Light modulation element, light modulator, light modulation module, light transmission device, and light transmission system

Abstract

To provide a light modulation element including a convex light guide and a segment electrode serving as a modulation electrode, in which the excellent optical characteristics are achieved.SOLUTION: A light modulation element includes a substrate including a multilayer part formed to have multiple layers, a light guide layer in the multilayer part where a light guide is formed, and a modulation electrode to control the light wave propagating in the light guide and divided into a plurality of segments along a light propagation direction of the light guide. In all the sections or some sections of the electrodes, distances L of the gaps between the adjacent segments measured along the extending direction of the light guide is constant. The multilayer part of the substrate includes the light guide layer, a first support layer disposed in contact with a lower surface of the light guide layer, a second support layer disposed in contact with a lower surface of the first support layer. A refractive index n0 of the light guide layer, a refractive index n1 of a first support layer, and a refractive index n2 of a second support layer satisfy the relations of n0>n1 and n2>n1.SELECTED DRAWING: Figure 5

Description

[Technical Field]

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

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

[0003] In recent years, in order to achieve even lower voltage operation and higher speed modulation while miniaturizing the optical modulator itself, optical modulators that use rib-type optical waveguides or ridge-type optical waveguides (hereinafter collectively referred to as convex optical waveguides) that are constructed by forming strip-shaped convex portions on the surface of a thin-film (or thin-plate) LN substrate (for example, a thickness of 20 μm or less) are being put into practical use in order to further strengthen the interaction between the signal electric field and the guided light in the substrate.

[0004] In recent years, it has been proposed to use a so-called segment electrode, which is a coplanar type modulation electrode divided into multiple segments along the light propagation direction of the optical waveguide, to achieve impedance matching between the modulation electrode and the drive circuit and velocity matching between the propagation velocity of high frequency waves in the modulation electrode and the light propagation velocity in the optical waveguide (Patent Documents 1, 2, and 3). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2022-148652 A JP 2016-194544 A JP 2020-181173 A Summary of the Invention [Problem to be solved by the invention]

[0006] The inventors of the present invention have discovered that a convex optical waveguide provided with segment electrodes as modulation electrodes has a problem in that optical characteristics such as the modulation extinction ratio vary even if the convex portions (i.e., ribs or ridges) that make up the optical waveguide are formed with high precision in a wafer process. The cause of this problem and a solution to it had not been discovered for a long time.

[0007] An object of the present invention is to realize good optical characteristics in an optical modulation element that uses a convex optical waveguide and segment electrodes as modulation electrodes. [Means for solving the problem]

[0008] One aspect of the present invention is an optical modulation element including: a substrate including a multilayer portion configured into multiple layers; an optical waveguide layer in the multilayer portion of the substrate, in which an optical waveguide is formed; and a modulating electrode formed on the optical waveguide layer, the modulating electrode being an electrode for controlling light waves propagating through the optical waveguide, the electrode being divided into a plurality of segments along the propagation direction of light in the optical waveguide, wherein a distance L measured in the extension direction of the optical waveguide between gaps between adjacent segments is constant in all sections of the electrode or in sections excluding a portion thereof; 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, and a second support layer in contact with a lower surface of the first support layer; and a refractive index n0 of the optical waveguide layer, a refractive index n1 of the first support layer, and a refractive index n2 of the second support layer satisfy the relationship n0>n1 and n2>n1. According to another aspect of the present invention, the distance L satisfies the relationship L>4×λ / n1, where λ is the wavelength of the light wave propagating through the optical waveguide and n1 is the refractive index of the first support layer. According to another aspect of the present invention, the thickness t1 of the first support layer satisfies the relationship t1<10×λ / n1, where λ is the wavelength of the light wave propagating through the optical waveguide and n1 is the refractive index of the first support layer. According to another aspect of the present invention, the multilayer portion of the substrate further includes a third support layer in contact with the lower surface of the second support layer, and the refractive index n0 of the optical waveguide layer, 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 the relationships n0>n1 and n3>n2>n1. According to another aspect of the present invention, a light absorbing material that absorbs light in the wavelength range of the light waves propagating through the optical waveguide is disposed on the rear surface of the substrate, opposite to the front surface of the optical waveguide layer. According to another aspect of the present invention, the light absorbing material is a carbon material, a black resin, or a metal filler. According to another aspect of the present invention, the substrate is formed by stacking a plurality of plates, each of which includes one or any number of the optical waveguide layer and a plurality of support layers including the first support layer and the second support layer. Another aspect of the present invention is an optical modulator comprising any one of the optical modulation elements described above, a housing that houses the optical modulation element, an optical fiber that inputs light to the optical modulation element, and an optical fiber that guides the light output by the optical modulation element to the outside of the housing. Another aspect of the present invention is an optical modulation module comprising any one of the optical modulation elements described above, a housing that houses the optical modulation element, an optical fiber that inputs light to the optical modulation element, an optical fiber that guides light output by the optical modulation element to the outside of the housing, and a drive circuit that drives the optical modulation element. Another aspect of the present invention is an optical transmission device comprising 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. Another aspect of the present invention is an optical transmission system including the optical transmitter and an optical fiber transmission line that transmits output light from the optical modulation element. [Effects of the Invention]

[0009] According to the present invention, it is possible to realize good optical characteristics in an optical modulation element that uses a convex optical waveguide and segment electrodes as modulation electrodes. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing the configuration of an optical modulator using an optical modulation element according to a first embodiment of the present invention. [Figure 2] FIG. 1 is a plan view of a light modulation element according to a first embodiment. [Figure 3] FIG. 3 is a side view of the light modulation element shown in FIG. [Figure 4] 3 is a diagram showing the configuration of a modulation section of the light modulation element shown in FIG. 2. FIG. [Figure 5] 5 is a VV cross-sectional view of the modulation section shown in FIG. [Figure 6] FIG. 10 is a side view of the light modulation element according to the second embodiment. [Figure 7] FIG. 10 is a cross-sectional view of a modulation section of an optical modulation element according to a second embodiment. [Figure 8] FIG. 10 is a side view of the light modulation element according to the third embodiment. [Figure 9] FIG. 10 is a cross-sectional view of a modulation section of an optical modulation element according to a third embodiment. [Figure 10] FIG. 10 is a diagram illustrating a configuration of an optical modulation module according to a fourth embodiment. [Figure 11] FIG. 10 is a diagram illustrating a configuration of an optical transmitting device according to a fifth embodiment. [Figure 12] FIG. 10 is a diagram illustrating a configuration of an optical transmission system according to a sixth embodiment. [Figure 13] FIG. 1 is a plan view showing an example of a conventional light modulation element. [Figure 14] 14 is a cross-sectional view taken along the line XIV-XIV of the conventional optical modulation element shown in FIG. [Figure 15] 15 is a cross-sectional view taken along the line XV-XV of the conventional optical modulation element shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0011] The inventors of the present invention have conducted extensive research into the variations in the optical characteristics of a convex optical waveguide provided with segment electrodes as modulation electrodes, and have discovered that the cause of these variations is interference of leaked light from the convex optical waveguide at the positions of the gaps between the segments (each part of the electrode divided at regular intervals) that make up the segment electrode.

