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

Abstract

The utility model 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 utility model relates to a kind of light modulation element, light modulator, light modulation module, optical transmission device and optical transmission system. BACKGROUND

[0002] In high-speed / high-capacity optical fiber communication systems, optical modulators equipped with optical modulation elements as optical waveguide elements are often used. The optical modulation elements as optical waveguide elements are composed of optical waveguides formed on semiconductor substrates such as InP or substrates having electro-optic effects such as LiNbO3 (hereinafter also referred to as LN) and control electrodes that control optical waves propagating in the optical waveguides. Among them, optical modulation elements using LN substrates are widely used in high-speed / high-capacity optical fiber communication systems because they have low optical loss and can achieve wideband optical modulation characteristics.

[0003] In recent years, in order to achieve further low-voltage driving and high-speed modulation while miniaturizing the optical modulator itself, optical modulators using rib-type optical waveguides or ridge-type optical waveguides (hereinafter collectively referred to as convex optical waveguides) are being put into practical use. The rib-type optical waveguide or ridge-type optical waveguide is composed of a convex portion formed in a strip shape on the surface of an LN substrate (e.g., thickness 20 μm or less) that has been thinned (or thinned) in order to further enhance the interaction between the signal electric field in the substrate and the waveguide light.

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

[0005] [Related Art Documents]

[0006] [Patent Literature]

[0007] [Patent Literature 1] Japanese Patent No. 2022-148652

[0008] [Patent Literature 2] Japanese Patent No. 2016-194544

[0009] [Patent Literature 3] Japanese Patent No. 2020-181173 SUMMARY

[0010] [Problems to be Solved by the Invention]

[0011] The inventors of the present application have found that in a convex optical waveguide provided with a segmented electrode as a modulation electrode, there is a problem that even if a convex portion (i.e., a rib or a ridge) constituting the optical waveguide is formed with high accuracy in a wafer process, a deviation in optical characteristics such as modulation extinction ratio occurs. This problem has not been found for a long time and no factor or solution has been found.

[0012] The present application has an object to achieve good optical characteristics in an optical modulation element using a convex optical waveguide and a segmented electrode as a modulation electrode.

[0013] [Technical means to solve the problem]

[0014] One embodiment of the present application is an optical modulation element including: a substrate including a multilayer portion configured as a plurality of layers; an optical waveguide constituted by a convex portion extending on an optical waveguide layer of the multilayer portion of the substrate; and a modulation electrode that is an electrode for controlling an optical wave propagating in the optical waveguide formed on the optical waveguide layer and is segmented into a plurality of segments along a propagation direction of light of the optical waveguide, wherein a gap between adjacent segments in an entire region or a region other than a part of the modulation electrode is constant, and wherein the multilayer portion of the substrate includes the optical waveguide layer, a first support layer in contact with a lower surface of the optical waveguide layer, a second support layer in contact with a lower surface of the first support layer, and a third support layer in contact with a lower surface of the second support layer, and wherein a refractive index n1 of the first support layer, a refractive index n2 of the second support layer, and a refractive index n3 of the third support layer have a relationship of n2 > n1 and n2 > n3.

[0015] According to another embodiment of the present application, the modulation electrode is segmented into a plurality of segments of the same length, and a gap between adjacent segments measured in an extension direction of the optical waveguide has a relationship of L > 4 x λ / n1 with respect to a wavelength λ of an optical wave propagating in the optical waveguide and a refractive index n1 of the first support layer.

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

[0017] According to another embodiment of the present application, a refractive index n2 and a thickness t2 of the second support layer have a relationship of t2 < t0 and n2 > n0 with respect to a refractive index n0 and a thickness t0 of the optical waveguide layer.

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

[0019] According to another embodiment of the present application, 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 of a wavelength band of light propagating in the optical waveguide.

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

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

[0022] Another embodiment of the present application is an optical modulator including: the optical modulation element according to any one of the above embodiments; a housing that accommodates the optical modulation element; an optical fiber that inputs light to the optical modulation element; and an optical fiber that guides light output from the optical modulation element to the outside of the housing.

[0023] Another embodiment of the present application is an optical modulation module including: the optical modulation element according to any one of the above embodiments; a housing that accommodates the optical modulation element; an optical fiber that inputs light to the optical modulation element; an optical fiber that guides light output from the optical modulation element to the outside of the housing; and a drive circuit that drives the optical modulation element.

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

[0025] Another embodiment of the present application is an optical transmission system including: the optical transmission device; and an optical fiber transmission path that transmits the output light of the optical modulation element.

[0026] [Effects of the Invention]

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

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

[0029] Figure 2This is a plan view of the optical modulation element in the first embodiment.

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

[0031] Figure 4 It means Figure 2 A diagram showing the structure of the modulation section of the optical modulation element.

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

[0033] Figure 6 (A) Figure 6 (B) is a cross-sectional view of the optical modulation element of the first variant of the first embodiment.

[0034] Figure 7 This is a side view of the optical modulation element of the second variation of the first embodiment.

[0035] Figure 8 This is a plan view of the optical modulation element in the second embodiment.

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

[0037] Figure 10 This is a plan view of the optical modulation element of a modified example of the second embodiment.

[0038] Figure 11 This is a diagram showing the structure of the optical modulation module in the third embodiment.

[0039] Figure 12 This is a diagram showing the structure of the optical transmitting device according to the fourth embodiment.

[0040] Figure 13 This is a diagram showing the structure of the optical transmission system according to the fifth embodiment.

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

[0042] Figure 15 yes Figure 14 The XV-XV cross-section of an existing optical modulation element is shown.

[0043] Figure 16 yes Figure 14 The XVI-XVI cross-section of an existing optical modulation element is shown.

