segments spaced at regular intervals. Similarly, the hot electrode 40b1 and the ground electrode 40b2 constituting the modulation electrode 40b are each divided into multiple sections of equal length along the light propagation direction of the arm waveguide 36a2, with the gaps between the segments spaced at regular intervals. The number of segments in each of the hot electrodes 40a1 and 40b1 and the ground electrodes 40a2 and 40b2 can be, for example, in the thousands. However, the number of segments can be any number depending on the light modulation characteristics required of the light modulator 1a.

[0128] Furthermore, the segments of hot electrodes 40a1 and 40b1 are electrically connected to each other via heat transfer paths 41a. Furthermore, the segments of ground electrode 40a2 are electrically connected to each other via ground transfer paths 41b, and the segments of ground electrode 40b2 are electrically connected to each other via ground transfer paths 41c. Thus, hot electrodes 40a1 and 40b1, which are connected to each other via heat transfer paths 41a, ground electrode 40a2 connected to ground transfer paths 41b, and ground electrode 40b2 connected to ground transfer paths 41c, collectively constitute coplanar electrodes.

[0129] Figure 5 (A) Figure 5 (B) is Figure 4 The VV cross-sectional arrow diagram of the modulation section 34a along the arm waveguide 36a1 is shown. Figure 5 (A) Figure 5In (B), the lower section (B) shows the structure of the light modulation element 1a in the VV cross section, and the upper section (A) is a graph showing the change in the refractive index of the arm waveguide 36a1 along the light propagation direction in the VV cross section.

[0130] like Figure 5 As shown in (A), in the arm waveguide 36a1, Figure 16 Similarly, in the arm waveguide 92a of the conventional optical modulation element 90, when a high-frequency signal is transmitted to the modulation electrode 40a, an electric field is applied to the arm waveguide 36a1 in the portion where the segments of the hot electrode 40a1 and the segments of the ground electrode 40a2 face each other, causing the refractive index to change (e.g., increase) by Δn from the refractive index n0 of the optical waveguide layer 22 (i.e., the optical substrate 24) (the substrate refractive index n0). Furthermore, in the gap portion where the segments do not face each other, no electric field is applied to the arm waveguide 36a1, and the refractive index remains at the substrate refractive index n0.

[0131] The portions of the arm waveguide 36a1 where the refractive index remains unchanged (i.e., the portions where the refractive index does not change from the substrate refractive index n0) occur at the gaps between the segments of the modulation electrode 40a that do not face each other represent disturbances in the refractive index variation of the arm waveguide 36a1 along the light propagation direction. Furthermore, similar to the conventional optical modulation element 90, these portions of the refractive index disturbance can cause light leakage from the arm waveguide 36a1 formed in the optical waveguide layer 22.

[0132] However, in this embodiment, the support layer 23 included in the substrate 20 is composed of three layers, namely, a first support layer 231, a second support layer 232, and a third support layer 233, each having different refractive indices, so that the leaked light can spread into the interior of the substrate 20 without interfering with each other. Furthermore, the refractive index n0 of the optical waveguide layer 22 in which the optical waveguide 26 is formed, the refractive index n1 of the first support layer 231, the refractive index n2 of the second support layer 232, and the refractive index n3 of the third support layer 233 are related by the following formula (1).

[0133] n0>n1、n2>n1、and n2>n3 (1)

[0134] That is, below the optical waveguide layer 22 in which the arm waveguide 36 a 1 is formed, a second support layer 232 having a high refractive index exists with a first support layer 231 having a lower refractive index than the optical waveguide layer 22 interposed therebetween.

[0135] The second support layer 232 with a high refractive index is sandwiched between the first support layer 231 and the third support layer 233 with a low refractive index. Therefore, the second support layer 232 with a high refractive index has a light confinement effect between the first support layer 231 and the third support layer 233 with a low refractive index.

[0136] With this structure, leakage light generated from the arm waveguide 36a1 formed in the optical waveguide layer 22 easily passes through the low-refractive-index first support layer 231. A portion of this leakage light then becomes waveguide mode light in the second support layer 232, which has a light-confining effect, and can propagate within the second support layer 232. Conversion to this waveguide mode can occur randomly, for example, due to disturbances in the interface between the second support layer 232 and other supporting layers. Furthermore, the propagation within the second support layer 232 and the random conversion to the waveguide mode reduce the coherence of this "leakage light."

