Optical waveguide element, optical waveguide device, and optical transmitter

Abstract

To realize a configuration capable of easily improving frequency characteristics at a low cost in a substrate having various crystal orientations in an optical waveguide element using an optical waveguide formed on the substrate.SOLUTION: An optical waveguide element is provided with an optical waveguide arranged on a substrate and a traveling wave type electrode giving phase change to a light wave in the optical waveguide. In an action portion in which a light wave is controlled, the electrode and the optical waveguide is configured so that phase change of a first action portion on a downstream side along a propagation direction of an electric signal is opposite in sign to phase change of a second action portion on an upstream side, and the first action portion is shorter than the second action portion. The electrode is composed of a center electrode and a ground electrode, the optical waveguide is a Mach-Zehnder type optical waveguide including a pair of two parallel waveguides, the center electrode is formed at the top of each of the parallel waveguides constituting the second action portion, and the ground electrode is formed at the top of each of the parallel waveguides constituting the first action portion.SELECTED DRAWING: Figure 33

Description

[Technical Field]

[0001] The present invention relates to an optical waveguide element, such as an optical modulation element, which is a functional element using an optical waveguide, and to an optical waveguide device and an optical transmitter using such an optical waveguide element. [Background technology]

[0002] Optical transmitters incorporating waveguide-type optical modulators are widely used in high-speed / large-capacity optical fiber communication systems. Among them, optical modulators using lithium niobate (LiNbO3) (hereinafter also referred to as LN) crystals with electro-optic effect as substrates are widely used in high-speed / large-capacity optical fiber communication systems because they have lower optical loss and can achieve broadband optical modulation characteristics compared to optical modulators using semiconductor materials such as indium phosphide (InP), silicon (Si), or gallium arsenide (GaAs).

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

[0004] On the other hand, the recent accelerated spread of Internet services has led to a further increase in communication traffic, and research is still ongoing into further miniaturization, broadening of bandwidth, and power saving of optical modulation elements.

[0005] From the viewpoint of broadband, optical modulators currently widely used for long-distance optical fiber communications use a traveling-wave electrode provided along an optical waveguide as a control electrode for controlling the light waves propagating in the optical waveguide. A traveling-wave electrode is composed of multiple conductor patterns arranged to form a distributed constant line, and one end where an electric signal (modulation signal) is input is terminated at a predetermined impedance at the opposite end. As a result, in the traveling-wave electrode, the electric signal propagates in one direction as a traveling wave, and this traveling wave generates an electric field in the optical waveguide, thereby controlling the light waves propagating in the optical waveguide.

[0006] Such traveling wave electrodes have the advantage over lumped constant type (pad type) electrodes in that the limitations on high frequency characteristics due to the time constant calculated from the inductance and capacitance can be ignored.

[0007] On the other hand, in a traveling-wave electrode, when an electrical signal propagates through the conductor pattern that constitutes the traveling-wave electrode, a non-negligible propagation loss may occur in the electrical signal. Furthermore, since this propagation loss is frequency-dependent, the effective voltage that contributes to controlling the light wave propagating through the optical waveguide, among the signal voltages propagating as the traveling wave, varies depending on the frequency of the electrical signal. Generally, the propagation loss increases as the frequency of the electrical signal increases, so the characteristics of the optical modulation element are such that the driving voltage increases as the frequency increases, and therefore the response sensitivity decreases as the frequency increases, limiting the operating frequency band.

[0008] As a technique for flattening the frequency response characteristics of an optical modulation element composed of an optical waveguide formed on a substrate, it is known to provide a flattening means formed by adjusting the polarization direction of the substrate in a part of the action part where the light wave in the optical waveguide is controlled by a control electrode (see Patent Document 1). In this optical modulation element, by adjusting the polarization direction of the flattening means as described above, the amount of induced phase generated in the action part by the control electrode is reduced in the flattening means, thereby flattening the frequency response characteristics.

[0009] The conventional optical modulation element described above has a simple structure that flattens the frequency response characteristics, which can be advantageous in terms of ease of manufacturing and cost. For example, the polarization direction can be easily adjusted by inserting the active portion of the substrate between two parallel plate electrodes that generate a high electric field.

[0010] However, in the above-mentioned conventional technology, depending on the desired polarization direction, it may be necessary to significantly tilt the substrate with respect to the high electric field. For example, in the case of an X-cut substrate made of lithium niobate (LN) crystal, the polarization adjustment direction in the planarization means may be close to parallel to the substrate surface. In this case, in order to ensure sufficient space to insert the tilted substrate between the parallel plate electrodes, it becomes necessary to widen the separation distance between the parallel plate electrodes according to the planar size of the substrate.

[0011] As a result, a larger voltage must be applied to the parallel plate electrodes, which may increase the size of the manufacturing equipment and result in a disadvantage in terms of manufacturing costs. Therefore, the above-mentioned conventional technology imposes limitations on the polarization direction that can be achieved at a desired cost, which may impose constraints on the design of the planarization means. In other words, the above-mentioned conventional technology has room for improvement in terms of the degree of freedom in designing the planarization means. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-284129 Summary of the Invention [Problem to be solved by the invention]

[0013] Given the above background, there is a need for a technology that can easily and inexpensively improve the frequency characteristics of optical waveguide elements that use optical waveguides formed on substrates, even for substrates with various crystal orientations. [Means for solving the problem]