[0044] [Explanation of reference numerals in the attached figures]

[0045] 1a, 1b, 1c, 90: light modulating element

[0046] 2: light modulator

[0047] 3: frame

[0048] 4: relay substrate

[0049] 5a, 5b: signal pin

[0050] 6a: input optical fiber

[0051] 6b: output optical fiber

[0052] 7a, 7b: support

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

[0054] 9: optical unit

[0055] 10: terminator

[0056] 20: substrate

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

[0058] 22: optical waveguide layer

[0059] 23: support layer

[0060] 231: first support layer

[0061] 232: second support layer

[0062] 233: third support layer

[0063] 24, 91: optical substrate

[0064] 25, 94: support substrate

[0065] 26: optical waveguide

[0066] 27: input waveguide

[0067] 28: branch waveguide

[0068] 29a, 29b: nested Mach-Zehnder type optical waveguide

[0069] 30: turnaround region

[0070] 31a, 31b: output waveguide

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

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

[0073] 34, 34a, 34b, 34c, 34d: modulation sections

[0074] 35: wire bonding

[0075] 36, 36a1, 36a2, 92a, 92b: arm waveguides

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

[0077] 40a1, 40b1, 93a1, 93b1: hot electrodes

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

[0079] 41a, 96a: heat transport paths

[0080] 41b, 41c, 96b, 96c: ground transport paths

[0081] 43: light absorbing material

[0082] 50: optical modulation module

[0083] 51: circuit substrate

[0084] 52: drive circuit

[0085] 55: optical transmission device

[0086] 56: light source

[0087] 57: modulator drive section

[0088] 58: modulation signal generation section

[0089] 60: optical transmission system

[0090] 61: optical fiber transmission path

[0091] 62: optical receiving device DETAILED DESCRIPTION

[0092] The inventors of the present application made earnest studies on the optical characteristic deviation of a convex light waveguide provided with segmented electrodes as modulation electrodes, and obtained the following insight: a factor of this deviation is interference of leaked light generated from the convex light waveguide at positions of each gap portion between segments (each portion of the electrode divided at a certain interval) constituting the segmented electrodes.

[0093] Figure 14 、 Figure 15 、 Figure 16is an explanatory view for explaining a factor of the optical characteristic deviation in the existing optical modulation element. Figure 14 is a plan view of an optical modulation element constituted by a convex optical waveguide in which a segmented electrode is provided as a modulation electrode, Figure 15 is Figure 14 is an XV-XV cross-sectional arrow view of the optical modulation element shown in Figure 16 is Figure 14 is an XVI-XVI cross-sectional arrow view of the optical modulation element shown in

[0094] Referring to Figure 14 , Figure 15 , Figure 16 , the existing optical modulation element 90 shown as an example has a Mach-Zehnder type 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 constituted by a convex optical waveguide, and modulation electrodes 93a and 93b which control optical waves propagating in each of two arm waveguides 92a and 92b of the Mach-Zehnder type optical waveguide 92. The other main surface (lower surface) of the optical substrate 91 is joined to a support substrate 94 (refer to Figure 15 , Figure 16 ). The support substrate 94 is generally a glass plate having a lower refractive index than the optical substrate 91.

[0095] The modulation electrode 93a has a hot electrode 93al and a ground electrode 93a2 facing each other with the arm waveguide 92a sandwiched in the main surface of the optical substrate 91. Similarly, the modulation electrode 93b has a hot electrode 93bl and a ground electrode 93b2 facing each other with the arm waveguide 92b sandwiched in the main surface of the optical substrate 91.

[0096] The modulation electrode 93a and the modulation electrode 93b are each constituted by a segmented electrode divided into a plurality of portions along the optical propagation direction of the arm waveguide 92a and the arm waveguide 92b. Specifically, the hot electrode 93al and the ground electrode 93a2 constituting the modulation electrode 93a are each divided into a plurality of portions (segments) of the same length along the optical propagation direction of the arm waveguide 92a. In addition, the hot electrode 93bl and the ground electrode 93b2 constituting the modulation electrode 93b are each divided into a plurality of segments of the same length along the optical propagation direction of the arm waveguide 92b, and arranged at a certain interval with gaps between the segments.

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

[0098] Moreover, such as 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 section where the hot electrode 93a1 and the ground electrode 93a2 face each other, by applying an electric field to the arm waveguide 92a, the refractive index changes (e.g., increases) Δn from the refractive index na (substrate refractive index na) of the optical substrate 91. In the gap section where the sections do not face each other, no electric field is applied to the arm waveguide 92a, so the refractive index remains at the substrate refractive index na.

[0099] The unchanging portions of refractive index generated at the non-opposing gaps in the segmented sections of the modulation electrodes 93a arranged along the arm waveguide 92a are discontinuities in the refractive index of the arm waveguide 92a along the direction of light propagation (disorders in refractive index change), and become factors that cause light leakage from the arm waveguide 92a.

[0100] Furthermore, the light leakage generated from each of the portions of the unchanging refractive index arranged along the arm waveguide 92a leaks out to the support substrate 94, which has a refractive index nb lower than that of the optical substrate 91, and is repeatedly reflected between the main surfaces of the support substrate 94, propagating in the left-right direction as shown in the figure while interfering with each other and reinforcing each other within the support substrate 94.

[0101] In particular, the segmented electrodes 93a used for modulation are generally divided into hundreds to thousands of segments, and therefore the number of gaps between the segments is also hundreds to thousands. As a result, in the arm waveguide 92a, the number of leaked lights generated from the equally spaced gaps between the segments is also hundreds to thousands. These leaked lights interfere with each other and reinforce each other within the support substrate 94, thus generating a leaked light intensity that cannot be ignored within the support substrate 94.

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

[0103] Moreover, a part of the high-intensity light leakage due to the interference can enter a part of the arm waveguide 92a, the arm waveguide 92b, and the other Mach-Zehnder optical waveguide 92, and couple with the signal light (or modulated light) propagating in the Mach-Zehnder optical waveguide 92. The light leakage coupled with the signal light (or modulated light) propagating in the Mach-Zehnder optical waveguide 92 is noise light, which degrades the extinction ratio and other optical characteristics of the light modulation action in the Mach-Zehnder optical waveguide 92, and causes deviation of the optical characteristics.

[0104] The present utility model is based on the above-mentioned insight into the factors of optical characteristic deviation, particularly suppresses the interference of light leakage in the support substrate with each other, suppresses the increase of light leakage intensity caused by the interference, thereby reducing the deviation of optical characteristics in the light modulation action.