[0137] The "leakage light" converted into the waveguide mode of the second supporting layer 232 then reaches the end of the substrate 20, depending on the intensity of the light confinement effect in the second supporting layer 232, and can be emitted from the end of the substrate 20 to the outside. As a result, the "leakage light" can be guided out of the substrate 20 without interfering with each other or reinforcing each other. In other words, mutual reinforcement caused by interference between the "leakage lights" can be suppressed (or prevented).

[0138] The modulation electrode 40b of the arm waveguide 36a2 and the modulation electrode of the arm waveguide of the Mach-Zehnder optical waveguide 32 in the other modulation section 34 are also constructed in the same manner as the modulation electrode 40a of the arm waveguide 36a1. The leakage light generated in these arm waveguides is also similar to the above-mentioned. The presence of the second support layer 232 can suppress the mutual enhancement caused by the interference of the leakage light.

[0139] Hereinafter, the arm waveguides of each of the Mach-Zehnder optical waveguides 32, including the arm waveguide 36a1 and the arm waveguide 36a2 of the Mach-Zehnder optical waveguide 32a, are collectively referred to as the arm waveguide 36. Furthermore, the modulation electrodes provided in the arm waveguides 36 of each of the modulation sections 34, including the modulation electrodes 40a and 40b provided in the arm waveguide 36a1 and the arm waveguide 36a2 in the modulation section 34a, are collectively referred to as the modulation electrodes 40.

[0140] Through the above-mentioned action, mutual reinforcement caused by interference within the support layer 23 is suppressed with respect to the leakage light generated from the arm waveguide 36 formed in the optical waveguide layer 22. As a result, even if this leakage light reaches the optical waveguide layer 22 again and is combined with the signal light propagating in the optical waveguide 26, the influence of the leakage light on the optical characteristics of the optical modulator 1a is suppressed to be smaller than that of the conventional optical modulator 90.

[0141] Here, in order to effectively suppress interference between leaked lights generated in the arm waveguides 36 of the Mach-Zehnder optical waveguide 32 formed in the optical waveguide layer 22 through the above-described action, it is important to suppress the mutual interference of these leaked lights within the first supporting layer 231 before reaching the second supporting layer 232. Specifically, the interference between leaked lights within the first supporting layer 231 depends on the spacing L between the gaps arranged at regular intervals between the segments constituting the modulation electrode 40 and / or the thickness t1 of the first supporting layer 231. Here, the spacing L between the gaps refers to the distance between the centers of the gaps in the longitudinal direction, along the corresponding arm waveguide 36.

[0142] More specifically, in order to suppress interference between leaked light within the first support layer 231, the spacing L of the gap between the segments constituting the modulation electrode 40 preferably satisfies the following formula (2) with respect to the wavelength λ of the light wave propagating in the optical waveguide 26 and the refractive index n1 of the first support layer 231, and more preferably satisfies the formula (3).

[0143] L>4×λ / n1 (2)

[0144] L>10×λ / n1 (3)

[0145] In order to suppress interference between leaked lights in the first supporting layer 231 , the thickness t1 of the first supporting layer 231 preferably satisfies the following formula (4), and more preferably satisfies the following formula (5).

[0146] t1<10×λ / n1 (4)

[0147] t1<4×λ / n1 (5)

[0148] Furthermore, if the thickness t2 of the second supporting layer 232 is set too thick, it will significantly affect the line impedance of the modulation electrode 40. Therefore, it is preferably thinner than the thickness t0 of the optical waveguide 26 of the optical waveguide layer 22. In other words, the thickness t2 of the second supporting layer 232 preferably satisfies the relationship expressed by the following equation (6) relative to the thickness t0 of the optical waveguide 26 of the optical waveguide layer 22.