[0012] Figures 13, 14, and 15 are explanatory diagrams for explaining the causes of the variations in the optical characteristics in conventional optical modulation elements. Figure 13 is a plan view of an optical modulation element configured with a convex optical waveguide provided with segment electrodes as modulation electrodes, and Figure 14 is a cross-sectional view taken along the line XIV-XIV of the optical modulation element shown in Figure 13. Figure 15 is a cross-sectional view taken along the line XV-XV of the optical modulation element shown in Figure 13.

[0013] 13, 14, and 15, a conventional optical modulation element 90 shown as an example has a Mach-Zehnder optical waveguide 92 formed on one main surface (top surface) of an optical substrate 91, which is an LN substrate having a thickness of several μm to several tens of μm, and configured as a convex optical waveguide, and modulation electrodes 93a and 93b for controlling light waves propagating through two arm waveguides 92a and 92b of the Mach-Zehnder optical waveguide 92. The other main surface (bottom surface) of the optical substrate 91 is bonded to a support substrate 94 (see FIGS. 14 and 15). The support substrate 94 is generally made of a material having a lower refractive index than the optical substrate 91, such as a glass plate.

[0014] The modulating electrode 93a has a hot electrode 93a1 and a ground electrode 93a2 that face each other across the arm waveguide 92a in the main surface of the optical substrate 91. Similarly, the modulating electrode 93b has a hot electrode 93b1 and a ground electrode 93b2 that face each other across the arm waveguide 92b in the main surface of the optical substrate 91.

[0015] The modulating electrodes 93a and 93b are configured as segment electrodes divided into a plurality of portions along the light propagation direction of the arm waveguides 92a and 92b. Specifically, the hot electrode 93a1 and the ground electrode 93a2 constituting the modulating electrode 93a are each divided into a plurality of portions (segments) of the same length along the light propagation direction of the arm waveguide 92a. Furthermore, the hot electrode 93b1 and the ground electrode 93b2 constituting the modulating electrode 93b are each divided into a plurality of segments of the same length along the light propagation direction of the arm waveguide 92b, with gaps between the segments spaced at regular intervals.

[0016] The segments of the hot electrodes 93a1 and 93b1 are electrically connected to each other by a hot transmission line 96a. The segments of the ground electrode 93a2 are electrically connected to each other by a ground transmission line 96b, and the segments of the ground electrode 93b2 are electrically connected to each other by a ground transmission line 96c. As a result, the hot electrodes 93a1 and 93b1 connected to each other by the hot transmission line 96a, the ground electrode 93a2 connected to the ground transmission line 96b, and the ground electrode 93b2 connected to the ground transmission line 96c together form a coplanar electrode.

[0017] As shown in the XV-XV cross-sectional view of FIG. 15, when a high-frequency signal is transmitted to the modulating electrode 93a in the arm waveguide 92a, for example, in a portion where a segment of the hot electrode 93a1 and a segment of the ground electrode 93a2 face each other, an electric field is applied to the arm waveguide 92a, causing the refractive index to change (for example, increase) by Δn from the refractive index na of the optical substrate 91 (substrate refractive index na), whereas in a gap portion where the segments do not face each other, no electric field is applied to the arm waveguide 92a, so the refractive index remains the substrate refractive index na.

[0018] Each of these refractive index unchanged portions occurring at the positions of the gaps where the segments of the modulating electrode 93a aligned along the arm waveguide 92a are not opposed to each other is a refractive index discontinuity (disturbance in refractive index change) along the light propagation direction of the arm waveguide 92a, and is a factor causing light leakage from the arm waveguide 92a.

[0019] The leakage light generated from each of the refractive index unchanged portions lined up along the arm waveguide 92a leaks out into the support substrate 94 having 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, where it interferes with and reinforces each other within the support substrate 94 and propagates in the left-right direction in the figure.

[0020] In particular, the segment electrodes serving as the modulating electrodes 93a are generally each divided into hundreds to thousands of segments, and therefore the number of gaps between the segments also amounts to hundreds to thousands. As a result, the number of leaked lights generated from the equally spaced gaps between the segments in the arm waveguide 92a also amounts to hundreds to thousands. These leaked lights interfere with and reinforce each other within the support substrate 94, and this can result in leaked light of a non-negligible intensity within the support substrate 94. The above phenomenon also occurs in the arm waveguide 92b on which the modulating electrode 93b is formed, and the leakage light from the arm waveguide 92b interferes and reinforces itself within the support substrate 94, further increasing the leakage light intensity within the support substrate 94 to a level that cannot be ignored.

[0021] Some of the high-intensity leaked light generated by the interference may enter the arm waveguides 92a, 92b and other parts of the Mach-Zehnder optical waveguide 92 and be coupled with the signal light (or modulated light) propagating through the Mach-Zehnder optical waveguide 92. Such leaked light coupled with the signal light (or modulated light) propagating through the Mach-Zehnder optical waveguide 92 is noise light, which deteriorates the 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.

[0022] The present invention has been made based on knowledge about the causes of the variations in optical characteristics as described above, and in particular, it suppresses interference between leaked light beams within the support substrate, suppresses an increase in the intensity of the leaked light due to such interference, and reduces variations in optical characteristics during optical modulation operation.

[0023] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. [1. First embodiment] First, a first embodiment of the present invention will be described. Fig. 1 is a diagram showing the configuration of an optical modulator 2 using an optical modulation element 1a according to the first embodiment of the present invention. The optical modulator 2 has the optical modulation element 1a and a relay substrate 4 inside a housing 3. The optical modulation element 1a has, for example, a DP-QPSK modulator configuration. A plate-shaped cover (not shown) is finally fixed to the opening of the housing 3, and the interior thereof is hermetically sealed.

[0024] The optical modulator 2 also has a signal pin 5a for inputting a high-frequency electrical signal used to modulate the optical modulation element 1a, and a signal pin 5b for inputting an electrical signal used to adjust the operating point of the optical modulation element 1a, etc.