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

[0106] [1. First Embodiment]

[0107] First, the first embodiment of the present utility model will be described. Figure 1 is a diagram showing the structure of the optical modulator 2 using the optical modulation element 1a of the first embodiment of the present utility model. The optical modulator 2 has the optical modulation element 1a and the relay substrate 4 inside the frame 3. The optical modulation element 1a is, for example, a Dual Polarization Quadrature Phase-Shift Keying (DP-QPSK) modulator structure. The frame 3 is finally fixed with a lid (not shown) as a plate body at the opening portion thereof, so that the inside thereof is hermetically sealed.

[0108] In addition, the optical modulator 2 has a signal pin 5a for inputting a high-frequency electric signal for modulation of the optical modulation element 1a, and a signal pin 5b for inputting an electric signal for operation point adjustment and the like of the optical modulation element 1a.

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

[0110] Here, the input optical fiber 6a and the output optical fiber 6b are fixed to the frame 3 via the support 7a and the support 7b as fixing members, respectively. The light input from the input optical fiber 6a is collimated by the lens 8a provided in the support 7a, and is input to the light modulating element la via the lens 8b. However, this is an example, and the input of the light to the light modulating element la can be performed in accordance with the related art, for example, by introducing the input optical fiber 6a into the frame 3 via the support 7a, and connecting the end surface of the introduced input optical fiber 6a to the end surface of the substrate 20 (described later) of the light modulating element la.

[0111] Further, the optical modulator 2 has an optical unit 9 that performs polarization wave synthesis on the two modulated lights output from the light modulating element la. The polarization wave synthesized light output from the optical unit 9 is condensed by the lens 8c provided in the support 7b, and is coupled to the output optical fiber 6b.

[0112] The relay substrate 4 transmits the high frequency electric signal input from the signal pin 5a and the electric signal for the purpose of adjustment of the operating point, etc. input from the signal pin 5b to the light modulating element la, by a conductor pattern (not shown) formed in the relay substrate 4. The conductor pattern on the relay substrate 4 is connected to the electrodes of the light modulating element la, respectively, for example, by wire bonding or the like. Further, the optical modulator 2 includes a terminator 10 having a prescribed impedance in the frame 3.

[0113] Figure 2 is a plan view showing an example of the structure of the light modulating element la. The light modulating element la has a substrate 20 configured as a multilayer. The substrate 20 is, for example, rectangular in plan view, and has two edges 21a, 21b extending in the upper and lower directions of the drawing and facing each other, and two edges 21c, 21d extending in the left and right directions of the drawing and facing each other. Figure 2

[0114] Figure 3 is a plan view showing an example of the structure of the light modulating element la. The light modulating element la has a substrate 20 configured as a multilayer. The substrate 20 is, for example, rectangular in plan view, and has two edges 21a, 21b extending in the upper and lower directions of the drawing and facing each other, and two edges 21c, 21d extending in the left and right directions of the drawing and facing each other. Figure 2 ​A side view of the light modulating element 1a is shown. The substrate 20 includes a light waveguide layer 22 and a support layer 23. In addition, in the present embodiment, the support layer 23 includes a first support layer 231 and a second support layer 232. In the present 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 light waveguide layer 22, and the support substrate 25 includes the support layer 23 composed of the first support layer 231 and the second support layer 232. The optical substrate 24 is, for example, an LN substrate having an X-cut that is thinned to a thickness of 20 μm or less (for example, 2 μm) and has an electro-optic effect. In addition, the support substrate 25 is, for example, a glass substrate including the first support layer 231 and the second support layer 232 composed of glass having different raw materials or compositions.

[0115] Furthermore, the substrate 20 is not necessarily composed of a plurality of plates as described above. The substrate 20 can be configured to be composed of a film formed in a layered manner on a suitable substrate. For example, the substrate 20 can be configured to include the first support layer 231 and the light waveguide layer 22 formed in a layered manner on a suitable plate constituting the second support layer 232 by a film forming process such as sputtering, evaporation, and / or crystal growth.

[0116] The light modulating element 1a has a light waveguide 26 formed on the light waveguide layer 22 of the substrate 20 (on the optical substrate 24 in the present embodiment) (the entire portion shown by the thick dashed line). Figure 2 The light waveguide 26 is a convex light waveguide (for example, a rib-type light waveguide or a ridge-type light waveguide) composed of a convex portion extending on the light waveguide layer 22, and performs, for example, coherent multi-value modulation such as over 100 GBaud.

[0117] Referring to Figure 2 , the light waveguide 26 includes an input waveguide 27 that receives input light from the input optical fiber 6a (arrow toward the right in the drawing) on the upper side of the edge 21a on the left in the drawing of the light waveguide layer 22, and a branch waveguide 28 that branches the input light into two lights having the same light amount. In addition, the light waveguide 26 includes two modulation sections, namely, so-called nested Mach-Zehnder type light waveguides 29a and 29b, that modulate each light branched by the branch waveguide 28.

[0118] The nested Mach-Zehnder type light waveguide 29a and the nested Mach-Zehnder type light waveguide 29b make the propagation direction of light turn back by 180 degrees in a turn-back region 30 on the right in the drawing of the light waveguide layer 22, and output the light from the edge 21a of the light waveguide layer 22 toward the left in the drawing through an output waveguide 31a and an output waveguide 31b.

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

[0120] In the illustrated upper portion of the optical waveguide layer 22, a bias electrode 33a for adjusting an operating point of the nested Mach-Zehnder optical waveguide 29a and the nested Mach-Zehnder optical waveguide 29b is formed at a position upstream of the folding region 30 along a propagation direction of the optical wave 26. Further, the Mach-Zehnder optical waveguide 32a, the Mach-Zehnder optical waveguide 32b, the Mach-Zehnder optical waveguide 32c, and the Mach-Zehnder optical waveguide 32d are provided with bias electrodes 33b and 33c for adjusting operating points thereof respectively.

[0121] Further, in the illustrated lower portion of the nested Mach-Zehnder optical waveguide 29a and the nested Mach-Zehnder optical waveguide 29b in which the optical wave is folded at the folding region 30, modulation sections 34a, 34b, 34c, and 34d are shown. The modulation sections 34a, 34b, 34c, and 34d are provided with modulation electrodes for performing a modulation operation on each of the four Mach-Zehnder optical waveguides 32a, 32b, 32c, and 32d. Hereinafter, the modulation sections 34a, 34b, 34c, and 34d will also be collectively referred to as the modulation section 34.