[0149] t2<t0 (6)

[0150] Furthermore, in order to make the second supporting layer 232 thin enough to satisfy equation (6) while ensuring light confinement in the second supporting layer 232 and actively guiding leaked light, the refractive index n2 of the second supporting layer 232 is preferably greater than the refractive index n0 of the optical waveguide layer 22. Specifically, the refractive index n2 of the second supporting layer 232 preferably satisfies the relationship given by equation (7) below relative to the refractive index n0 of the optical waveguide layer 22.

[0151] n2>n0 (7)

[0152] <First Modification>

[0153] As a first modification of the light modulator 1a, Figure 6 (A) Figure 6 As shown in (B), the substrate 20 can be constructed so that the light confinement effect in the second support layer 232 is weakened, and the leakage light from the arm waveguide 36 is converted into the waveguide mode of the second support layer 232 and propagates within the second support layer 232, then becomes a non-waveguide mode and leaks to other support layers. This structure is suitable for situations such as where a support layer with a large light confinement effect affects the high-frequency electric field. Here, Figure 6 (A) Figure 6 (B) shows the structure of the substrate 20 of the first embodiment. Figure 5 (A) Figure 5 (B) corresponds to a diagram showing the structure of a modified example of the substrate 20 .

[0154] In this case, the coherence of the "leakage light" that leaks from the second supporting layer 232 into the third supporting layer 233 or the first supporting layer 231 in a non-guided mode is also reduced as described above. Therefore, mutual reinforcement caused by interference between the leaked lights within the third supporting layer 233 or the first supporting layer 231 is less likely to occur. In other words, in this case, the presence of the second supporting layer 232 also suppresses mutual reinforcement caused by interference between the "leakage lights."

[0155] In such Figure 6 (A) Figure 6 When the substrate 20 is configured such that the light confinement effect in the second supporting layer 232 is weakened as shown in (B), that is, when the "leakage light" that has propagated through the second supporting layer 232 is configured to leak out again from the second supporting layer 232, it is preferable that the main direction of leakage of the propagated "leakage light" is not toward the first supporting layer close to the signal light, but toward the third supporting layer 233 that is further away. Therefore, the refractive index n1 of the first supporting layer 231, the refractive index n2 of the second supporting layer 232, and the refractive index n3 of the third supporting layer 233 preferably have the relationship expressed by the following formula (8).

[0156] (n2-n3)<(n2-n1) (8)

[0157] As a result, the "leakage light" that has propagated through the second supporting layer 232 primarily leaks toward the third supporting layer 233. This prevents the "leakage light" from leaking toward the first supporting layer 231 and optically coupling with the optical waveguide 26 of the optical waveguide layer 22. Consequently, this leakage light is prevented from adversely affecting the optical characteristics of the light modulation element 1a.

[0158] As a specific example, the wavelength λ of the light wave propagating through the optical waveguide 26 is 1.55 μm, and the spacing L between the segments forming the modulation electrode 40 is 50 μm to 100 μm. Furthermore, the thickness t0 of the optical waveguide layer 22 in the optical waveguide 26 portion is 1 μm to 2 μm, and the refractive index n0 of the optical waveguide layer 22 at the wavelength λ is 2.2. Furthermore, the first support layer 231 is composed, for example, of SiO2, has a thickness t1 of 3 μm, and a refractive index n1 of 1.48 at the wavelength λ. Furthermore, the second support layer 232 can be composed, for example, of a high-refractive-index material such as TiO2 or Ta2O5, or a semiconductor material such as Si or Ge. The thickness t2 of the second support layer 232 is, for example, 0.2 μm to 3 μm. The refractive index n2 of the second support layer 232 at the wavelength λ is 2.35 when the second support layer 232 is composed of TiO2, 2.1 when the second support layer 232 is composed of Ta2O5 or the like, 3.4 when the second support layer 232 is composed of Si, and 4.4 when the second support layer 232 is composed of Ge. Furthermore, the third support layer 233 is composed of glass, for example, and has a thickness t3 of 300 μm. The refractive index n3 at the wavelength λ is 1.55.