[0014] One aspect of the present invention comprises a substrate, an optical waveguide disposed in or on the substrate, and an electrode provided along the optical waveguide that acts on the optical waveguide to generate a phase change in an optical wave propagating through the optical waveguide, wherein the electrode is a traveling wave electrode, and the electrode and the optical waveguide are configured such that, in an action section where the optical wave is controlled by the electrode, the phase change that occurs in a first action section that is within a predetermined distance from a downstream end along the propagation direction of a traveling wave of an electrical signal propagating through the electrode has an opposite sign to the phase change that occurs in a second action section that is within a predetermined distance from an input end of the electrical signal on the upstream side along the propagation direction, and the length of the acting portion is shorter than the length of the portion of the second acting portion that acts on the optical waveguide, and the electrodes are composed of two center electrodes and a plurality of ground electrodes formed within the plane of the substrate so as to sandwich each of the two center electrodes at a predetermined distance from the center electrode, the optical waveguide is a Mach-Zehnder optical waveguide including two paired parallel waveguides, and the acting portion composed of the two parallel waveguides formed on the substrate has the center electrodes formed on each of the parallel waveguides that constitute the second acting portion, and the ground electrodes formed on each of the parallel waveguides that constitute the first acting portion, which is an optical waveguide element. According to another aspect of the present invention, each of the central electrodes is linear over the entire active portion of the parallel waveguide, and the first active portion and the second active portion of each of the parallel waveguides are linear extending along the extending direction of the central electrode, and the first active portion of each of the parallel waveguides is linear in the extending direction. the center electrode and is located at a position spaced apart from the second acting portion in a direction perpendicular to the extending direction. According to another aspect of the present invention, each of the parallel waveguides is linear throughout the entire operation section, and a portion of each of the center electrodes along the first operation portion of the parallel waveguide is linear and is located at a position separated from a portion above the second operation portion in a direction orthogonal to the extending direction of the parallel waveguide. According to another aspect of the present invention, the electrodes are configured such that a voltage attenuation coefficient of an electrical signal in the first operation portion is different from a voltage attenuation coefficient in the second operation portion. According to another aspect of the present invention, a ratio of a length of the first operation portion to a length of the second operation portion has a relationship of [r:(1 + r), where r = 2, 1.5, 1.2, 1.0, or 0.8]. According to another aspect of the present invention, a ratio of a length of the first operation portion to a length of the second operation portion is 1:2. When the length of the first operation portion is L and a voltage attenuation coefficient of an electrical signal in the electrode is α, a value A given by [A = exp(−αL)] is in a range of [0.12 < A ≤ 0.88]. According to another aspect of the present invention, a ratio of a length of the first operation portion to a length of the second operation portion is 1:2. When the length of the first operation portion is L and a voltage attenuation coefficient of an electrical signal in the electrode is α, a value A given by [A = exp(−αL)] is in a range of [0.27 < A ≤ 0.83].

Advantages of the Invention

[0015] According to the present invention, in an optical waveguide element using an optical waveguide formed on a substrate, frequency characteristics can be improved easily and at low cost even in a substrate having various crystal orientations.

Brief Description of the Drawings

[0016] [Figure 1] It is a diagram showing a configuration of an optical modulation element according to a first embodiment of the present invention. [Figure 2] It is a diagram showing a configuration of a conventional optical modulation element. [Figure 3]2 is a diagram for explaining the operation of the light modulation element shown in FIG. [Figure 4] 3 is an explanatory diagram for explaining an improvement effect of the light modulation element shown in FIG. 1 over the conventional light modulation element shown in FIG. 2. FIG. [Figure 5] 3 is an explanatory diagram for explaining an improvement effect of the frequency response of the light modulation element shown in FIG. 1 over the conventional light modulation element shown in FIG. 2. FIG. [Figure 6] FIG. 3 is a diagram showing a configuration of a light modulation element according to a first modified example of the first embodiment. [Figure 7] 7 is a diagram showing an improvement effect of frequency response in the light modulation element shown in FIG. 6. FIG. [Figure 8] FIG. 10 is a diagram showing a configuration of a light modulation element according to a second modified example of the first embodiment. [Figure 9] 9 is a diagram showing an improvement effect of frequency response in the light modulation element shown in FIG. 8. FIG. [Figure 10] FIG. 10 is a diagram showing a configuration of a light modulation element according to a third modified example of the first embodiment. [Figure 11] 7 is a first diagram showing an improvement effect of frequency response in the light modulation element shown in FIG. 6. FIG. [Figure 12] 7 is a second diagram showing the effect of improving the frequency response in the light modulation element shown in FIG. 6. FIG. [Figure 13] 7 is a third diagram showing the effect of improving the frequency response in the light modulation element shown in FIG. 6. FIG. [Figure 14] FIG. 4 is a diagram showing the configuration of a light modulation element according to a second embodiment of the present invention. [Figure 15] FIG. 10 is a diagram showing the configuration of a light modulation element according to a third embodiment of the present invention. [Figure 16] 16 is a cross-sectional view of the light modulation element shown in FIG. 15 taken along the line XVI-XVI. [Figure 17] 17 is a cross-sectional view of the light modulation element shown in FIG. 15 taken along the line XVII-XVII. [Figure 18] FIG. 10 is a diagram showing the configuration of a light modulation element according to a modified example of the third embodiment. [Figure 19]19 is a cross-sectional view of the light modulation element shown in FIG. 18 taken along the line XIX-XIX. [Figure 20] 20 is a cross-sectional view of the light modulation element shown in FIG. 18 taken along the line XX-XX. [Figure 21] FIG. 10 is a diagram showing the configuration of a light modulation element according to a fourth embodiment of the present invention. [Figure 22] 22 is a cross-sectional view of the light modulation element shown in FIG. 21 taken along the line XXII-XXII. [Figure 23] 23 is a cross-sectional view of the light modulation element shown in FIG. 21 taken along the line XXIII-XXIII. [Figure 24] FIG. 10 is a diagram showing the configuration of a light modulation element according to a fifth embodiment of the present invention. [Figure 25] 25 is a cross-sectional view of the optical modulation element shown in FIG. 24 taken along the line XXV-XXV. [Figure 26] 26 is a cross-sectional view of the light modulation element shown in FIG. 24 taken along the line XXVI-XXVI. [Figure 27] FIG. 10 is a diagram showing the configuration of a light modulation element according to a sixth embodiment of the present invention. [Figure 28] 28 is a cross-sectional view taken along the arrows XXVIII-XXVIII of the light modulation element shown in FIG. 27. [Figure 29] 29 is a cross-sectional view of the light modulation element shown in FIG. 27 taken along the line XXIX-XXIX. [Figure 30] FIG. 10 is a diagram showing the configuration of a light modulation element according to a seventh embodiment of the present invention. [Figure 31] 31 is a cross-sectional view of the light modulation element shown in FIG. 30 taken along the line XXXI-XXXI. [Figure 32] 31 is a cross-sectional view of the light modulation element shown in FIG. 30 taken along the line XXXII-XXXII. [Figure 33] FIG. 13 is a diagram showing the configuration of a light modulation element according to an eighth embodiment of the present invention. [Figure 34] FIG. 13 is a diagram showing the configuration of a light modulation element according to a ninth embodiment of the present invention. [Figure 35] FIG. 20 is a diagram showing the configuration of a light modulation element according to a tenth embodiment of the present invention. [Figure 36] 36 is a cross-sectional view of the light modulation element shown in FIG. 35 taken along the line XXXVI-XXXVI. [Figure 37] FIG. 22 is a diagram showing the configuration of an optical modulation device according to an eleventh embodiment of the present invention. [Figure 38] FIG. 22 is a diagram illustrating a configuration of an optical transmitting device according to a twelfth embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. [First embodiment] First, a first embodiment of the present invention will be described. Fig. 1 is a diagram showing the configuration of an optical waveguide element according to the first embodiment of the present invention. In this embodiment, the optical waveguide element is an optical modulation element 100 that performs optical modulation using, for example, a Mach-Zehnder optical waveguide.