[0122] High-frequency electric signals for performing a modulation operation on each of the Mach-Zehnder optical waveguides 32 are input from the relay substrate 4 via wire bonds 35 shown on the right side. These high-frequency electric signals propagate in the modulation electrodes formed in each of the modulation sections 34 and are terminated by a termination resistor (not shown) provided in the terminator 10 shown on the lower side.

[0123] In order to avoid complication of the drawing and to make it easy to understand, the detailed configuration of the electrodes formed in the modulation sections 34a, 34b, 34c, and 34d is not described in Figure 2 In each of the modulation sections 34, a segmented electrode divided into a plurality of segments along a propagation direction of the optical wave of the optical waveguide is formed as the modulation electrode, as in the prior art shown in Figure 14

[0124] As an example, the configuration of the modulation electrode in the modulation section 34a is shown in Figure 4 The modulation electrodes of the other modulation sections 34b, 34c, and 34d also have the same configuration as that of the modulation electrode in the modulation section 34a.​Figure 4 It is constructed in the same way.

[0125] exist Figure 4 In the process, modulation electrode 40a and modulation electrode 40b control the light waves propagating in the arm waveguides 36a1 and 36a2 of the Mach-Zehnder type optical waveguide 32a, respectively.

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

[0127] The modulation electrodes 40a and 40b are respectively configured as segmented electrodes divided into multiple parts 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 parts (segments) of the same length along the light propagation direction of the arm waveguide 36a1, with the gaps between the segments arranged at a certain interval. Similarly, the hot electrode 40b1 and the ground electrode 40b2 constituting the modulation electrode 40b are each divided into multiple segments of the same length along the light propagation direction of the arm waveguide 36a2, with the gaps between the segments arranged at a certain interval. The number of segments for each of the hot electrode 40a1, hot electrode 40b1, and ground electrode 40a2, ground electrode 40b2 is, for example, on the order of several thousand. However, the number of segments can be arbitrary depending on the required light modulation characteristics of the optical modulation element 1a.

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

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

[0130] As shown in (A), in the arm waveguide 36al, when a high-frequency signal is transmitted to the modulation electrode 40a, the refractive index is varied (e.g., increased) by Δn from the refractive index n0 of the optical waveguide layer 22 (i.e., the optical substrate 24) (substrate refractive index n0) by applying an electric field to the arm waveguide 36al at the portion where the segments of the hot electrode 40al and the segments of the ground electrode 40a2 face each other. Figure 5 Figure 16 As shown in (A), in the arm waveguide 36al, when a high-frequency signal is transmitted to the modulation electrode 40a, the refractive index is varied (e.g., increased) by Δn from the refractive index n0 of the optical waveguide layer 22 (i.e., the optical substrate 24) (substrate refractive index n0) by applying an electric field to the arm waveguide 36al at the portion where the segments of the hot electrode 40al and the segments of the ground electrode 40a2 face each other.

[0131] The portions where no variation in the refractive index occurs (i.e., the portions where the refractive index is not varied from the substrate refractive index n0) in the arm waveguide 36al at the positions of the gaps where the segments of the modulation electrode 40a do not face each other in the arm waveguide 36al are disturbances in the refractive index variation of the arm waveguide 36al in the light propagation direction. Also, in such disturbances in the refractive index, light leakage can occur from the arm waveguide 36al formed in the optical waveguide layer 22, as in the case of the prior-art optical modulation element 90.

[0132] However, in the present embodiment, in particular, the support layer 23 included in the substrate 20 is composed of three layers of the first support layer 231, the second support layer 232, and the third support layer 233 having different refractive indexes, so that the light leakage is spread into the substrate 20 without interfering with each other. Also, the refractive index n0 of the optical waveguide layer 22 in which the optical waveguide 26 is formed and the refractive indexes n1 of the first support layer 231, n2 of the second support layer 232, and n3 of the third support layer 233 have the following relation of Expression (1).

[0133] n0 > n1, n2 > n1, and n2 > n3 (1)

[0134] That is, under the optical waveguide layer 22 in which the arm waveguide 36al is formed, the second support layer 232 having a higher refractive index than the optical waveguide layer 22 is present with the first support layer 231 having a lower refractive index than the optical waveguide layer 22 interposed therebetween.

[0135] Also, the second support layer 232 having a higher refractive index is interposed between the first support layer 231 having a lower refractive index and the third support layer 233. Therefore, the second support layer 232 having a higher refractive index becomes a layer having a light confinement effect between the first support layer 231 and the third support layer 233 having lower refractive indexes.

[0136] ​By the above structure, the leakage light generated from the arm waveguide 36al formed in the optical waveguide layer 22 easily passes through the first support layer 231 having a low refractive index, and a part of these leakage lights becomes waveguide mode light of the second support layer 232 having a light confinement effect and can propagate in the second support layer 232. The conversion to the waveguide mode can occur randomly, for example, due to a disorder of the boundary surface between the second support layer 232 and other support layers, and the like. Further, by the propagation within the second support layer 232 and by the random conversion to the waveguide mode, the coherence of the "leakage light" is reduced.

[0137] Thereafter, the "leakage light" converted to the waveguide mode of the second support layer 232 reaches the end portion of the substrate 20 in dependence on the intensity degree of the light confinement effect in the second support layer 232, and can be emitted to the outside from the end portion of the substrate 20, for example. Thereby, the "leakage light" can be guided to the outside of the substrate 20 without interfering with each other and without being enhanced by each other. That is, the mutual enhancement caused by the interference between the "leakage light" can be suppressed (or prevented).

[0138] The modulation electrodes 40b of the arm waveguides of the Mach-Zehnder type optical waveguide 32 in the other modulation sections 34 and the modulation electrodes of the arm waveguides of the Mach-Zehnder type optical waveguide 32 are also similarly constituted as the modulation electrode 40a of the arm waveguide 36al, and the leakage light generated in these arm waveguides is also similarly suppressed from being mutually enhanced by the interference with each other by the presence of the second support layer 232.