[0159] Furthermore, in this embodiment and the following embodiments, the interval L between adjacent segments of each modulation electrode 40 does not necessarily need to be constant throughout its entire section (i.e., its entirety). It suffices that the interval L between adjacent segments is constant throughout the entire section or except for a portion of the section of the modulation electrode 40. Similarly, the length of each segment of each modulation electrode 40 does not necessarily need to be the same throughout its entire section. It suffices that the length of each segment is constant throughout the entire section or except for a portion of the section of the modulation electrode 40. For example, if the modulation electrode 40 is divided into hundreds to thousands of segments, the segment length and / or the interval between adjacent segments in one or more sections of the modulation electrode 40 may differ from the segment length and / or the interval between adjacent segments in other sections.

[0160] <Second Modification>

[0161] While the entire substrate 20 is configured as a multilayer structure in the first embodiment, this does not necessarily have to be the case. For example, as long as the substrate 20 is configured as a multilayer structure at least below the modulation section 34 where the modulation electrodes 40 serving as segment electrodes are formed, the aforementioned interference suppression effect of light leakage can be achieved.

[0162] That is, the substrate 20 includes at least a multilayer portion configured as a plurality of layers, and the multilayer portion only needs to include the optical waveguide layer 22, the first supporting layer 231 in contact with the lower surface of the optical waveguide layer 22, the second supporting layer 232 in contact with the lower surface of the first supporting layer 231, and the third supporting layer 233 in contact with the lower surface of the second supporting layer 232.

[0163] For example, as a second variation, Figure 7 As shown, the substrate 20 can be configured as follows: in the layer structure observed from the side 21a, the substrate 20 as a whole has an optical waveguide layer 22, a first support layer 231 and a third support layer 233, and the second support layer 232 is only provided in the lower part of the modulation portion 34 where the modulation electrode 40 is formed.

[0164] [2. Second embodiment]

[0165] Next, a second embodiment of the present invention, light modulator 1b, will be described. Light modulator 1b has the same structure as light modulator 1a, but differs in that a light-absorbing material is disposed on the end surface of substrate 20. Light modulator 1b can be incorporated into light modulator 2 in place of light modulator 1a.

[0166] Figure 8 is a plan view of the light modulating element 1b and is Figure 2 The plan view of the light modulator 1a shown in FIG. Figure 9 is a side view of the light modulator 1b viewed from the side 21a, and is Figure 3 The figure is equivalent to the side view of the light modulator 1a as viewed from the side 21a. Figure 8 and Figure 9 In the Figure 2 and Figure 3 Same components as Figure 2 and Figure 3 The same symbols as in the above are used to represent Figure 2 and Figure 3 Description.

[0167] The optical modulator 1b has the same structure as the optical modulator 1a, but a light-absorbing material 43 is disposed on at least a portion of the end face of the substrate 20. This light-absorbing material 43 absorbs light in the wavelength band of the light waves propagating through the optical waveguide 26. For example, the portion where the light-absorbing material 43 is disposed can be the end face of the second supporting layer 232 of the substrate 20, where light leaking from the arm waveguide 36, which includes the modulation electrode 40 serving as a segmented electrode, can propagate. In this embodiment, the light-absorbing material 43 is particularly disposed on the end face portion of the second supporting layer 232 where this leaking light can reach. Specifically, the light-absorbing material 43 is disposed on the end face portion of the second supporting layer 232, which corresponds to the modulation section 34 and is located downstream of the end face portion along the propagation direction of the light waves propagating through the arm waveguide 36.

[0168] Thus, in the optical modulator 1b, leakage light generated from the arm waveguide 36 of the optical waveguide layer 22 and propagating through the second supporting layer 232 is absorbed by the light-absorbing material 43 and attenuated upon reaching the end face of the second supporting layer 232. Consequently, the intensity of the leakage light reflected at the end face of the second supporting layer 232 is reduced, and the effect of this leakage light on the optical characteristics of the optical modulator 1b is more effectively suppressed compared to the conventional optical modulator 90.

[0169] In addition, Figure 9 In the embodiment, the light absorbing material 43 is extended over the entire thickness direction of the substrate 20, but it only needs to be provided at least on the end surface portion of the second support layer 232. Figure 9 As shown, the light-absorbing material 43 is extended over substantially the entire thickness direction of the substrate 20, and the leakage light propagating in the first support layer 231 and the third support layer 233 can also be absorbed by the light-absorbing material 43, thereby more effectively suppressing the influence of the leakage light on the optical characteristics of the light modulator 1b.