[0025] Furthermore, the optical modulator 2 has an input optical fiber 6a for inputting light into the housing 3 and an output optical fiber 6b for guiding the light modulated by the optical modulation element 1a to the outside of the housing 3, both on the same surface of the housing 3.

[0026] Here, the input optical fiber 6a and the output optical fiber 6b are fixed to the housing 3 via supports 7a and 7b, which are fixing members. Light input from the input optical fiber 6a is collimated by a lens 8a arranged in the support 7a, and then input to the light modulation element 1a via the lens 8b. However, this is just one example, and light can also be input to the light modulation element 1a according to conventional technology, 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 20a (described later) of the light modulation element 1a.

[0027] The optical modulator 2 also has an optical unit 9 that polarization-combines the two modulated lights output from the optical modulation elements 1a. The polarization-combined light output from the optical unit 9 is collected by a lens 8c disposed within the support 7b and coupled to the output optical fiber 6b.

[0028] The relay board 4 relays the high-frequency electrical signal input from the signal pin 5a and the electrical signal for adjusting the operating point input from the signal pin 5b to the optical modulation element 1a by a conductor pattern (not shown) formed on the relay board 4. The conductor patterns on the relay board 4 are connected to the electrodes of the optical modulation element 1a by, for example, wire bonding. The optical modulator 2 also includes a terminator 10 having a predetermined impedance inside the housing 3.

[0029] Fig. 2 is a plan view showing an example of the configuration of the light modulation element 1a. The light modulation element 1a has a substrate 20a configured with multiple layers. The substrate 20a is, for example, rectangular in plan view and has two opposing left and right sides 21a and 21b extending in the vertical direction in Fig. 2, and two opposing top and bottom sides 21c and 21d extending in the horizontal direction in Fig.

[0030] FIG. 3 is a side view of the side 21a of the light modulation element 1a shown in FIG. 2. The substrate 20a includes an optical waveguide layer 22 and a support layer 23a. In this embodiment, the support layer 23a includes a first support layer 23a1 and a second support layer 23a2. In this embodiment, the substrate 20a is formed by stacking a plurality of plates, for example. Specifically, the substrate 20a is formed by stacking an optical substrate 24 and a support substrate 25a. The optical substrate 24 includes the optical waveguide layer 22, and the support substrate 25a includes the support layer 23a, which is made of the first support layer 23a1 and the second support layer 23a2. The optical substrate 24 is, for example, an X-cut LN substrate having an electro-optic effect, which has been thinned and processed to a thickness of 20 μm or less (e.g., 2 μm). The support substrate 25a is, for example, a glass substrate including a first support layer 23a1 and a second support layer 23a2 made of glass having different materials or compositions.

[0031] The substrate 20a does not necessarily have to be composed of multiple plates as described above. The substrate 20a may be composed of a film formed in layers on an appropriate substrate. For example, the substrate 20a may include a first support layer 23a1 and an optical waveguide layer 22 formed in layers by a film formation process such as sputtering, vapor deposition, and / or crystal growth on an appropriate plate constituting the second support layer 23a2.

[0032] The optical modulation element 1a has an optical waveguide 26 (the entirety of the thick dotted line shown in FIG. 2) formed on the optical waveguide layer 22 of the substrate 20a (on the optical substrate 24 in this embodiment). The optical waveguide 26 is a convex optical waveguide (for example, a rib-type optical waveguide or a ridge-type optical waveguide) formed by a convex portion extending on the optical waveguide layer 22, and performs coherent multilevel modulation of, for example, more than 100 GBaud.

[0033] 2, optical waveguide 26 includes an input waveguide 27 that receives input light (indicated by an arrow pointing rightward in the figure) from input optical fiber 6a on the upper side of left side 21a of optical waveguide layer 22 in the figure, and branching waveguide 28 that branches the input light into two light beams having the same light intensity. Optical waveguide 26 also includes so-called nested Mach-Zehnder optical waveguides 29a and 29b as two modulation sections that modulate the respective light beams branched by branching waveguide 28.

[0034] In the nested Mach-Zehnder optical waveguides 29a and 29b, the propagation direction of light is turned back by 180 degrees in a turning region 30 on the right side of the optical waveguide layer 22 in the figure, and the light is output to the left in the figure from the side 21a of the optical waveguide layer 22 by the output waveguides 31a and 31b.

[0035] Each of the nested Mach-Zehnder optical waveguides 29a and 29b includes two Mach-Zehnder optical waveguides 32a and 32b, and two Mach-Zehnder optical waveguides 32c and 32d 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 waveguide 32. Each of the Mach-Zehnder optical waveguides 32 includes two arm waveguides.

[0036] In the illustrated upper portion of the optical waveguide layer 22, a bias electrode 33a for adjusting the operating points of the nested Mach-Zehnder optical waveguides 29a and 29b is formed upstream of the turning region 30 along the propagation direction of the light waves of the optical waveguide 26. In addition, bias electrodes 33b and 33c for adjusting the operating points of the Mach-Zehnder optical waveguides 32a, 32b and 32c, 32d are provided.

[0037] Furthermore, modulation electrodes for causing the four Mach-Zehnder optical waveguides 32a, 32b, 32c, and 32d to perform modulation operations are formed in modulation sections 34a, 34b, 34c, and 34d shown in the lower part of the nested Mach-Zehnder optical waveguides 29a and 29b folded back at the folding region 30. Hereinafter, the modulation sections 34a, 34b, 34c, and 34d will also be collectively referred to as modulation section 34.

[0038] High frequency electrical signals for causing each of the Mach-Zehnder optical waveguides 32 to perform a modulation operation are input from the relay substrate 4 via wire bonding 35 on the right side of the figure. These high frequency electrical signals propagate through the modulation electrodes formed in each of the modulation sections 34 and are terminated by termination resistors (not shown) provided in the terminator 10 shown below in the figure.

[0039] To avoid cluttering the drawing and to facilitate understanding, the details of the electrodes formed in the modulation sections 34a, 34b, 34c, and 34d are not shown in Fig. 2. Each of the modulation sections 34 is formed with a segment electrode, which is divided into multiple segments along the propagation direction of light in the optical waveguide, as a modulation electrode, similar to the prior art shown in Fig. 13.

[0040] As an example, the configuration of the modulating electrodes in the modulating section 34a is shown in Fig. 4. The modulating electrodes in the other modulating sections 34b, 34c, and 34d are configured in the same manner as in Fig. 4. In FIG. 4, the modulating electrodes 40a and 40b respectively control the light waves propagating through the arm waveguides 36a1 and 36a2 of the Mach-Zehnder optical waveguide 32a.