[0139] Hereinafter, the arm waveguides 36al, 36a2 of the Mach-Zehnder type optical waveguide 32a and the arm waveguides of each of the Mach-Zehnder type optical waveguides 32 will be collectively referred to as the arm waveguides 36. Further, the modulation electrodes 40a, 40b provided to the arm waveguides 36al, 36a2 in the modulation section 34a and the modulation electrodes provided to the arm waveguides 36 in each of the modulation sections 34 will be collectively referred to as the modulation electrodes 40.

[0140] By the above action, for the leakage light generated from the arm waveguides 36 formed in the optical waveguide layer 22, the mutual enhancement by the interference in the support layer 23 is suppressed, and as a result thereof, even in the case where these leakage lights reach the optical waveguide layer 22 again and are combined with the signal light propagating in the optical waveguide 26, the influence of the leakage light on the optical characteristics of the optical modulating element la is suppressed to be smaller than that of the conventional optical modulating element 90.

[0141] Here, in order to effectively suppress the interference between the leak lights generated in the arm waveguides 36 of the Mach-Zehnder type optical waveguide 32 formed in the optical waveguide layer 22 by the above-described action, it is important to suppress the mutual interference of these leak lights within the first support layer 231 before reaching the second support layer 232. The interference between the leak lights within the first support layer 231 depends on, in particular, the interval L of the gaps arranged at a certain interval between the respective segments constituting the modulation electrode 40 from each other and / or the thickness tl of the first support layer 231. Here, the interval L of the gaps from each other refers to the distance between the centers in the length direction of the respective gaps along the corresponding arm waveguide 36.

[0142] More specifically, in order to suppress the interference between the leak lights within the first support layer 231, the interval L of the gaps of the segments constituting the modulation electrode 40 is preferably such that the following expression (2) is satisfied with respect to the wavelength λ of the optical wave propagating in the optical waveguide 26 and the refractive index nl of the first support layer 231, and more preferably such that expression (3) is satisfied.

[0143] L > 4 x λ / nl (2)

[0144] L > 10 x λ / nl (3)

[0145] In addition, in order to suppress the interference between the leak lights within the first support layer 231, the thickness tl of the first support layer 231 is preferably such that the following expression (4) is satisfied, and more preferably such that expression (5) is satisfied.

[0146] tl < 10 x λ / nl (4)

[0147] tl < 4 x λ / nl (5)

[0148] In addition, the thickness t2 of the second support layer 232 is preferably thinner than the thickness to of the optical waveguide 26 of the optical waveguide layer 22 if it is set too thick, since it greatly affects the line impedance of the modulation electrode 40. That is, the thickness t2 of the second support layer 232 is preferably in the following expression (6) with respect to the thickness to of the optical waveguide 26 of the optical waveguide layer 22.

[0149] t2 < to (6)

[0150] In addition, in order to make the second support layer 232 thin to satisfy expression (6) while actively guiding the leak lights by light confinement in the second support layer 232, the refractive index n2 of the second support layer 232 is preferably greater than the refractive index no of the optical waveguide layer 22. That is, the refractive index n2 of the second support layer 232 is preferably in the following expression (7) with respect to the refractive index no of the optical waveguide layer 22.

[0151] n2 > no (7)

[0152] <First Modification>

[0153] As a first modification of the optical modulation element 1a, as shown in Figure 6 (A) of FIG. 16A, Figure 6 (B) of FIG. 16B, the substrate 20 can be configured so that the light confinement effect in the second support layer 232 is weakened, and the light leakage from the arm waveguide 36 is converted into a waveguide mode of the second support layer 232 and propagates within the second support layer 232, and then becomes a non-waveguide mode and leaks out to the other support layers. This structure is suitable for cases where a support layer with a large light confinement effect affects a high-frequency electric field or the like. Here, Figure 6 (A) of FIG. 17A, Figure 6 (B) of FIG. 17B are diagrams showing the structure of the modified example of the substrate 20, corresponding to Figure 5 (A) of FIG. 15A, Figure 5 (B) of FIG. 15B showing the structure of the substrate 20 of the first embodiment.

[0154] In this case, the coherence of the "leaked light" that becomes a non-waveguide mode and leaks out from the second support layer 232 to the third support layer 233 or the first support layer 231 is also reduced as described above, and thus mutual enhancement due to interference between the leaked light inside the third support layer 233 or the first support layer 231 is less likely to occur. That is, in this case, by the presence of the second support layer 232, mutual enhancement due to interference between the "leaked light" is also suppressed.

[0155] In the case where the substrate 20 is configured so that the light confinement effect in the second support layer 232 is weakened, as shown in Figure 6 (A) of FIG. 18A, Figure 6 (B) of FIG. 18B, that is, in the case where the "leaked light" that has propagated within the second support layer 232 leaks out from the second support layer 232 again, it is preferable that the main direction in which the "leaked light" that has propagated within the second support layer 232 leaks out be the direction of the third support layer 233 that is farther than the first support layer 231 that is close to the signal light. Therefore, 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 preferably have the following relation of Expression (8).

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

[0157] Thus, the "leaked light" that has propagated within the second support layer 232 mainly leaks out in the direction of the third support layer 233, and thus the "leaked light" is less likely to leak out in the direction of the first support layer 231 and optically couple with the optical waveguide 26 of the optical waveguide layer 22. As a result, the "leaked light" is less likely to adversely affect the optical characteristics of the optical modulation element 1a.

[0158] As a specific example, for example, the wavelength λ of the optical wave propagating in the optical waveguide 26 is 1.55 μm, and the interval L of the gap between the segments constituting the modulation electrode 40 is 50 μm or more and 100 μm or less. In addition, the thickness t0 of the portion of the optical waveguide 26 of the optical waveguide layer 22 is 1 μm or more and 2 μm or less, and the refractive index n0 of the optical waveguide layer 22 at the wavelength λ is 2.2. In addition, the first support layer 231 is constituted of, for example, SiO2, and has a thickness t1 of 3 μm and a refractive index n1 at the wavelength λ of 1.48. In addition, the second support layer 232 can be constituted of, for example, 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 or more and 3 μm or less. The refractive index n2 of the second support layer 232 at the wavelength λ is, in the case where the second support layer 232 is constituted of TiO2, 2.35, in the case where the second support layer 232 is constituted of Ta2O5 or the like, 2.1, in the case where the second support layer 232 is constituted of Si, 3.4, and in the case where the second support layer 232 is constituted of Ge, 4.4. In addition, the third support layer 233 is constituted of, for example, glass, and has a thickness t3 of 300 μm and a refractive index n3 at the wavelength λ of 1.55.