[0170] The end surface of the substrate 20 where the light-absorbing material 43 is disposed has little influence on the electrical properties of the modulation electrode 40 or the waveguide properties of the optical waveguide 26 . Therefore, various materials including metal materials can be selected as the material of the light-absorbing material 43 .

[0171] For example, the light absorbing material 43 may be a carbon material such as carbon black, a black resin such as cashew oil, or a metal filler such as Ag. These light absorbing materials 43 may be applied to the end surface of the substrate 20 using, for example, a suitable resin as an adhesive and then cured, thereby being disposed on the end surface.

[0172] Modifications

[0173] As described above, the light absorbing material 43 is preferably provided on an end surface portion of the second supporting layer 232 that can propagate leaked light from the arm waveguide 36 and that is downstream of the modulation section 34 along the propagation direction of the light wave propagating in the arm waveguide 36 .

[0174] Therefore, for example, the arrangement position of the modulation part 34 in the optical waveguide layer 22 of the substrate 20 is as follows: Figure 10 In the case where it is located upstream of the return area 30 along the propagation direction of the light wave in the optical waveguide 26 as shown, the light-absorbing material 43 is preferably arranged on the end face portion of the second support layer 232 in the edge 21b of the modulation section 34 that is downstream along the propagation direction of the light wave propagating in the arm waveguide 36.

[0175] [3. Third embodiment]

[0176] Next, a third embodiment of the present invention will be described. This embodiment is an optical modulation module 50 using the optical modulation element 1a described in the first embodiment. Figure 11 FIG. 5 is a diagram showing the structure of the optical modulation module 50 of this embodiment. Figure 11 In the Figure 1 The same components as those of the light modulator 2 of the first embodiment shown in FIG. Figure 1 The same symbols as those shown in the Figure 1 Description.

[0177] The optical modulation module 50 has Figure 1 The optical modulator 2 shown has the same structure as the one shown, but differs in that it includes a circuit substrate 51 instead of the relay substrate 4. The circuit substrate 51 includes a drive circuit 52. The drive circuit 52 generates a high-frequency electrical signal for driving the optical modulator element 1a based on, for example, a modulation signal supplied from the outside via the signal pin 5a, and outputs the generated high-frequency electrical signal to the optical modulator element 1a.

[0178] The optical modulation module 50 having the above-mentioned structure includes an optical modulation element 1a in the same manner as the optical modulator 2 of the first embodiment. Therefore, as with the optical modulator 2, the influence of leakage light from the arm waveguide 36 provided with the modulation electrode 40 as a segmented electrode on the optical characteristics of the optical modulation element 1a can be reduced, thereby achieving good optical modulation operation.

[0179] In this embodiment, the light modulation module 50 includes the light modulation element 1 a as an example. However, the light modulation element 1 b according to the second embodiment or the light modulation element according to the modification of the first embodiment or the modification of the second embodiment may be included.

[0180] [4. Fourth embodiment]

[0181] Next, a fourth embodiment of the present invention will be described. This embodiment is an optical transmission device 55 equipped with the optical modulator 2 of the first embodiment. Figure 12 This diagram shows the structure of an optical transmitter 55 according to this embodiment. The optical transmitter 55 includes an optical modulator 2, a light source 56 for inputting light into the optical modulator 2, a modulator driver 57, and a modulation signal generator 58. Alternatively, the optical modulation module 50 according to the third embodiment may be used in place of the optical modulator 2 and the modulator driver 57. Furthermore, the optical modulator 2 may include the optical modulation element 1b according to the second embodiment or an optical modulation element according to a modified example of the first embodiment or a modified example of the second embodiment, in place of the optical modulation element 1a.

[0182] The modulation signal generating unit 58 is an electronic circuit that generates an electrical signal for causing the optical modulator 2 to perform a modulation operation. Based on the transmission data provided from the outside, it generates a high-frequency signal, i.e., a modulation signal, for causing the optical modulator 2 to perform an optical modulation operation according to the modulation data, and outputs it to the modulator driving unit 57.