[0018] The optical modulation element 100 includes an optical waveguide 104 formed on a substrate 102, and a control electrode 106 that controls light waves propagating through the optical waveguide 104. The substrate 102 is a substrate that has an electro-optic effect. For example, the substrate 102 is an X-cut substrate made of LN, with the Z axis pointing upward in the figure and the Y axis pointing rightward in the figure.

[0019] The optical waveguide 104 is, for example, a Mach-Zehnder optical waveguide, and has a pair of parallel waveguides 104a and 104b. The control electrode 106 is a traveling-wave electrode in which an electric signal (hereinafter also referred to as a modulation signal) propagates in one direction as a traveling wave.

[0020] The control electrode 106 is provided along the length direction (extension direction) of the two parallel waveguides 104a, 104b, and applies an electric field to the parallel waveguides 104a, 104b, thereby generating a phase change in the light waves propagating through these parallel waveguides 104a, 104b. As a result, for example, input light incident from the end of the lower optical waveguide 104 on the left side of the substrate 102 in the figure (indicated by the white arrow pointing to the right in the figure) is modulated, and the modulated light is output from the end of the upper optical waveguide 104 in the figure (indicated by the white arrow pointing to the left in the figure).

[0021] Specifically, the control electrode 106 is composed of a center electrode 106a and ground electrodes 106b and 106c. One end of the center electrode 106a is connected to a signal source 110 that generates a modulation signal, and the other end is terminated by a terminator 112 having a predetermined impedance. The center electrode 106a is disposed between the parallel waveguides 104a and 104b within the plane of the substrate 102 and is arranged along the parallel waveguides 104a and 104b. As a result, the center electrode 106a, together with the ground electrodes 106b and 106c that face each other across the parallel waveguides 104a and 104b, applies an electric field to the parallel waveguides 104a and 104b, respectively. As a result, electric fields in opposite directions are applied to the parallel waveguides 104a and 104b, and phase changes of opposite signs occur in the light waves propagating through the parallel waveguides 104a and 104b.

[0022] Here, the portion of the control electrode 106 that controls the light waves of the parallel waveguides 104a and 104b, i.e., the portion that imparts a phase change to the light waves of the parallel waveguides 104a and 104b, constitutes an action portion 108 (the portion within the center electrode 106a indicated by the dashed arrow in the figure). The action portion 108 has a first action portion 108c (the range indicated by the dashed arrow in the figure) that is within a predetermined distance from the downstream end 108e along the propagation direction of the traveling wave propagating through the control electrode 106, and a second action portion 108a (the range indicated by the dashed arrow in the figure) that is within a predetermined distance from the upstream input end 108d of the electrical signal along the propagation direction. Here, the predetermined distance range refers to a range in which the sign of the phase change of the light generated in each of the parallel waveguides 104a and 104b is the same when a DC voltage is applied between the center electrode 106a and the ground electrodes 106b and 106c.

[0023] In particular, the control electrode 106 and the optical waveguide 104 are configured so that the phase change occurring in each of the light waves of the parallel waveguides 104a and 104b at the first action portion 108c of the action section 108 has an opposite sign to the phase change occurring in each of the parallel waveguides 104a and 104b at the second action portion 108a.

[0024] Furthermore, the lengths of the first acting portion 108c and the second acting portion 108a measured along the extension direction of the acting section 108 are such that the first acting portion 108c is shorter than the second acting portion 108a. That is, the length of the portion of the first acting portion 108c that acts on the parallel waveguides 104a and 104b is shorter than the length of the portion of the second acting portion 108a that acts on the parallel waveguides 104a and 104b.

[0025] In this embodiment, the action portion 108 includes a turning portion 108b (the range indicated by the dashed-dotted arrow in the drawing) which is a first turning portion where the propagation direction of light is reversed.

[0026] Specifically, the second action portion 108a of the action unit 108 is configured as a straight portion extending in the +Y direction from the input end 108d along the propagation direction of the light waves in the parallel waveguides 104a and 104b. The first action portion 108c, which is connected to the second action portion 108a across the folded portion 108b, is configured as a straight portion extending in the -Y direction along the propagation direction of the light waves in the parallel waveguides 104a and 104b and terminating at the end 108e.

[0027] As a result, in the action portion 108, the phase change in the first action portion 108c has an opposite sign to the phase change in the second action portion 108a.

[0028] For example, when focusing on one parallel waveguide 104a constituting the action portion 108, the electric field from the center electrode 106a to the ground electrode 106b is in the +Z direction in the second action portion 108a and in the -Z direction in the first action portion 108c. As a result, in the parallel waveguide 104a, the phase change in the first action portion 108c has an opposite sign to the phase change in the second action portion 108a. Similarly, when focusing on the other parallel waveguide 104b, the electric field from the center electrode 106a to the ground electrode 106c is in the -Z direction in the second action portion 108a and in the +Z direction in the first action portion 108c. As a result, in the parallel waveguide 104b, the phase change in the first action portion 108c has an opposite sign to the phase change in the second action portion 108a.

[0029] The configuration of the control electrode 106 shown in FIG. 1 is intended to provide a simplified schematic explanation of the configuration of the action portion 108, and in reality, for example, the entire center electrode 106a, including the portion on the signal source 110 side and the portion on the terminator 112 side, together with the ground electrodes 106b and 106c, may be formed as a coplanar transmission line having a predetermined impedance (the same applies to the following embodiments and modified examples).

[0030] Next, the operation of the optical modulation element 100 will be described. First, for comparison, the operation of an optical modulation element using a conventional, general Mach-Zehnder optical waveguide having a linear (non-folded) parallel waveguide will be described. FIG. 2 is a diagram showing an example of the configuration of such a conventional optical modulation element 200. The optical modulation element 200 has an optical waveguide 204 and a control electrode 206 formed on a substrate 202 made of, for example, X-cut LN. The optical waveguide 204 is a Mach-Zehnder optical waveguide including two linear parallel waveguides 204a and 204b extending in the Y-axis direction. The control electrode 206 has a center electrode 206a and ground electrodes 206b and 206c provided along the parallel waveguides 204a and 204b.