[0159] Further, in the present embodiment and each of the embodiments shown below, the interval L of the gap between the segments of each of the modulation electrodes 40 is not necessarily constant in the entire range (i.e., the entirety) thereof, as long as the interval L of the gap between the segments is constant in the entire range or in a range other than a part of the modulation electrode 40. Similarly, the length of each segment of each of the modulation electrodes 40 is not necessarily constant in the entire range thereof, as long as the length of each segment is constant in the entire range or in a range other than a part of the modulation electrode 40. For example, in the case where the modulation electrode 40 is divided into several hundreds to several thousands of segments, the length of the segments and / or the interval L of the gap between the segments can be different in one range or a plurality of ranges in the modulation electrode 40 from the length of the segments and / or the interval L of the gap between the segments in other ranges.

[0160] <Second Modification Example>

[0161] The substrate 20 is constituted of a plurality of layers in the first embodiment, but is not necessarily constituted of a plurality of layers as a whole. For example, the substrate 20 can function or have an effect of suppressing interference of light leakage as long as it is constituted of a plurality of layers at least in the lower portion of the modulation portion 34 in which the modulation electrode 40 serving as a segmented electrode is formed.

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

[0163] For example, as a second modification example, as shown in FIG. 6, the substrate 20 can be configured such that, in a layer structure viewed from the edge 21a, the substrate 20 has the optical waveguide layer 22, the first support layer 231, and the third support layer 233 in the entire substrate 20, and has the second support layer 232 only in a lower portion of the modulation portion 34 in which the modulation electrode 40 is formed. Figure 7

[0164] [2. Second Embodiment]

[0165] Next, the optical modulation element 1b according to the second embodiment of the present application will be described. The optical modulation element 1b has the same configuration as the optical modulation element 1a, but differs from the optical modulation element 1a in that the light-absorbing material is provided to the end surface of the substrate 20. The optical modulation element 1b can be used instead of the optical modulation element 1a by being mounted in the optical modulator 2.

[0166] Figure 8 is a plan view of the optical modulation element 1b, and is a view corresponding to the plan view of the optical modulation element 1a shown in FIG. 2. In addition, in Figure 2 Figure 9 is a side view of the optical modulation element 1b viewed from the edge 21a, and is a view corresponding to the side view of the optical modulation element 1a viewed from the edge 21a shown in FIG. 3. Furthermore, in Figure 3 Figure 8 Figure 9 , the same components as those in Figure 2 Figure 3 are denoted by the same symbols as those in Figure 2 Figure 3 , and the above description of Figure 2 Figure 3 is applied.

[0167] ​​​​​​​The light modulating element 1b has the same structure as the light modulating element 1a, but the end surface of the substrate 20 is provided with a light absorbing material 43 at least in a portion thereof, the light absorbing material 43 absorbing light of a wavelength band of the light wave propagating in the optical waveguide 26. The portion provided with the light absorbing material 43 can be, for example, a portion of the end surface of the second support layer 232 in the end surface of the substrate 20, in which light leakage from the arm waveguide 36 provided with the modulation electrode 40 as a segmented electrode can propagate. In the present embodiment, the light absorbing material 43 is particularly provided in the end surface portion of the second support layer 232 to which the light leakage can reach. Specifically, the light absorbing material 43 is provided in the end surface of the second support layer 232 corresponding to the portion downstream of the modulation section 34 in the propagation direction of the light wave propagating in the arm waveguide 36.

[0168] Thus, in the light modulating element 1b, the light leakage generated from the arm waveguide 36 of the optical waveguide layer 22 and propagating in the second support layer 232 is attenuated by being absorbed by the light absorbing material 43 when reaching the end surface of the second support layer 232. Therefore, the intensity of the light leakage reflected at the end surface among the light leakage propagating in the second support layer 232 is weakened, and the influence of these light leakages on the optical characteristics of the light modulating element 1b is more effectively suppressed compared to the conventional light modulating element 90.

[0169] Further, in the present embodiment, the light absorbing material 43 is provided so as to extend substantially entirely in the thickness direction of the substrate 20, but it is sufficient to provide the light absorbing material 43 at least in the end surface portion of the second support layer 232. However, by providing the light absorbing material 43 so as to extend substantially entirely in the thickness direction of the substrate 20 as shown in FIG. 4B, the light leakage propagating in the first support layer 231 and the third support layer 233 can also be absorbed by the light absorbing material 43, and thus the influence of these light leakages on the optical characteristics of the light modulating element 1b can be more effectively suppressed. Figure 9 Figure 9 Further, in the present embodiment, the light absorbing material 43 is provided so as to extend substantially entirely in the thickness direction of the substrate 20, but it is sufficient to provide the light absorbing material 43 at least in the end surface portion of the second support layer 232. However, by providing the light absorbing material 43 so as to extend substantially entirely in the thickness direction of the substrate 20 as shown in FIG. 4B, the light leakage propagating in the first support layer 231 and the third support layer 233 can also be absorbed by the light absorbing material 43, and thus the influence of these light leakages on the optical characteristics of the light modulating element 1b can be more effectively suppressed.

[0170] The end surface of the substrate 20 provided with the light absorbing material 43 is less affected by the material provided in this portion on the electrical characteristics of the modulation electrode 40 or the waveguide characteristics of the optical waveguide 26 in terms of position, and thus 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 can 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 can be applied to the end surface of the substrate 20 using an appropriate resin as a binder and hardened, for example, to be provided in the end surface.

[0172]

[0173] ​​As in the above-described configuration, the light-absorbing material 43 is preferably provided in the end face portion of the second support layer 232 in the edge 21b located downstream of the light wave propagating in the arm waveguide 36, in the case where the configuration position of the modulation section 34 in the optical waveguide layer 22 of the substrate 20 is located further upstream than the turnaround region 30 along the propagation direction of the light wave in the optical waveguide 26, for example.