[0183] The modulator driving unit 57 amplifies the modulation signal input from the modulation signal generating unit 58 and outputs four sets of high-frequency electrical signals. The four sets of high-frequency electrical signals are used to drive each of the modulation electrodes 40 arranged in the four Mach-Zehnder optical waveguides 32 of the optical modulation element 1a included in the optical modulator 2.

[0184] These high-frequency electrical signals are input to the signal pin 5a of the optical modulator 2 to drive the optical modulation element 1a. As a result, light output from the light source 56 is modulated by the optical modulator 2, for example, DP-QPSK, and output from the optical transmitter 55 as modulated light.

[0185] In the optical transmission device 55 , since the optical modulator 2 or the optical modulation module 50 including the optical modulation element 1 a , the optical modulation element 1 b , or the optical modulation element 1 c is used, good modulation characteristics can be achieved and good optical transmission can be performed.

[0186] [5. Fifth embodiment]

[0187] Next, a fifth embodiment of the present invention will be described. This embodiment is an optical transmission system 60 using the optical transmission device 55 of the fourth embodiment. Figure 13This diagram shows the structure of an optical transmission system 60 according to this embodiment. The optical transmission system 60 includes: an optical transmitter 55 according to the fourth embodiment; an optical fiber transmission path 61 for transmitting the modulated optical signal, which is the output light of the optical modulator 2 or the optical modulation module 50 included in the optical transmitter 55; and an optical receiver 62 for receiving the optical signal transmitted by the optical fiber transmission path 61. The optical transmission system 60 transmits optical signals using the optical transmitter 55, which uses the optical modulator 2 or the optical modulation module 50 including the optical modulation element 1a, the optical modulation element 1b, or their modified examples, and thus exhibits excellent optical transmission performance.

[0188] [6. Other Implementation Methods]

[0189] In the first to third embodiments, the optical waveguide layer 22 forming the optical waveguide 26 is included in the optical substrate 24, which is an LN substrate. However, the optical waveguide layer 22 does not necessarily need to be made of LN. The optical waveguide layer 22 may also be made of a semiconductor material such as InP.

[0190] In the above embodiment, the multi-layered substrate 20 is formed by laminating a plurality of plates. However, this is an example, and as described above, the substrate 20 may be formed by a film formed in a layer on an appropriate substrate.

[0191] In the above embodiment, the substrate 20 is formed by stacking an optical substrate 24, which is a plate body constituting the optical waveguide layer 22, and a support substrate 25, which is a plate body constituting the first support layer 231 and the second support layer 232. However, the optical substrate 24 and the support substrate 25 are merely examples of plates constituting the substrate 20, and the distribution of layers included in each of the multiple plates is arbitrary. In other words, when the substrate 20 is formed by stacking multiple plates, each plate body can include one of the multiple support layers, such as the optical waveguide layer 22 and the first support layer 231, or any number of multiple layers.

[0192] The present invention is not limited to the configurations of the above-described embodiments, and can be implemented in various forms without departing from the spirit and scope of the present invention.

[0193] [7. Structures Supported by the Above-mentioned Embodiments]

[0194] The above-described embodiment and modifications support the following configurations.

[0195] (Structure 1) An optical modulation element includes: a substrate including a multilayer portion; an optical waveguide formed of protrusions extending on an optical waveguide layer of the multilayer portion of the substrate; and a modulation electrode for controlling light waves propagating in the optical waveguide formed on the optical waveguide layer, the modulation electrode being formed by dividing the modulation electrode into a plurality of segments along the propagation direction of light in the optical waveguide. A spacing L between adjacent segments, measured in the direction in which the optical waveguide extends, is constant throughout or except for a portion of the modulation electrode. The multilayer portion of the substrate includes the optical waveguide layer, a first supporting layer in contact with a lower surface of the optical waveguide layer, a second supporting layer in contact with a lower surface of the first supporting layer, and a third supporting layer in contact with a lower surface of the second supporting layer. The refractive index n1 of the first supporting layer, the refractive index n2 of the second supporting layer, and the refractive index n3 of the third supporting layer satisfy the relationship n2>n1 and n2>n3.