[0031] Center electrode 206a is provided between parallel waveguides 204a and 204b along the parallel waveguides. Ground electrodes 206b and 206c are disposed opposite center electrode 206a across parallel waveguides 204a and 204b, respectively. Control electrode 206 is a traveling-wave electrode. One end of center electrode 206a is connected to signal source 210 that generates a modulation signal, and the other end is terminated by terminator 212 having a predetermined impedance. The portion of parallel waveguides 204a and 204b whose lightwave is controlled by control electrode 206 constitutes action section 208 (the range indicated by the dashed arrow in the figure) with length L. Here, the upstream end of action section 208 along the propagation direction of the traveling wave propagating through control electrode 206 is referred to as input end 208a, and the downstream end is referred to as end 208b.

[0032] In the above configuration, the induced phase difference φ1 generated between the parallel waveguides 204a and 204b throughout the entire length of the action portion 208 can be obtained by integrating the phase difference generated in the infinitesimal section ds along the axis s (top of the figure) extending in the extension direction of the action portion 208, as shown in the following equation.

number

number

[0033] In the above equation, V0 is the voltage amplitude of the electrical signal at input end 208a of action section 208, α is the voltage attenuation coefficient (attenuation ratio per unit length) of the electrical signal at control electrode 206, and Δφ is the amount of induced phase difference per unit voltage and length generated between parallel waveguides 204a and 204b. Also, A represents the voltage attenuation ratio at end 208b relative to input end 208a of control electrode 206 (the ratio of the signal voltage output from end 208b to the signal voltage input to input end 208a).

[0034] Fig. 3 is an explanatory diagram for explaining the action of the light modulation element 100. Fig. 3 shows the length of the action portion 108 constituting the light modulation element 100 shown in Fig. 1, measured along the action portion 108.

[0035] In the following, the "length" of the optical waveguide, control electrode, and action portion refers to the length measured along the optical waveguide, control electrode, and action portion, respectively. Furthermore, the "induced phase difference" refers to the phase difference generated between two parallel waveguides by the control electrode, and the phase change generated in the parallel waveguides refers to the change in the optical phase generated in each parallel waveguide by the control electrode. In other words, an induced phase difference occurs between the two parallel waveguides as a result of the phase change generated in each parallel waveguide by the control electrode.

[0036] 3, the action section 108 is configured such that the first action section 108c has a length L and the second action section 108a has a length 2L. Since the sign of the phase change in the first action section 108c is opposite to that of the second action section 108a, in DC operation where the loss of the control electrode 106 can be ignored, the induced phase difference generated in the region of length L on the right half of the second action section 108a in the figure is canceled out by the induced phase difference generated in the first action section 108c. Therefore, in the above DC operation, the light modulation element 100 is equivalent to the light modulation element 200 shown in FIG. 2.

[0037] However, for high-frequency electrical signals, the control electrode 106 has a propagation loss, and therefore the induced phase amount in the first acting portion 108c does not completely cancel out the induced phase amount in the portion of the second acting portion 108a with length L on the right side in the figure. For this reason, the optical modulation element 100 shown in Fig. 3 exhibits frequency characteristics different from those of the optical modulation element 200 shown in Fig. 2.

[0038] In FIG. 3, if the control electrode 106 has a voltage attenuation coefficient α similar to that of the control electrode 206, the induced phase difference φ2 generated between the parallel waveguides 104a and 104b is given by the following equation:

number

[0039] In the first line of equation (3), the integral of the first term is the integral along axis s1 (the axis shown at the bottom in the figure) extending in the extension direction of second acting portion 108a of acting portion 108, and the integral of the second term is the integral along axis s2 (the axis shown at the top in the figure) extending in the extension direction of first acting portion 108c of acting portion 108. Note that folded portion 108b in acting portion 108 is a portion where the sign of the phase change is opposite in each of parallel waveguides 104a, 104b, and the amount of phase difference generated is smaller than that in second acting portion 108a and first acting portion 108c, so the amount of induced phase generated in folded portion 108b is ignored in equation (3).

[0040] Here, in order to quantitatively grasp the difference in the amount of induced phase difference generated in the optical modulation element 100 of Figure 3 compared to the optical modulation element 200 of Figure 2, the ratio φ2 / φ1 of φ2 in equation (3) to φ1 in equation (1) is taken, resulting in the following equation.

number

[0041] 4 is a diagram showing the relationship between φ2 / φ1 and A in equation (4). In FIG. 4, the horizontal axis is A, the vertical axis is φ2 / φ1, and line 400 represents φ2 / φ1 expressed in equation (4). As shown in the figure, φ2 / φ1 reaches a maximum value of 1.25 when A=0.5. That is, the optical modulation element 100 shown in FIG. 3 has an induced phase difference at a frequency where the voltage attenuation coefficient α of the control electrode 106 satisfies A=0.5 (i.e., the response at that frequency) that is 1.25 times better than the optical modulation element 200 in FIG. 2.

[0042] Here, A is the voltage attenuation rate per length L of the control electrodes 106, 206, and A=0.5 corresponds to a state in which the S21 component is -6 dB (=20 log(0.5)) in the transfer function expressed in S-parameter notation. Also, φ2 / φ1=1.25 corresponds to a 2 dB (=20 log(1.25)) improvement in frequency response.

[0043] FIG. 5 shows the results of a simulation of the frequency response of the optical modulation element 100 shown in FIG. 3. The horizontal axis represents the frequency of the electrical signal input to the control electrode 106. The vertical axis represents the so-called EOE response, that is, the ratio of the power of the modulated light output from the optical waveguide 104 to the power of the electrical signal applied to the control electrode 106. The EOE response shown in FIG. 5 is normalized by the value of the EOE response at a frequency of 0 Hz, that is, a DC signal. In the calculation of FIG. 5, the voltage attenuation coefficient α of the control electrode 106 was approximated by the following equation: where α0 is a constant and f is the frequency.

number

[0044] Line 500 and line 502 in Fig. 5 respectively represent the response characteristics of the light modulation element 100 shown in Fig. 3 and the light modulation element 200 shown in Fig. 2. In the example of Fig. 5, α0 in equation (5) is adjusted so that the A value of the light modulation element 100 shown in Fig. 3 becomes 0.5 at frequency f1 that provides a 3 dB bandwidth of the EOE response in the light modulation element 200 with the conventional configuration shown in Fig. 2. Such adjustment of the value of α0 can be performed, for example, by adjusting the cross-sectional area of ​​the center electrode 106a of the control electrode 106.