[0041] The modulating electrode 40a has a hot electrode 40a1 and a ground electrode 40a2 that face each other with one arm waveguide 36a1 sandwiched between them on the surface of the optical waveguide layer 22. Similarly, the modulating electrode 40b has a hot electrode 40b1 and a ground electrode 40b2 that face each other with the other arm waveguide 36a2 sandwiched between them on the surface of the optical waveguide layer 22.

[0042] The modulating electrodes 40a and 40b are each configured as segment 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 modulating electrode 40a are each divided into multiple sections (segments) of the same length along the light propagation direction of the arm waveguide 36a1, with gaps between the segments spaced at regular intervals. The hot electrode 40b1 and the ground electrode 40b2 constituting the modulating electrode 40b are each divided into multiple segments of the same length along the light propagation direction of the arm waveguide 36a2, with 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 is, for example, on the order of several thousand. However, the number of segments may be any number depending on the light modulation characteristics desired for the optical modulation element 1a.

[0043] The segments of hot electrodes 40a1 and 40b1 are electrically connected to each other by hot transmission line 41a. The segments of ground electrode 40a2 are electrically connected to each other by ground transmission line 41b, and the segments of ground electrode 40b2 are electrically connected to each other by ground transmission line 41c. As a result, hot electrodes 40a1 and 40b1 connected to each other by hot transmission line 41a, ground electrode 40a2 connected to ground transmission line 41b, and ground electrode 40b2 connected to ground transmission line 41c together form a coplanar electrode.

[0044] Fig. 5 is a VV cross-sectional view taken along the arm waveguide 36a1 in the modulation section 34a shown in Fig. 4. In Fig. 5, the lower part (B) shows the configuration of the optical modulation element 1a in the VV cross-section, and the upper part (A) is a graph showing the change in refractive index of the arm waveguide 36a1 in the VV cross-section along the light propagation direction.

[0045] 5A, in the arm waveguide 36a1, similarly to the arm waveguide 92a of the optical modulation element 90 according to the conventional technology shown in FIG. 15, when a high-frequency signal is transmitted to the modulating electrode 40a, an electric field is applied to the arm waveguide 36a1 in a portion where a segment of the hot electrode 40a1 and a segment of the ground electrode 40a2 face each other, causing the refractive index to change (for example, increase) by Δn from the refractive index n0 (substrate refractive index n0) of the optical waveguide layer 22 (i.e., the optical substrate 24). In addition, in a gap portion where the segments do not face each other, no electric field is applied to the arm waveguide 36a1, so the refractive index remains the substrate refractive index n0.

[0046] Each of the refractive index unchanged portions (i.e., portions where the refractive index does not change from the substrate refractive index n0) that occur in the gaps in the arm waveguide 36a1 where the segments of the modulation electrode 40a are not opposed to each other is a disturbance in the refractive index change along the light propagation direction of the arm waveguide 36a1. In each of these refractive index disturbance portions, light may leak from the arm waveguide 36a1 formed in the optical waveguide layer 22, as in the optical modulation element 90 according to the prior art described above.

[0047] However, to prevent these leaked lights from interfering with each other, in this embodiment, in particular, the support layer 23a included in the substrate 20a is configured with two layers, a first support layer 23a1 and a second support layer 23a2, which have different refractive indices. 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 23a1, and the refractive index n2 of the second support layer 23a2 have the relationship of the following formula (1). n0>n1 and n2>n1 (1)

[0048] As a result, leakage light generated from the arm waveguide 36a1 formed in the optical waveguide layer 22 enters the first support layer 23a1, which has a refractive index n1 smaller than the refractive index n0 of the optical waveguide layer 22, and then enters the second support layer 23a2, which has a refractive index n2 larger than that of the first support layer 23a1.

[0049] At this time, the refraction angle θ2 of the leaked light when passing through the boundary surface between the first support layer 23a1 and the second support layer 23a2 is n1 / n2 (<1) times smaller than the incident angle θ1. Therefore, the spread angle of each leaked light incident into the second support layer 23a2 is smaller than the spread angle of the leaked light in the first support layer 23a1, and overlapping of the leaked lights in the second support layer 23a2 is suppressed, thereby suppressing interference between the leaked lights.

[0050] The modulating electrode 40b of the arm waveguide 36a2 and the modulating electrodes of the arm waveguides of the Mach-Zehnder optical waveguides 32 in the other modulating sections 34 are configured similarly to the modulating electrode 40a of the arm waveguide 36a1 described above, and similarly to the above, overlapping of leaked light beams generated in these arm waveguides is suppressed within the second support layer 23a2, thereby suppressing interference between the leaked light beams.

[0051] Hereinafter, the arm waveguides of the Mach-Zehnder optical waveguide 32, including the arm waveguides 36a1 and 36a2 of the Mach-Zehnder optical waveguide 32a, will be collectively referred to as arm waveguides 36. Furthermore, the modulating electrodes provided in the arm waveguides 36 in each of the modulating sections 34, including the modulating electrodes 40a and 40b provided in the arm waveguides 36a1 and 36a2 in the modulating section 34a, will be collectively referred to as modulating electrodes 40.

[0052] The above-mentioned action suppresses the buildup of intensities of the leaked lights generated from the arm waveguides 36 formed in the optical waveguide layer 22 due to interference between the leaked lights while they are propagating through the second support layer 23a2. As a result, even if the leaked lights reach the optical waveguide layer 22 again and are multiplexed with the signal light propagating through the optical waveguide 26, the influence of the leaked lights on the optical characteristics of the optical modulation element 1a is suppressed to be smaller than that of the conventional optical modulation element 90.

[0053] 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 by the above-mentioned action, it may be important to prevent these leaked lights from interfering with each other in the first support layer 23a1 before they reach the second support layer 23a2. Specifically, the interference between leaked lights in the first support layer 23a1 depends on the interval L between the gaps arranged at regular intervals between the segments constituting the modulating electrode 40 and / or the thickness t1 of the first support layer 23a1. Here, the interval L between the gaps refers to the distance along the corresponding arm waveguide 36 between the centers of the gaps in the longitudinal direction.