[0196] The optical modulator element with Structure 1 guides light leakage from the optical waveguide, generated by gaps between segments of the segmented electrodes (i.e., the light modulation electrodes) formed by dividing the optical waveguide along the propagation direction of light, into the second supporting layer, thereby suppressing mutual enhancement due to interference. Consequently, the optical modulator element with Structure 1 can reduce the impact of this leakage light on the optical properties of the optical modulator element, achieving excellent optical characteristics.

[0197] (Structure 2) An optical modulator according to Structure 1, wherein the modulation electrode is divided into a plurality of segments of the same length, and the spacing L of the gaps between adjacent segments measured in the extension direction of the optical waveguide has a relationship of L>4×λ / n1 with respect to the wavelength λ of the light wave propagating in the optical waveguide and the refractive index n1 of the first supporting layer.

[0198] The light modulation element of structure 2 can further suppress the mutual enhancement caused by the interference between the leakage lights, thereby achieving better optical characteristics.

[0199] (Structure 3) The optical modulation element according to Structure 1 or 2, wherein the thickness t1 of the first supporting layer satisfies the relationship t1<10×λ / n1 with respect to the wavelength λ of the light wave propagating in the optical waveguide and the refractive index n1 of the first supporting layer.

[0200] The light modulation element of structure 3 can suppress mutual enhancement caused by interference between leaked lights in the first supporting layer, thereby achieving better optical characteristics.

[0201] (Structure 4) The optical modulation element according to any one of Structures 1 to 3, wherein the refractive index n2 and thickness t2 of the second supporting layer have a relationship of t2<t0 and n2>n0 with respect to the refractive index n0 and thickness t0 of the optical waveguide layer.

[0202] The light modulation element of Structure 4 can prevent the material used in the second supporting layer from affecting the electrical characteristics of the electrode formed on the optical waveguide layer, while effectively guiding the leaked light from the optical waveguide to the second supporting layer.

[0203] (Structure 5) The optical modulation element according to any one of Structures 1 to 4, wherein the refractive index n1 of the first supporting layer, the refractive index n2 of the second supporting layer, and the refractive index n3 of the third supporting layer have a relationship of (n2-n3)<(n2-n1).

[0204] The light modulation element of the structure 5 can guide the leakage light guided from the optical waveguide to the second supporting layer to the third supporting layer, thereby preventing the leakage light from being re-coupled with the optical waveguide.

[0205] (Structure 6) The optical modulation element according to any one of Structures 1 to 5, wherein a light absorbing material is provided on at least a portion of the end face of the substrate, and the light absorbing material absorbs light in a wavelength band of the light wave propagating in the optical waveguide.

[0206] The intensity of the light leakage reaching the end face of the substrate through the optical modulator of structure 6 can be reduced by the light-absorbing material arranged on the end face, thereby effectively reducing the influence of the light leakage on the optical characteristics of the optical modulator and achieving better optical characteristics.

[0207] (Structure 7) The light modulation element according to Structure 6, wherein the light absorbing material is a carbon material, a black resin, or a metal filler.

[0208] The light modulation element having Structure 7 can effectively reduce the intensity of the leaked light reaching the end face of the substrate, thereby achieving better optical characteristics as a light modulation element.

[0209] (Structure 8) An optical modulation element according to any one of Structures 1 to 6, wherein the substrate is formed by stacking a plurality of plates, each of which includes one or two adjacent layers of the optical waveguide layer, the first supporting layer, the second supporting layer, and the third supporting layer.

[0210] The light modulation element of Structure 8 can easily form a substrate including an optical waveguide layer for forming an optical waveguide and a plurality of supporting layers.

[0211] (Structure 9) An optical modulator comprises: an optical modulator according to any one of Structures 1 to 8; a frame that houses the optical modulator; an optical fiber that inputs light to the optical modulator; and an optical fiber that guides the light output by the optical modulator to the outside of the frame.