[0045] As shown in Figure 5, in the optical modulation element 100 shown in Figure 3, the EOE response is improved by 2 dB at the above frequency f1, and as a result, the frequency f2 that provides a 3 dB bandwidth (i.e., operating frequency band) of the EOE response of the optical modulation element 100 is improved by approximately three times compared to f1 of the conventional optical modulation element 200. As shown in Fig. 5, the optical modulation element 100 shown in Fig. 3 improves the EOE response in regions other than DC. In particular, it can be seen that the EOE response is improved by approximately 2 dB in a wide region from around frequency f1 to a wide band. As a result, the 3 dB bandwidth of the EOE response of the optical modulation element 100 is broadened to a band equivalent to the 5 dB bandwidth of the conventional optical modulation element 200.

[0046] 2 and the configuration of the light modulation element 100 shown in FIG. 3, the action section 108 of the light modulation element 100 has a first action portion 108c added thereto, and the length of the second action portion 108a is extended by the same length as the first action portion 108c. The above-mentioned improvement in frequency response is provided by the extended portion of the second action portion 108a and the first action portion 108c. In other words, the length of the first action portion 108c must be shorter than the length of the second action portion 108a, and therefore, the length of the control electrode 106 at the first action portion 108c must be shorter than the length at the second action portion 108a.

[0047] In the optical modulation element 100 having the above configuration, the control electrode 106 and the parallel waveguides 104a, 104b are configured so that the phase change of light generated in the parallel waveguides 104a, 104b at the first action portion 108c of the action unit 108 has an opposite sign to the phase change of light generated in the parallel waveguides 104a, 104b at the second action portion 108a. Therefore, in the optical modulation element 100, the induced phase difference generated between the parallel waveguides 104a, 104b at the second action portion 108a is reduced by the induced phase difference generated at the first action portion 108c. Then, in the optical modulation element 100, the induced phase difference canceled by the first action portion 108c out of the induced phase difference at the second action portion 108a has frequency characteristics, and as a result, the operating frequency band realized by the entire action unit 108 is expanded.

[0048] The above-mentioned improvement in the frequency band is achieved by the arrangement of the control electrode 106 and the parallel waveguides 104a and 104b that constitute the active portion 108, and there is no need to adjust the polarization direction of the substrate as in conventional optical modulation elements. Therefore, with the optical modulation element 100, it is possible to improve the frequency characteristics easily and at low cost even for substrates with various crystal orientations.

[0049] Furthermore, in the optical modulation element 100, the first acting portion 108c is folded back 180 degrees relative to the second acting portion 108a within the plane of the substrate 102, so that the phase change in the first acting portion 108c has an opposite sign to the phase change in the second acting portion 108a. This allows the optical modulation element 100 to expand the operating frequency band with a simple configuration.

[0050] 3, the ratio of the length of the first acting portion 108c to the length of the second acting portion 108a constituting the acting section 108 is 1:2, but the configuration of the optical waveguide element according to the present invention is not limited to this. Modified examples of the optical modulation element 100 will be described below.

[0051] <First Modification> First, a first modified example of the optical modulation element 100 according to the first embodiment of the present invention will be described. The optical waveguide element according to this modified example has the same configuration as the optical modulation element 100 according to the first embodiment, but the ratio of the length of the first acting portion to the length of the second acting portion constituting the acting part is r:(1+r) instead of 1:2, where r is any real number.

[0052] Fig. 6 is a diagram showing the configuration of an optical modulation element 100-1 according to a first modified example. In Fig. 6, the same components as those in the optical modulation element 100 shown in Fig. 1 and Fig. 3 are indicated by the same reference numerals as those shown in Fig. 1 and Fig. 3, and the above explanations for Fig. 1 and Fig. 3 are to be cited.

[0053] The optical modulation element 100-1 has a similar configuration to the optical modulation element 100, but differs in that it has a control electrode 106-1 instead of the control electrode 106. The control electrode 106-1 has a similar configuration to the control electrode 106, but instead of the control electrode 106 having the center electrode 106a and the ground electrodes 106b and 106c, it has a center electrode 106-1a and ground electrodes 106-1b and 106-1c.

[0054] The center electrode 106-1a and the ground electrodes 106-1b and 106-1c have the same configuration as the center electrode 106a and the ground electrodes 106b and 106c, but the length of extension along the parallel waveguides 104a and 104b is different from that of the center electrode 106-1a and the ground electrodes 106-1b and 106-1c.

[0055] As a result, the action portion 108-1, which is the portion where the control electrode 106-1 controls the light waves of the parallel waveguides 104a and 104b, is configured so that the length of the first action portion 108-1c is rL and the length of the second action portion 108-1a is L+rL, unlike the action portion 108 of the optical modulation element 100 shown in Fig. 1. In other words, the length ratio of the first action portion 108-1c to the second action portion 108-1a is r:(r+1).

[0056] 1, the first action portion 108-1c and the second action portion 108-1a are respectively within a predetermined distance range (the range indicated by the dashed-dotted arrow in the figure) from the end 108-1e, which is the downstream end along the propagation direction of the traveling wave propagating through the control electrode 106-1, and within a predetermined distance range (the range indicated by the dashed-dotted arrow in the figure) from the input end 108-1d of the electrical signal, which is the upstream end along the propagation direction. The action portion 108-1 also has a folded portion 108-1b similar to the folded portion 108b of the action portion 108. Here, the predetermined distance range refers to a range in which the phase change of the light generated in each of the parallel waveguides 104a and 104b has the same sign when a DC voltage is applied between the center electrode 106-1a and the ground electrodes 106-1b and 106-1c.

[0057] In FIG. 6, the induced phase difference φ generated between the parallel waveguides 104a and 104b 2-1 is given by the following equation:

number

[0058] Fig. 7 shows the results of a simulation of the frequency response of the optical modulation element 100-1 shown in Fig. 6. As in Fig. 5, the horizontal axis represents the frequency of the electrical signal input to the control electrode 106, and the vertical axis represents the EOE response. Also in Fig. 7, as in Fig. 5, α is assumed to follow equation (5), and α0 in equation (5) is adjusted so that the A value of the optical modulation element 100-1 shown in Fig. 6 (i.e., the voltage attenuation rate within the range of length L from the input end 108-1d of the second acting portion 108-1a) becomes 0.5 at frequency f1 that provides the 3 dB bandwidth of the optical modulation element 200 with the conventional configuration shown in Fig. 2.