[0174] Therefore, for example, in the case where the configuration position of the modulation section 34 in the optical waveguide layer 22 of the substrate 20 is located further upstream than the turnaround region 30 along the propagation direction of the light wave in the optical waveguide 26, as shown in Figure 10 the light-absorbing material 43 is preferably provided in the end face portion of the second support layer 232 in the edge 21b located downstream of the light wave propagating in the arm waveguide 36.

[0175] [3. Third Embodiment]

[0176] Next, the third embodiment of the present application will be described. This embodiment is a light modulation module 50 using the optical modulation element 1a shown in the first embodiment. Figure 11 is a view showing the configuration of the light modulation module 50 of this embodiment. In Figure 11 , the same components as those of the optical modulator 2 of the first embodiment shown in Figure 1 will be denoted by the same symbols as those shown in Figure 1 , and the above description of Figure 1 will be applied.

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

[0178] The light modulation module 50 having the above-described configuration, like the optical modulator 2 of the first embodiment, includes the optical modulation element 1a, and therefore, like the optical modulator 2, it is possible to reduce the influence of the light leakage 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, thereby achieving a good light modulation operation.

[0179] Further, in this embodiment, the light modulation module 50 is provided as an example to include the optical modulation element 1a, but it can also be provided to include the optical modulation element 1b of the second embodiment or the optical modulation element of the modified example of the first embodiment or the modified example of the second embodiment.

[0180] [4. Fourth Embodiment]

[0181] Next, the fourth embodiment of the present application will be described. This embodiment is an optical transmission device 55 in which the optical modulator 2 of the first embodiment is mounted. Figure 12 is a view showing the configuration of the optical transmission device 55 of this embodiment. The optical transmission device 55 has the optical modulator 2, a light source 56 that emits light to the optical modulator 2, a modulator drive section 57, and a modulation signal generation section 58. In addition, the optical modulator module 50 of the third embodiment can be used instead of the optical modulator 2 and the modulator drive section 57. Further, the optical modulator 2 can include the optical modulation element 1b of the second embodiment or the optical modulation element of the modified example of the first embodiment or the modified example of the second embodiment instead of the optical modulation element 1a.

[0182] The modulation signal generation section 58 is an electronic circuit that generates an electric signal for causing the optical modulator 2 to perform a modulation operation, and generates a high-frequency signal, i.e., a modulation signal, for causing the optical modulator 2 to perform an optical modulation operation in accordance with the modulation data, based on transmission data supplied from the outside, and outputs the modulation signal to the modulator drive section 57.

[0183] The modulator drive section 57 amplifies the modulation signal input from the modulation signal generation section 58 and outputs four sets of high-frequency electric signals for driving each of the modulation electrodes 40 of the optical modulation element 1a included in the optical modulator 2, which are provided in the four Mach-Zehnder type optical waveguides 32.

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

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

[0186] [5. Fifth Embodiment]

[0187] Next, the fifth embodiment of the present application will be described. This embodiment is an optical transmission system 60 that uses the optical transmission device 55 of the fourth embodiment. Figure 13is a view showing the configuration of an optical transmission system 60 according to the present embodiment. The optical transmission system 60 has the optical transmitting apparatus 55 according to the fourth embodiment, an optical fiber transmission path 61 that transmits the output light, i.e., the modulated optical signal, of the optical modulator 2 or the optical modulation module 50 included in the optical transmitting apparatus 55, and an optical receiving apparatus 62 that receives the optical signal transmitted by the optical fiber transmission path 61. The optical transmission system 60 transmits the optical signal by using the optical transmitting apparatus 55 of the optical modulator 2 or the optical modulation module 50 that includes the optical modulation element 1a, the optical modulation element 1b, or their modified examples, and thus has good optical transmission performance.

[0188] [6. Other Embodiments]

[0189] As for the optical waveguide layer 22 that forms the optical waveguide 26, it is provided in the optical substrate 24 that is an LN substrate in the first to third embodiments, but it can not necessarily be composed of LN. The optical waveguide layer 22 can also be composed of a semiconductor material such as InP.

[0190] In the present embodiment, the substrate 20 configured in multiple layers is provided so as to be configured by stacking a plurality of plate bodies. However, this is an example, and as described above, the substrate 20 can also be configured by a film body formed in layers on a suitable substrate.

[0191] In the present embodiment, the substrate 20 is provided so as to have the optical substrate 24 that is a plate body configuring the optical waveguide layer 22 and the support substrate 25 that is a plate body configuring the first support layer 231 and the second support layer 232 stacked. However, the optical substrate 24 and the support substrate 25 are examples of the plate bodies configuring the substrate 20, and the allocation of the layers included in each of the plurality of plate bodies is arbitrary. That is, in the case where the substrate 20 is configured by stacking a plurality of plate bodies, each of the plate bodies can include one or an arbitrary number of layers of the optical waveguide layer 22 and the plurality of support layers such as the first support layer 231.

[0192] Further, the present application is not limited to the configuration of the present embodiment, and can be implemented in various manners without departing from the gist thereof.

[0193] [7. Configuration Supported by the Above Embodiments]

[0194] The above embodiments and modified examples support the following configurations.

[0195] A light modulating element according to Structure 1, wherein the modulation electrode is divided into a plurality of segments of the same length and formed, and a gap between the adjacent segments measured in the extending direction of the optical waveguide has a constant interval L, in the entire section or a section other than a part of the modulation electrode, and the first support layer has a thickness t1, and the thickness t1 has a relationship of t1 < 10 x 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 support layer.

[0196] With the light modulating element of Structure 1, the gap between the segments of the segment electrode, i.e., the light modulating electrode, formed by being divided into a plurality along the propagating direction of the light of the optical waveguide, can guide the light leakage from the optical waveguide to the second support layer, and thus the mutual enhancement caused by the interference can be suppressed. Thus, in the light modulating element of Structure 1, the influence of these light leakages on the optical characteristics of the light modulating element can be reduced, and good optical characteristics can be achieved.

[0197] A light modulating element according to Structure 1, wherein the modulation electrode is divided into a plurality of segments of the same length and formed, and a gap between the adjacent segments measured in the extending direction of the optical waveguide has a constant interval L, in the entire section or a section other than a part of the modulation electrode, and the first support layer has a thickness t1, and the thickness t1 has a relationship of t1 < 10 x 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 support layer.