[0212] According to the light modulator of Structure 9, since the light modulation element of any one of Structures 1 to 8 is used, a light modulator having excellent optical characteristics can be realized.

[0213] (Structure 10) A light modulation module includes: the light modulation element according to any one of Structures 1 to 8; and a driving circuit that drives the light modulation element.

[0214] According to the light modulation module of Structure 10, since the light modulation element of any one of Structures 1 to 8 is used, a light modulation module having excellent optical characteristics can be realized.

[0215] (Configuration 11) An optical transmission device includes: 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 modulation element to perform a modulation operation.

[0216] According to the optical transmission device of Structure 11, since the optical modulator or optical modulation module using the optical modulation element of any one of Structures 1 to 8 is used, good optical transmission characteristics can be achieved.

[0217] (Structure 12) An optical transmission system includes: the optical transmitting device according to Structure 11; and an optical fiber transmission path that propagates the output light of the optical modulation element.

[0218] According to the optical transmission system of Structure 12, since the optical transmission device using the optical modulation element of any one of Structures 1 to 7 is used, good optical transmission characteristics can be achieved.

Claims

1. A light modulation element, characterized in that: include: a substrate including a multilayer portion configured as a plurality of layers; an optical waveguide composed of a protrusion extending on the optical waveguide layer of the multilayer portion of the substrate; as well as The modulation electrode is an electrode for controlling the light wave propagating in the optical waveguide formed on the optical waveguide layer, and is formed by dividing the modulation electrode into a plurality of segments along the propagation direction of light in the optical waveguide. In the entire section or except for a part of the section of the modulation electrode, the interval L between the adjacent segments measured in the extending direction of the optical waveguide is constant. The multilayer portion of the substrate includes the optical waveguide layer, a first supporting layer in contact with a lower surface of the optical waveguide layer, a second supporting layer in contact with a lower surface of the first supporting layer, and a third supporting layer in contact with a lower surface of the second supporting layer. The refractive index n1 of the first supporting layer, the refractive index n2 of the second supporting layer, and the refractive index n3 of the third supporting layer are The relationship of n2>n1 and n2>n3.

2. The light modulation element according to claim 1, wherein The modulation electrode is divided into a plurality of segments of the same length. The spacing L between the adjacent segments measured in the extending direction of the optical waveguide has a value of The relationship L>4×λ / n1.

3. The light modulation element according to claim 1, wherein The thickness t1 of the first supporting layer has a value of The relationship of t1<10×λ / n1.

4. The light modulation element according to claim 1, wherein The refractive index n2 and thickness t2 of the second supporting layer are relative to the refractive index n0 and thickness t0 of the optical waveguide layer. The relationship is t2<t0 and n2>n0.

5. The light modulation element according to claim 1, wherein The refractive index n1 of the first supporting layer, the refractive index n2 of the second supporting layer, and the refractive index n3 of the third supporting layer are The relationship is (n2-n3)<(n2-n1).

6. The light modulation element according to claim 1, wherein A light absorbing material is provided on at least a portion of the end surface of the substrate, and the light absorbing material absorbs light in a wavelength band of the light wave propagating through the optical waveguide.

7. The light modulation element according to claim 6, wherein The light absorbing material is carbon material, black resin or metal filler.

8. The light modulation element according to claim 1, wherein The substrate is formed by stacking multiple plates. The plate body includes one or two adjacent layers among the optical waveguide layer, the first supporting layer, the second supporting layer and the third supporting layer.

9. An optical modulator, characterized in that: include: The light modulation element according to claim 1; a frame housing the light modulation element; an optical fiber for inputting light to the light modulation element; as well as An optical fiber guides the light output from the light modulation element to the outside of the housing.

10. An optical modulation module, characterized in that: include: The light modulation element according to claim 1; as well as The driving circuit drives the light modulation element.

11. An optical transmitting device, characterized in that: include: The optical modulator according to claim 9 or the optical modulation module according to claim 10; as well as The electronic circuit generates an electrical signal for causing the light modulating element to perform a modulation operation.

12. An optical transmission system, characterized in that: include: The optical transmitting device according to claim 11; and an optical fiber transmission path for transmitting the output light of the optical modulation element.

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