[0059] Lines 700, 702, 704, 706, and 708 shown in Fig. 7 represent the EOE responses when r = 2, 1.5, 1.2, 1.0, and 0.8, respectively. Here, line 706, which represents the EOE response when r = 1, is the EOE response when the length ratio of the first acting portion 108-1c to the second acting portion 108-1a is 1:2, i.e., the same configuration as the light modulation element 100 shown in Fig. 3, and corresponds to line 500 shown in Fig. 5. Furthermore, line 502 shown in Fig. 7 is the same as line 502 in Fig. 5, and represents the EOE response of the conventional light modulation element 200 shown in Fig. 2.

[0060] Figure 7 shows that as the value of r increases, the EOE response exhibits greater peaking at lower frequencies. In other words, by selecting an appropriate value of r during design, the magnitude of the peaking can be controlled to expand the operating frequency band.

[0061] <Second Modification> Next, a second modified example of the optical waveguide element according to the first embodiment of the present invention will be described. The optical waveguide element according to this modified example has a similar configuration to the optical modulation element 100 according to the first embodiment, but the voltage attenuation coefficient α of the control electrode constituting the action part is not constant throughout the action part, but is different between the first action part and the second action part.

[0062] Fig. 8 is a diagram showing the configuration of a light modulation element 100-2 according to a second modified example. In Fig. 8, the same components as those in the light modulation element 100 shown in Fig. 1 and Fig. 3 are indicated by the same reference numerals as those shown in Fig. 1 and Fig. 3, and the above-mentioned explanations for Fig. 1 and Fig. 3 are used.

[0063] The optical modulation element 100-2 has a configuration similar to that of the optical modulation element 100, but includes a control electrode 106-2 instead of the control electrode 106. The optical modulation element 100-2 also includes an action portion 108-2 configured by the control electrode 106-2 and the parallel waveguides 104a and 104b, instead of the action portion 108 configured by the control electrode 106 and the parallel waveguides 104a and 104b. The action portion 108-2 has a configuration similar to that of the action portion 108, but includes a second action portion 108-2a, a folded portion 108-2b, and a first action portion 108-2c, instead of the second action portion 108a, the folded portion 108b, and the first action portion 108c.

[0064] The second acting portion 108-2a and the first acting portion 108-2c have the same configuration as the second acting portion 108a and the first acting portion 108c, but the voltage attenuation coefficient α of the control electrode 106-2 in the second acting portion 108-2a and the first acting portion 108-2c are different from each other. That is, the control electrode 106-2 has the same configuration as the control electrode 106, but includes a center electrode 106-2a instead of the center electrode 106a, and is configured so that the voltage attenuation coefficient α1 of the portion corresponding to the second acting portion 108-2a is different from the voltage attenuation coefficient α2 of the portion corresponding to the first acting portion 108-2c.

[0065] Such a configuration can be realized, for example, by making the cross-sectional area of ​​the center electrode 106-2a constituting the control electrode 106-2 different between the portion corresponding to the second acting portion 108-2a and the portion corresponding to the first acting portion 108-2c. Here, of the ends of the acting portion 108-2, the upstream end along the propagation direction of the traveling wave propagating through the control electrode 106-2 is referred to as input end 108-2d, and the downstream end is referred to as end 108-2e.

[0066] In FIG. 8, the induced phase difference φ generated between the parallel waveguides 104a and 104b 2-2 is given by the following equation:

number

number

[0067] Fig. 9 shows the results of a simulation of the frequency response of the optical modulation element 100-2 shown in Fig. 8. As in Fig. 5, the horizontal axis represents the frequency of the electrical signal input to the control electrode 106-2, and the vertical axis represents the EOE response. Also in Fig. 9, as in Fig. 5, α1 and α2 are each proportional to the square root of the frequency of the electrical signal. Therefore, m can be given as a parameter that does not have frequency dependency.

[0068] 1, 3 and 6, and is set to comply with equation (5). At frequency f1 that provides a 3 dB bandwidth for the optical modulation element 200 of the conventional configuration shown in FIG. 2, α0 in equation (5) is adjusted so that the A value of the optical modulation element 100-2 shown in FIG. 8 (i.e., the voltage attenuation rate within the range of length L from the input end 108-2d of the second acting portion 108-2a) becomes 0.5.

[0069] 9 show the EOE responses when m=2.5, 2.0, 1.5, 1.0, and 0.5, respectively. Here, line 906, which shows the EOE response when m=1, is the EOE response when the voltage attenuation coefficient α2 in the first acting portion 108-2c is equal to the voltage attenuation coefficient α1 (=α) in the second acting portion 108-2a, that is, when the configuration is the same as that of the light modulation element 100 shown in FIG. 3, and corresponds to line 500 shown in FIG.

[0070] Figure 9 shows that as the value of m increases, the EOE response exhibits greater peaking at lower frequencies. In other words, by selecting an appropriate value of m during design, the magnitude of the peaking can be controlled to expand the operating frequency band.

[0071] <Third Modification> Next, a third modified example of the optical waveguide element according to the first embodiment of the present invention will be described. The optical waveguide element according to this modified example has a configuration similar to that of the optical modulation element 100-2 shown in Fig. 8, but furthermore, the distance (gap) between the central conductor and the ground conductor is different between the first and second action parts of the action section, and the ratio of the length of the first action part to the length of the second action part is 1:n instead of 1:2, where n is any real number.

[0072] Fig. 10 is a diagram showing the configuration of an optical modulation element 100-3 according to a third modified example. In Fig. 10, the same components as those of the optical modulation element 100-2 shown in Fig. 8 are denoted by the same reference numerals as those shown in Fig. 8, and the description of Fig. 8 above is used.

[0073] The light modulation element 100-3 has a configuration similar to that of the light modulation element 100-2 shown in FIG. 8, but includes an action portion 108-3 including a control electrode 106-3 instead of the action portion 108-2 including the control electrode 106-2.

[0074] The control electrode 106-3 includes a center electrode 106-3a and ground electrodes 106-3b and 106-3c similar to the center electrode 106-2a and ground electrodes 106b and 106c included in the control electrode 106-2.