[0198] With the light modulating element of Structure 2, the mutual enhancement caused by the interference between the light leakages can be further suppressed, and thus better optical characteristics can be achieved.

[0199] A light modulating element according to Structure 1 or 2, wherein the first support layer has a thickness t1, and the thickness t1 has a relationship of t1 < 10 x 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 support layer.

[0200] With the light modulating element of Structure 3, the mutual enhancement caused by the interference between the light leakages in the first support layer can be suppressed, and thus better optical characteristics can be achieved.

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

[0202] With the optical modulation element of Structure 4, it is possible to prevent the material used in the second support layer from affecting the electrical characteristics of the electrode formed in the optical waveguide layer, while effectively guiding the light leakage from the optical waveguide to the second support 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 support layer and the refractive index n2 of the second support layer and the refractive index n3 of the third support layer have a relationship of (n2 - n3) < (n2 - n1).

[0204] With the optical modulation element of Structure 5, it is possible to guide the light leakage from the optical waveguide to the second support layer to the third support layer, thereby suppressing the case where the light leakage is coupled again to the optical waveguide.

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

[0206] With the optical modulation element of Structure 6, for the intensity of the light leakage that has reached the end surface of the substrate, it is possible to reduce the intensity by the light-absorbing material provided at the end surface, thereby effectively reducing the influence of the light leakage on the optical characteristics of the optical modulation element, and achieving better optical characteristics.

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

[0208] With the optical modulation element of Structure 7, it is possible to effectively reduce the intensity of the light leakage that has reached the end surface of the substrate, thereby achieving better optical characteristics as an optical modulation element.

[0209] (Structure 8) The optical modulation element according to any one of Structures 1 to 6, wherein the substrate is a laminate of a plurality of plate bodies each including one or two adjacent ones of the optical waveguide layer, the first support layer, the second support layer, and the third support layer.

[0210] With the optical modulation element of Structure 8, it is possible to easily configure a substrate including the optical waveguide layer for forming an optical waveguide and a plurality of support layers.

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

[0212] With the optical modulator of Structure 9, since the optical modulation element of any one of Structures 1 to 8 is used, an optical modulator having good optical characteristics can be realized.

[0213] (Structure 10) An optical modulation module including: the optical modulation element according to any one of Structures 1 to 8; and a drive circuit that drives the optical modulation element.

[0214] With the optical modulation module of Structure 10, since the optical modulation element of any one of Structures 1 to 8 is used, an optical modulation module having good optical characteristics can be realized.

[0215] (Structure 11) An optical transmission apparatus including: the optical modulator according to Structure 9 or the optical modulation module according to Structure 10; and an electronic circuit that generates an electric signal for causing the optical modulation element to perform a modulation action.

[0216] With the optical transmission apparatus of Structure 11, since the optical modulator or the optical modulation module that utilizes the optical modulation element of any one of Structures 1 to 8 is used, good optical transmission characteristics can be realized.

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

[0218] With the optical transmission system of Structure 12, since the optical transmission apparatus that utilizes the optical modulation element of any one of Structures 1 to 7 is used, good optical transmission characteristics can be realized.

Claims

1. An optical modulation element, characterized by comprising: including: a substrate including a multilayer portion configured as a plurality of layers; an optical waveguide configured by a protrusion extending on an optical waveguide layer of the multilayer portion of the substrate; and a modulation electrode that controls an optical wave propagating in the optical waveguide formed on the optical waveguide layer, and is formed by being divided into a plurality of segments along a propagation direction of light of the optical waveguide, a gap between the adjacent segments is constant with respect to a direction in which the optical waveguide extends, the multilayer portion of the substrate includes the optical waveguide layer, a first support layer that is in contact with a lower surface of the optical waveguide layer, a second support layer that is in contact with a lower surface of the first support layer, and a third support layer that is in contact with a lower surface of the second support layer, a refractive index n1 of the first support layer, a refractive index n2 of the second support layer, and a refractive index n3 of the third support layer have a relationship of n2 > n1 and n2 > n3.

2. The optical modulation element according to claim 1, wherein the modulation electrode is formed by being divided into a plurality of segments having the same length, a gap of a gap between the adjacent segments measured in the direction in which the optical waveguide extends has a relationship of L > 4 x λ / n1 with respect to a wavelength λ of the optical wave propagating in the optical waveguide and a refractive index n1 of the first support layer.

3. The optical modulation element according to claim 1, wherein a thickness t1 of the first support layer has a relationship of t1 < 10 x λ / n1 with respect to a wavelength λ of the optical wave propagating in the optical waveguide and a refractive index n1 of the first support layer.

4. The optical modulation element according to claim 1, wherein a refractive index n2 of the second support layer and a thickness t2 have a relationship of t2 < t0 and n2 > n0 with respect to a refractive index n0 of the optical waveguide layer and a thickness t0.

5. The optical modulation element according to claim 1, wherein the refractive index n1 of the first support layer, the refractive index n2 of the second support layer, and the refractive index n3 of the third support layer have a relationship of (n2 - n3) < (n2 - n1).

6. The optical modulation element according to claim 1, wherein at least a part of an end surface of the substrate is provided with a light-absorbing material that absorbs light of a wavelength band of the optical wave propagating in the optical waveguide.

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

8. The optical modulation element according to claim 1, wherein the substrate is a laminate of a plurality of plate bodies, the plate bodies respectively include one or two layers of the optical waveguide layer, the first support layer, the second support layer, and the third support layer. including: the optical modulation element according to claim 1; a frame that houses the optical modulation element; 9. An optical modulator, comprising: an optical fiber that inputs light to the optical modulation element; and an optical fiber that guides light output from the optical modulation element to the outside of the frame. including: the optical modulation element according to claim 1; and ​ 10. An optical modulation module, characterized by comprising: ​ ​ ​ A drive circuit drives the light modulating element.

11. An optical transmitting device, characterized in that, Comprising: The optical modulator as claimed in claim 9 or the optical modulation module as claimed in claim 10; And An electronic circuit generates an electric signal for causing the light modulating element to perform a modulation action.

12. An optical transmission system, characterized by, Comprising: The optical transmission device as claimed in claim 11; And an optical fiber transmission path propagates the output light of the light modulating element.

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