[0054] More specifically, in order to suppress interference between leaked lights within the first support layer 23a1, the spacing L between the gaps between the segments constituting the modulating electrode 40 preferably satisfies the following formula (2), and more preferably formula (3), in terms of the wavelength λ of the light wave propagating through the optical waveguide 26 and the refractive index n1 of the first support layer 23a1: L>4×λ / n1 (2) L>10×λ / n1 (3)

[0055] Furthermore, in order to suppress interference between leaked lights within the first support layer 23a1, the thickness t1 of the first support layer 23a1 preferably satisfies the following formula (4), and more preferably satisfies formula (5). t1<10×λ / n1 (4) t1<4×λ / n1 (5)

[0056] In this embodiment, for 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 constituting the modulating electrode 40 is 50 μm. The optical waveguide layer 22 has a thickness t0 of 1 μm in the optical waveguide 26 and a refractive index n0 of 2.2 at the wavelength λ. The first support layer 23a1 is made of, for example, SiO2, has a thickness t1 of 3 μm, and a refractive index n1 of 1.48 at the wavelength λ. The second support layer 23a2 is made of, for example, alkali-free glass, has a thickness t2 of 300 μm, and a refractive index n2 of 1.56 at the wavelength λ. Instead of alkali-free glass, the second support layer 23a2 may be made of a semiconductor material such as Si (refractive index 3.5 at the wavelength λ).

[0057] In this embodiment, the entire substrate 20a is configured as a multilayer structure, but the entire substrate 20a does not necessarily have to be configured as a multilayer structure. For example, if the substrate 20a is configured as a multilayer structure at least below the modulation section 34 where the modulation electrodes 40, which are segment electrodes, are formed, the above-mentioned function or effect of suppressing interference of leaked light can be achieved.

[0058] That is, the substrate 20a includes at least a multilayer portion configured in multiple layers, and the multilayer portion includes an optical waveguide layer 22, a first support layer 23a1 that contacts the underside of the optical waveguide layer 22, and a second support layer 23a2 that contacts the underside of the first support layer 23a1.

[0059] In this embodiment and the following embodiments, the distance L between the gaps between adjacent segments of each of the modulating electrodes 40 does not necessarily have to be constant throughout its entire section (i.e., the entirety), as long as the distance L between the gaps between adjacent segments is constant throughout the entire section of the modulating electrode 40 or throughout sections excluding some of the sections. Similarly, the length of each of the segments of each of the modulating electrodes 40 does not necessarily have to be the same throughout its entire section, as long as the length of each segment is constant throughout the entire section of the modulating electrode 40 or throughout sections excluding some of the sections. For example, when the modulating electrode 40 is divided into hundreds to thousands of segments, the length of the segments and / or the distance between the gaps between adjacent segments in one section or multiple sections of the modulating electrode 40 may be different from the length of the segments and / or the distance between the gaps between adjacent segments in other sections.

[0060] [2. Second Embodiment] Next, a light modulation element 1b according to a second embodiment of the present invention will be described. The light modulation element 1b has a configuration similar to that of the light modulation element 1a, but differs in that it has a substrate 20b instead of the substrate 20a. The light modulation element 1b can be mounted on an optical modulator 2 and used in place of the light modulation element 1a.

[0061] The plan view of the light modulation element 1b is the same as the plan view of the light modulation element 1a shown in FIGS. 2 and 4, and therefore the explanations of FIGS. 2 and 4 and the above-mentioned FIGS.

[0062] Fig. 6 is a side view of the side 21a of the optical modulation element 1b, and corresponds to Fig. 3 for the optical modulation element 1a according to the first embodiment. Fig. 7 is a cross-sectional view taken along the arm waveguide 36a1 in the modulation section 34a of the optical modulation element 1b, and corresponds to Fig. 5 for the optical modulation element 1a according to the first embodiment.

[0063] In Figures 6 and 7, components that are the same as those shown in Figures 3 and 5 are indicated by the same reference numerals as those shown in Figures 3 and 5, and the explanations for Figures 3 and 5 above are incorporated herein.

[0064] 6 and 7, the substrate 20b constituting the light modulation element 1b has a configuration similar to that of the substrate 20a, but differs in that it includes a support layer 23b instead of the support layer 23a. The support layer 23b has a configuration similar to that of the support layer 23a, but differs in that it includes a third support layer 23a3 in contact with the lower surface of the second support layer 23a2, in addition to the first support layer 23a1 and the second support layer 23a2. The substrate 20b is configured, for example, by stacking a support substrate 25b including the first support layer 23a1, the second support layer 23a2, and the third support layer 23a3 on the lower surface of the optical substrate 24, instead of the support substrate 25a.

[0065] Note that, like the substrate 20a, the substrate 20b does not necessarily have to be composed of multiple plates. The substrate 20b may be composed of a film formed in layers on an appropriate substrate. For example, the substrate 20b may include the second support layer 23a2, the first support layer 23a1, and the optical waveguide layer 22 formed in layers by a film formation process such as sputtering, vapor deposition, and / or crystal growth on an appropriate plate constituting the third support layer 23a3.

[0066] As in Figure 5, in Figure 7, the lower part (B) shows the configuration of the optical modulation element 1b in a cross section along the arm waveguide 36a1, and the upper part (A) is a graph showing the change in refractive index of the arm waveguide 36a1 along the light propagation direction in the above cross section.

[0067] In the optical modulation element 1b, the support layer 23b included in the substrate 20b is composed of three layers, namely, a first support layer 23a1, a second support layer 23a2, and a third support layer 23a3, which have different refractive indices. 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 23a1, the refractive index n2 of the second support layer 23a2, and the refractive index n3 of the third support layer 23a3 satisfy the relationship of the following formula (6). n0>n1, n2>n1, and n3>n2>n1 (6)

[0068] As a result, the leaked light generated from the arm waveguide 36a1 formed in the optical waveguide layer 22 is refracted twice in a direction narrowing the spread of the leaked light, when passing through the boundary surface between the first support layer 23a1 and the second support layer 23a2 and when passing through the boundary surface between the second support layer 23a2 and the third support layer 23a3. Therefore, the spread angle of each leaked light incident on the third support layer 23a3 is smaller than the spread angle in the second support layer 23a2 of the optical modulation element 1a according to the first embodiment, further suppressing overlap between the leaked lights and more effectively suppressing interference between the leaked lights.

[0069] The above-mentioned effect occurs in the arm waveguide 36a2 and each of the arm waveguides 36 of the Mach-Zehnder optical waveguides 32 in the other modulation sections 34 in the same way.

[0070] Due to the above-mentioned action, an increase in the intensity of the leaked light generated from the arm waveguides 36 formed in the optical waveguide layer 22 due to interference between the leaked light while the leaked light propagates in the third support layer 23a3 is further suppressed compared to the optical modulation element 1a according to the first embodiment. As a result, even if the leaked light reaches the optical waveguide layer 22 again and is multiplexed with the signal light propagating through the optical waveguide 26, the influence of the leaked light on the optical characteristics of the optical modulation element 1b is further suppressed compared to the conventional optical modulation element 90.