[0075] The action portion 108-3 also includes a second action portion 108-3a, a folded portion 108-3b, and a first action portion 108-3c that have the same configurations as the second action portion 108-2a, folded portion 108-2b, and first action portion 108-2c that the action portion 108-2 includes.

[0076] However, the gap (distance) g1 between the center electrode 106-3a of the control electrode 106-3 and the ground electrodes 106-3b and 106-3c in the second action portion 108-3a and the gap g2 between the center electrode 106-3a and the ground electrodes 106-3b and 106-3c in the first action portion 108-3c are different values.

[0077] Furthermore, the length of the second action portion 108-3a is 2L, while the length of the first action portion 108-3c is L / n, where n is any real number. As with the light modulation element 100-2 shown in FIG. 8, the voltage attenuation coefficient of the control electrode 106-3 in the second action portion 108-3a is α1, and the voltage attenuation coefficient of the control electrode 106-3 in the first action portion 108-3c is α2. Such voltage attenuation coefficients can be adjusted, for example, by adjusting the thickness of the center electrode 106-3a.

[0078] Of the ends of the action portion 108-3, the upstream end along the propagation direction of the traveling wave propagating through the control electrode 106-3 is referred to as input end 108-3d, and the downstream end is referred to as end 108-3e.

[0079] In FIG. 10, the induced phase difference φ generated between the parallel waveguides 104a and 104b 2-3 is given by the following equation:

number

number

[0080] 11, 12, and 13 show simulation results of the frequency response of the optical modulation element 100-2 shown in FIG. 10, showing the frequency response when g = n = 1, 2, and 4, respectively. As in FIG. 5, the horizontal axes in FIGS. 11, 12, and 13 represent the frequency of the electrical signal input to the control electrode 106-3, and the vertical axes represent the EOE response. Also, in FIGS. 11, 12, and 13, as in FIG. 5, α1 and α2 are each proportional to the square root of the frequency of the electrical signal. Therefore, m is a parameter that does not depend on frequency.

[0081] 1, 3 and 6, and is set to comply with equation (5). At frequency f1 that provides a 3 dB bandwidth for the optical modulation element 200 of the conventional configuration shown in FIG. 2, α0 in equation (5) is adjusted so that the A value of the optical modulation element 100-3 shown in FIG. 10 (i.e., the voltage attenuation rate within the range of length L from the input end 108-3d of the second action part 108-3a) becomes 0.5.

[0082] Lines 1100, 1102, 1104, 1106, and 1108 shown in Fig. 11 represent the EOE responses when m = 2.5, 2.0, 1.5, 1.0, and 0.5, respectively, when g = n = 1. Fig. 11 shows the EOE response when g = n = 1, that is, the EOE response when the optical modulation element 100-3 shown in Fig. 10 has the same configuration as the optical modulation element 100-2 shown in Fig. 8, and therefore lines 1100, 1102, 1104, 1106, and 1108 are the same as lines 900, 902, 904, 906, and 908 shown in Fig. 9. Here, line 1106 showing the EOE response when m = 1 in Fig. 11 corresponds to line 500 shown in Fig. 5.

[0083] 12 and 13 show the EOE responses when g = n = 2 and g = n = 4, respectively. Specifically, lines 1200, 1202, 1204, 1206, and 1208 in Fig. 12 show the EOE responses when m = 2.5, 2.0, 1.5, 1.0, and 0.5, respectively, when g = n = 2. Lines 1300, 1302, 1304, 1306, and 1308 in Fig. 13 show the EOE responses when m = 2.5, 2.0, 1.5, 1.0, and 0.5, respectively, when g = n = 4.

[0084] 11, 12, and 13, the larger the value of g (or n) is set while maintaining the relationship of g = n, the less dependent the frequency response is on the value of m. In other words, the frequency response is stabilized against fluctuations in the value of m.

[0085] [Second embodiment] Next, a second embodiment of the present invention will be described. The optical modulation element according to this embodiment has a similar configuration to the optical modulation element 100 according to the first embodiment shown in Fig. 1, but differs in that the second acting portion of the acting part has a folded portion.

[0086] 14 is a diagram showing the configuration of an optical modulation element 1400 according to a second embodiment of the present invention. The optical modulation element 1400 includes an optical waveguide 1404 formed on a substrate 1402, and a control electrode 1406 that controls light waves propagating through the optical waveguide 1404. The substrate 1402 is an X-cut substrate made of, for example, LN, similar to the substrate 102 of the optical modulation element 100 shown in FIG. 1, with the Z axis pointing upward in the figure and the Y axis pointing rightward in the figure.

[0087] The optical waveguide 1404 is, for example, a Mach-Zehnder optical waveguide, similar to the optical waveguide 104 of the optical modulation element 100 shown in Fig. 1, and has a pair of parallel waveguides 1404a and 1404b. The control electrode 1406 is a traveling-wave electrode in which an electric signal (hereinafter also referred to as a modulation signal) propagates in one direction as a traveling wave.

[0088] The control electrode 1406 is provided along the extension direction of the two parallel waveguides 1404 a and 1404 b, and applies an electric field to the parallel waveguides 1404 a and 1404 b, thereby generating a phase change in the light waves propagating through these parallel waveguides 1404 a and 1404 b. As a result, for example, input light incident on the lower right end of the substrate 1402 in the optical waveguide 1404 is modulated, and the modulated light is output from the upper left end in the figure.

[0089] The control electrode 1406 is composed of a center electrode 1406a and ground electrodes 1406b and 1406c, with one end of the center electrode 1406a connected to a signal source 1410 that generates a modulation signal and the other end terminated by a terminator 1412 having a predetermined impedance. The center electrode 1406a is disposed between and along the parallel waveguides 1404a and 1404b within the plane of the substrate 102. As a result, the center electrode 1406a, together with the ground electrodes 1406b and 1406c that face each other across the parallel waveguides 1404a and 1404b, applies an electric field to the parallel waveguides 1404a and 1404b, respectively. As a result, electric fields in opposite directions are applied to the parallel waveguides 1404a and 1404b, and phase changes in opposite directions occur in the light waves propagating through the parallel waveguides 1404a and 1404b, respectively.