[0071] In addition, for the optical modulation element 1b, the preferred conditions for the spacing L of the gaps between the segments constituting the modulation electrode 40 along the corresponding arm waveguide 36 and the thickness t1 of the first support layer 23a1 are as shown in the above-mentioned formulas (2) to (5).

[0072] In this embodiment, for 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 constituting the modulating electrode 40 is 50 μm. The optical waveguide layer 22 has a thickness t0 of 1 μm in the optical waveguide 26 and a refractive index n0 of 2.2 at the wavelength λ. The first support layer 23a1 is made of, for example, SiO2, has a thickness t1 of 3 μm, and a refractive index n1 of 1.48 at the wavelength λ. The second support layer 23a2 is made of, for example, alkali-free glass, has a thickness t2 of 200 μm, and a refractive index n2 of 1.56 at the wavelength λ. The third support layer 23a3 is made of Si, has a thickness t3 of 300 μm, and a refractive index n3 of 3.5 at the wavelength λ.

[0073] In this embodiment, similarly to the substrate 20a of the first embodiment, the substrate 20b can achieve the above-mentioned function or effect of suppressing interference of leaked light if it is configured in multiple layers at least below the modulation section 34 where the modulation electrode 40, which is a segment electrode, is formed.

[0074] That is, the substrate 20b includes at least a multilayer portion configured in multiple layers, and the multilayer portion may include an optical waveguide layer 22, a first support layer 23a1 that is in contact with the underside of the optical waveguide layer 22, a second support layer 23a2 that is in contact with the underside of the first support layer 23a1, and a third support layer 23a3 that is in contact with the underside of the second support layer 23a2.

[0075] 3. Third Embodiment Next, an optical modulation element 1c according to a third embodiment of the present invention will be described. The optical modulation element 1c has a similar configuration to the optical modulation element 1a, but a light-absorbing material that absorbs light in the wavelength range of the light waves propagating through the optical waveguide 26 is arranged on the back surface of the substrate 20a, opposite the front surface of the optical waveguide layer 22. The optical modulation element 1c can be mounted in the optical modulator 2 and used in place of the optical modulation element 1a.

[0076] The plan view of the light modulation element 1c is the same as the plan view of the light modulation element 1a shown in FIGS. 2 and 4, and therefore the explanations of FIGS. 2 and 4 and the above-mentioned FIGS.

[0077] Fig. 8 is a side view of the side 21a of the optical modulation element 1c, and corresponds to Fig. 3 for the optical modulation element 1a according to the first embodiment. Fig. 9 is a cross-sectional view taken along the arm waveguide 36a1 in the modulation section 34a of the optical modulation element 1c, and corresponds to Fig. 5 for the optical modulation element 1a according to the first embodiment. Note that the graph of the refractive index in the arm waveguide 36a1 in Fig. 9 is the same as that in Fig. 5, and is therefore omitted.

[0078] In Figures 8 and 9, components that are the same as those shown in Figures 3 and 5 are indicated by the same reference numerals as those shown in Figures 3 and 5, and the explanations for Figures 3 and 5 above are incorporated herein.

[0079] 8 and 9, in the optical modulation element 1c, a light absorbing material 43 that absorbs light in the wavelength range of the light waves propagating through the optical waveguide 26 is arranged on the entire back surface 42 of the substrate 20a, which faces the front surface of the optical waveguide layer 22. As a result, in the optical modulation element 1c, leaked light generated from the arm waveguide 36 of the optical waveguide layer 22 is absorbed and attenuated by the light absorbing material 43 when it propagates through the second support layer 23a2 and reaches the back surface 42. As a result, in the optical modulation element 1c, the intensity of the leaked light that reaches the optical waveguide layer 22 again is significantly reduced, and the influence of this leaked light on the optical characteristics of the optical modulation element 1c is more effectively suppressed than in the conventional optical modulation element 90.

[0080] The light-absorbing material 43 may be, for example, a carbon material such as carbon black, a black resin such as cashew oil, or a metal filler such as Ag. The light-absorbing material 43 may be disposed on the rear surface 42 of the substrate 20a by applying the material to the rear surface 42 using an appropriate resin as a binder and then curing the applied material.

[0081] In this embodiment, the light absorbing material 43 is disposed over the entire rear surface 42 of the substrate 20a, but it may be applied to a part of the rear surface 42. For example, if the light absorbing material 43 is disposed at least in an area of ​​the rear surface 42 that corresponds to the lower part of the modulation section 34 where the modulation electrode 40, which is a segment electrode, is formed, the above-mentioned effect of attenuating leaked light can be achieved.

[0082] Furthermore, when mounting the light modulation element 1c in the housing 3, the light modulation element 1c can be fixed to the housing 3, for example, by providing an adhesive layer between the surface of the light-absorbing material 43 arranged on the back surface 42 of the substrate 20a and the surface of the back surface 42 on which the light-absorbing material 43 is not applied, and the housing 3.

[0083] [4. Fourth Embodiment] Next, a fourth embodiment of the present invention will be described. This embodiment is an optical modulation module 50 including the optical modulation element 1a according to the first embodiment. Fig. 10 is a diagram showing the configuration of the optical modulation module 50 according to this embodiment. In Fig. 10, the same components as those in the optical modulator 2 according to the first embodiment shown in Fig. 1 are indicated by the same reference numerals as those shown in Fig. 1, and the above description of Fig. 1 is incorporated herein.

[0084] 1, the optical modulation module 50 has a similar configuration to the optical modulator 2 shown in Fig. 1, but differs in that it includes a circuit board 51 instead of the relay board 4. The circuit board 51 includes a drive circuit 52. The drive circuit 52 generates a high-frequency electrical signal that drives the optical modulation 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 modulation element 1a.

[0085] The optical modulation module 50 having the above configuration includes the optical modulation element 1a, similar to the optical modulator 2 according to the first embodiment described above. Therefore, similar to the optical modulator 2, the influence of leakage light from the arm waveguide 36 provided with the modulation electrode 40, which is a segment electrode, on the optical characteristics of the optical modulation element 1a can be reduced, thereby realizing good optical modulation operation.

[0086] In this embodiment, the optical modulation module 50 is provided with an optical modulation element 1a as an example, but may also be provided with an optical modulation element 1b according to the second embodiment or an optical modulation element 1c according to the third embodiment.

[0087] [5. Fifth Embodiment] Next, a fifth embodiment of the present invention will be described. This embodiment is an optical transmission device 55 equipped with the optical modulator 2 according to the first embodiment. FIG. 11 is a diagram showing the configuration of the optical transmission device 55 according to this embodiment. This optical transmission device 55 has the optical modulator 2, a light source 56 that inputs light to the optical modulator 2, a modulator driver 57, and a modulation signal generator 58. Note that the optical modulation module 50 according to the fourth embodiment can also be used instead of the optical modulator 2 and the modulator driver 57. Furthermore, the optical modulator 2 may include an optical modulation element 1b or an optical modulation element 1c instead of the optical modulation element 1a.

[0088] The modulation signal generating unit 58 is an electronic circuit that generates an electrical signal to cause the optical modulator 2 to perform modulation operations. Based on transmission data provided from the outside, the modulation signal generating unit 58 generates a modulation signal, which is a high-frequency signal to cause the optical modulator 2 to perform optical modulation operations in accordance with the modulation data, and outputs the modulation signal to the modulator driving unit 57.

[0089] 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 for driving each of the modulation electrodes 40 provided on the four Mach-Zehnder optical waveguides 32 of the optical modulation element 1a included in the optical modulator 2.

[0090] 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, the light output from the light source 56 is modulated, for example, by DP-QPSK by the optical modulator 2, and is output from the optical transmitter 55 as modulated light.

[0091] The optical transmitter 55 uses the optical modulator 2 or the optical modulation module 50 that includes the optical modulation element 1a, 1b, or 1c, and therefore can achieve good modulation characteristics and perform good optical transmission.

[0092] [6. Sixth Embodiment] Next, a sixth embodiment of the present invention will be described. This embodiment is an optical transmission system 60 using the optical transmitting device 55 according to the fifth embodiment. Fig. 12 is a diagram showing the configuration of the optical transmission system 60 according to this embodiment. This optical transmission system 60 includes the optical transmitting device 55 according to the fifth embodiment, an optical fiber transmission line 61 that transmits a modulated optical signal that is output light from the optical transmitting device 55, and an optical receiving device 62 that receives the optical signal transmitted by the optical fiber transmission line 61. The optical transmission system 60 has good optical transmission performance because it transmits an optical signal using the optical transmitting device 55 that uses the optical modulator 2 or optical modulation module 50 that includes the optical modulation element 1a, 1b, or 1c.

[0093] 7. Other Embodiments In the first to third embodiments described above, the optical waveguide layer 22 in which the optical waveguide 26 is formed is included in the optical substrate 24, which is an LN substrate, but it does not necessarily have to be made of LN. The optical waveguide layer 22 may also be made of a semiconductor material such as InP.

[0094] In the above-described embodiment, the multi-layered substrates 20a and 20b are formed by stacking a plurality of plates. However, this is merely an example, and as described above, the substrates 20a and 20b may be formed by films formed in layers on an appropriate substrate.

[0095] In the above-described embodiment, the substrate 20a is formed by laminating the optical substrate 24, which serves as a plate constituting the optical waveguide layer 22, and the support substrate 25a, which serves as a plate constituting the first support layer 23a1 and the second support layer 23a2. However, the optical substrate 24 and the support substrate 25a are merely examples of the plates constituting the substrate 20a, and the layers included in each of the multiple plates may be distributed arbitrarily. In other words, when a substrate such as the substrates 20a and 20b is formed by laminating multiple plates, each of the plates may include one or any number of multiple layers selected from the optical waveguide layer 22 and the multiple support layers, such as the first support layer 23a1.

[0096] 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 of the present invention.

[0097] 8. Configurations Supported by the Above Embodiments The above-described embodiment supports the following configurations.

[0098] (Configuration 1) An optical modulation element including: a substrate including a multilayer portion configured into multiple layers; an optical waveguide layer in the multilayer portion of the substrate, in which an optical waveguide is formed; and a modulating electrode formed on the optical waveguide layer, the modulating electrode being an electrode for controlling light waves propagating through the optical waveguide, the electrode being divided into a plurality of segments along the propagation direction of light in the optical waveguide, wherein in all sections of the electrode or sections excluding a part thereof, a distance L measured in the extension direction of the optical waveguide between gaps between adjacent segments is constant; 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, and a second support layer in contact with a lower surface of the first support layer, wherein a refractive index n0 of the optical waveguide layer, a refractive index n1 of the first support layer, and a refractive index n2 of the second support layer satisfy the relationship n0>n1 and n2>n1. According to the optical modulation element of configuration 1, it is possible to suppress constructive interference of leaked light from the optical waveguide, which occurs due to gaps between segments of the optical modulation electrode, which is a segment electrode formed by dividing it into a plurality of parts along the propagation direction of light in the optical waveguide. As a result, the optical modulation element of configuration 1 can reduce the influence of such leaked light on the optical characteristics of the optical modulation element, and can achieve good optical characteristics.

[0099] (Configuration 2) The optical modulation element according to configuration 1, wherein the spacing L satisfies the relationship L>4×λ / n1, where λ is the wavelength of the light wave propagating through the optical waveguide and n1 is the refractive index of the first support layer. According to the light modulation element of configuration 2, constructive interference between leaked lights can be suppressed, and better optical characteristics can be achieved.

[0100] (Configuration 3) An optical modulation element described in configuration 1 or 2, wherein the thickness t1 of the first support layer satisfies the relationship t1<10×λ / n1, where λ is the wavelength of the light wave propagating through the optical waveguide and n1 is the refractive index of the first support layer. According to the light modulation element of Configuration 3, constructive interference between leaked lights in the first support layer can be suppressed, thereby achieving better optical characteristics.

[0101] (Configuration 4) An optical modulation element described in any one of configurations 1 to 3, wherein the multilayer portion of the substrate further includes a third support layer in contact with the lower surface of the second support layer, and the refractive index n0 of the optical waveguide layer, 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 the relationship n0>n1 and n3>n2>n1. According to the light modulation element of configuration 4, by configuring the support layer with three layers, constructive interference between leaked lights can be further suppressed, and even better optical characteristics can be achieved.

[0102] (Configuration 5) An optical modulation element according to any one of configurations 1 to 4, wherein a light-absorbing material that absorbs light in the wavelength range of the light waves propagating through the optical waveguide is arranged on the back surface of the substrate opposite the front surface of the optical waveguide layer. According to the optical modulation element of configuration 5, the intensity of the leaked light that reaches the back surface of the substrate is reduced by the light-absorbing material arranged on the back surface, thereby effectively reducing the influence of the leaked light on the optical characteristics of the optical modulation element, thereby achieving even better optical characteristics.

[0103] (Configuration 6) The light modulation element according to configuration 5, wherein the light absorbing material is a carbon material, a black resin, or a metal filler. According to the light modulation element of Configuration 6, the intensity of the leaked light that reaches the rear surface of the substrate can be effectively reduced, and even better optical characteristics as a light modulation element can be achieved.

[0104] (Structure 7) An optical modulation element described in any one of structures 1 to 6, wherein the substrate is formed by stacking a plurality of plates, each of which includes one or any number of the optical waveguide layer and a plurality of support layers including the first support layer and the second support layer. According to the optical modulation element of Configuration 7, it is possible to easily form a substrate including an optical waveguide layer in which an optical waveguide is formed and a plurality of support layers.

[0105] (Configuration 8) An optical modulator comprising: an optical modulation element according to any one of configurations 1 to 7; a housing that houses the optical modulation element; an optical fiber that inputs light to the optical modulation element; and an optical fiber that guides the light output by the optical modulation element to the outside of the housing. According to the optical modulator of configuration 8, since the optical modulation element of any one of configurations 1 to 7 is used, an optical modulator having good optical characteristics can be realized.

[0106] (Configuration 9) An optical modulation module comprising: an optical modulation element according to any one of configurations 1 to 7; a housing that houses the optical modulation element; an optical fiber that inputs light to the optical modulation element; an optical fiber that guides the light output by the optical modulation element to the outside of the housing; and a drive circuit that drives the optical modulation element. According to the optical module of configuration 9, since the optical modulation element of any one of configurations 1 to 7 is used, an optical module having good optical characteristics can be realized.

[0107] (Configuration 10) An optical transmitter comprising the optical modulator according to configuration 8 or the optical modulation module according to configuration 9, and an electronic circuit that generates an electrical signal for causing the optical modulation element to perform a modulation operation. According to the optical transmitter of configuration 10, an optical modulator or an optical modulation module using an optical modulation element of any one of configurations 1 to 7 is used, so that good optical transmission characteristics can be realized.

[0108] (Configuration 11) An optical transmission system including the optical transmitter according to configuration 10, and an optical fiber transmission line for transmitting output light from the optical modulation element. According to the optical transmission system of the eleventh configuration, an optical transmitter using an optical modulation element of any one of the first to seventh configurations is used, and therefore, good optical transmission characteristics can be realized. [Explanation of symbols]

[0109] 1a, 1b, 1c, 90...optical modulation element, 2...optical modulator, 3...housing, 4...relay board, 5a, 5b...signal pin, 6a...input optical fiber, 6b...output optical fiber, 7a, 7b...support, 8a, 8b, 8c...lens, 9...optical unit, 10...terminator, 20a, 20b...substrate, 21a, 21b, 21c, 21d...side, 22...optical waveguide layer, 23a, 23b...support layer, 23a1...first support layer, 23a2...second support layer, 23a3...third support layer, 24, 91...optical substrate, 25a, 25b, 94...support substrate, 26...optical waveguide, 27...input waveguide, 28... Branch waveguide, 29a, 29b... Nested Mach-Zehnder type optical waveguide, 30... Folding region, 31a, 31b... Output waveguide, 32, 32a, 32b, 32c, 32d, 92... Ma 3. A-Zehnder type optical waveguide, 33a, 33b, 33c... bias electrodes, 34, 34a, 34b, 34c, 34d... modulation section, 35... wire bonding, 36, 36a1, 36a2, 92a, 92b... arm waveguides, 40, 40a, 40b, 93a, 93b... modulation electrodes, 40a1, 40b1, 93a1, 93b1... hot electrodes, 40a2, 40b2, 93 a2, 93b2...ground electrode, 41a, 96a...hot transmission path, 41b, 41c, 96b, 96c...ground transmission path, 42...back surface, 43...light absorbing material, 50...optical modulation module, 51...circuit board, 52...drive circuit, 55...optical transmitter, 56...light source, 57...modulator driver, 58...modulation signal generator, 60...optical transmission system, 61...optical fiber transmission path, 62...optical receiver.

Claims

1. a substrate including a multilayer portion configured in multiple layers; an optical waveguide layer in the multilayer portion of the substrate, in which an optical waveguide is formed; a modulation electrode formed on the optical waveguide layer to control a light wave propagating through the optical waveguide, the modulation electrode being divided into a plurality of segments along the propagation direction of light in the optical waveguide; Including, In the entire section of the electrode or in a section excluding a part thereof, the distance L between the gaps between 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 support layer in contact with a lower surface of the optical waveguide layer, and a second support layer in contact with a lower surface of the first support layer; The refractive index n0 of the optical waveguide layer, the refractive index n1 of the first support layer, and the refractive index n2 of the second support layer are n0>n1 and n2>n1 have a relationship of Light modulation element.

2. The distance L is related to the wavelength λ of the light wave propagating through the optical waveguide and the refractive index n1 of the first support layer, and is expressed as follows: L>4×λ / n1 have a relationship of The light modulation element according to claim 1 .

3. The thickness t1 of the first support layer is expressed as follows, where λ is the wavelength of the light wave propagating through the optical waveguide and n1 is the refractive index of the first support layer: t1<10×λ / n1 have a relationship of The light modulation element according to claim 1 .

4. the multilayer portion of the substrate further includes a third support layer in contact with a lower surface of the second support layer; The refractive index n0 of the optical waveguide layer, 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 are n0>n1 and n3>n2>n1 have a relationship of The light modulation element according to claim 1 .

5. a light-absorbing material that absorbs light in the wavelength range of the light wave propagating through the optical waveguide is disposed on a back surface of the substrate that faces the front surface of the optical waveguide layer; The light modulation element according to claim 1 .

6. The light-absorbing material is a carbon material, a black resin, or a metal filler. The light modulation element according to claim 5 .

7. the substrate is formed by stacking a plurality of plates, Each of the plates includes one or any number of layers selected from the optical waveguide layer and a plurality of support layers including the first support layer and the second support layer. The light modulation element according to claim 1 .

8. The light modulation element according to claim 1; a housing that houses the light modulation element; an optical fiber for inputting light to the optical modulation element; an optical fiber that guides the light output from the optical modulation element to the outside of the housing; An optical modulator comprising:

9. The light modulation element according to claim 1; a housing that houses the light modulation element; an optical fiber for inputting light to the optical modulation element; an optical fiber that guides the light output from the optical modulation element to the outside of the housing; a drive circuit for driving the light modulation element; An optical modulation module comprising:

10. an optical modulator according to claim 8 or an optical modulation module according to claim 9; an electronic circuit for generating an electrical signal for causing the optical modulation element to perform a modulation operation; An optical transmitting device comprising:

11. 11. An optical transmission system comprising: the optical transmitter according to claim 10; and an optical fiber transmission line for transmitting output light from the optical modulation element.

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