[0090] Here, the portion of the control electrode 1406 that controls the light waves in the parallel waveguides 1404a and 1404b, i.e., the portion that imparts a phase change to the light waves in the parallel waveguides 1404a and 1404b, constitutes an action portion 1408 (the portion within the central electrode 1406a indicated by the dashed arrow in the figure). The action portion 1408 has a first action portion 1408c (the range indicated by the dashed arrow in the figure) that is within a predetermined distance from an end 1408e, which is the downstream end along the propagation direction of the traveling wave propagating through the control electrode 1406, and a second action portion 1408a (the range indicated by the dashed arrow in the figure) that is within a predetermined distance from an input end 1408d of the electrical signal on the upstream side along the propagation direction. Here, the range of a predetermined distance refers to the range in which the sign of the phase change of the light generated in each of the parallel waveguides 1404a and 1404b is the same when a DC voltage is applied between the center electrode 1406a and the ground electrodes 1406b and 1406c.

[0091] 1 and 3, the action portion 1408 includes a turning portion 1408b (indicated by the dashed-dotted arrow in the figure) which is a first turning portion where the propagation direction of light is reversed. As a result, the control electrode 1406 and the optical waveguide 1404 are configured so that the phase change generated in the light waves of the parallel waveguides 1404a and 1404b in the first action portion 1408c of the action portion 1408 has an opposite sign to the phase change generated in the second action portion 1408a.

[0092] In particular, in the optical modulation element 1400, the second action portion 1408a of the action portion 1408 includes a turn-back portion 1408f (the range indicated by the dashed-dotted arrow in the figure), which is a second turn-back portion where the propagation direction of light is reversed. The pair of parallel waveguides 1404a and 1404b intersect at the turn-back portion 1408f. As a result, in the second action portion 1408a, the signs of the phase changes generated in the light waves of the parallel waveguides 1404a and 1404b before and after the turn-back portion 1408f are not reversed, and the control electrode 1406 induces phase changes in a fixed direction in each of the parallel waveguides 1404a and 1404b.

[0093] Therefore, in the optical modulation element 1400, as in the optical modulation element 100, a phase change in a certain direction occurs in the parallel waveguides 1404a and 1404b in the second action part 1408a of the action section 1408, and a phase change of the opposite sign to these phase changes occurs in the parallel waveguides 1404a and 1404b in the first action part 1408c.

[0094] The optical modulation element 1400 having the above configuration is configured, similarly to the optical modulation element 100, so that the phase change generated in the light waves of the parallel waveguides 1404a, 1404b in the first acting portion 1408c of the acting section 1408 has an opposite sign to the phase change generated in the second acting portion 1408a. Therefore, based on the same principle as the optical modulation element 100, the operating frequency band is expanded compared to the configuration of the conventional optical modulation element 200 shown in Figure 2.

[0095] Furthermore, in the light modulation element 1400, the second acting portion 1408a of the acting portion 1408 is configured to include the folded portion 1408f, so that the size in the left-right direction of the figure can be reduced compared to the light modulation element 100 (Figure 1) according to the first embodiment.

[0096] [Third embodiment] Next, a third embodiment of the present invention will be described. The optical modulation element according to this embodiment differs from the optical modulation element 100 according to the first embodiment shown in Fig. 1 in that the action portion does not include a folded portion and extends linearly. Two parallel waveguides forming a pair intersect in the middle of the action portion, so that the phase changes occurring in the two parallel waveguides on either side of the intersection have opposite signs.

[0097] 15 is a diagram showing the configuration of an optical modulation element 2400 according to the third embodiment of the present invention. The optical modulation element 2400 includes an optical waveguide 2404 formed on a substrate 2402, and a control electrode 2406 that controls light waves propagating through the optical waveguide 2404. The substrate 2402 is an X-cut substrate made of, for example, LN, similar to the substrate 102 of the optical modulation element 100 shown in FIG. 1, with the Z axis pointing upward in the figure and the Y axis pointing rightward in the figure.

[0098] The optical waveguide 2404 is, for example, a Mach-Zehnder optical waveguide, similar to the optical waveguide 104 of the optical modulation element 100 shown in Fig. 1, and has a pair of parallel waveguides 2404a and 2404b. Unlike the parallel waveguides 104a and 104b, the parallel waveguides 2404a and 2404b do not have any folded portions and extend linearly in the Y-axis direction.

[0099] The control electrode 2406 is composed of center electrodes 2406a-1 and 2406a-2 and ground electrodes 2406b, 2406c, and 2406d. The ground electrodes 2406b, 2406c, and 2406d are formed at a predetermined distance from the center electrodes 2406a-1 and 2406a-2. As a result, the center electrode 2406a-1, together with the ground electrodes 2406b and 2406c, constitutes a distributed parameter line, and the center electrode 2406a-2, together with the ground electrodes 2406c and 2406d, constitutes a distributed parameter line.

[0100] One end of center electrode 2406a-1 is connected to signal source 2410a that generates a modulation signal, and the other end is terminated by terminator 2412a having a predetermined impedance. Furthermore, one end of center electrode 2406a-2 is connected to signal source 2410b that generates a modulation signal, and the other end is terminated by terminator 2412b having a predetermined impedance. This allows control electrode 2406 to function as a traveling-wave electrode in which an electrical signal (hereinafter also referred to as a modulation signal) propagates in one direction as a traveling wave in each of center electrodes 2406a-1 and 2406a-2. Here, signal sources 2410a and 2410b output the same modulation signal, but the signal voltage of the modulation signal output by signal source 2410b is -Vs, which is the inverse of the signal voltage +Vs of the modulation signal output by signal source 2410a.

[0101] The center electrodes 2406a-1 and 2406a-2 are provided along the extension direction of the two parallel waveguides 2404a and 2404b, and apply an electric field to the parallel waveguides 2404a and 2404b, respectively, to generate a phase change in the light waves propagating through these parallel waveguides 2404a and 2404b. As a result, for example, input light incident on the left end of the optical waveguide 2404 on the substrate 2402 in the figure is modulated, and the modulated light is output from the right end in the figure.

[0102] Here, the portion of the control electrode 2406 that controls the light waves in the parallel waveguides 2404a and 2404b, i.e., the portion that imparts a phase change to the light waves in the parallel waveguides 2404a and 2404b, constitutes an action portion 2408 (the portion within the range indicated by the dashed arrow in the figure in the center electrode 2406a). The action portion 2408 has a first action portion 2408c (the range indicated by the dashed arrow in the figure) that is within a predetermined distance from an end 2408e, which is the downstream end along the propagation direction of the traveling wave propagating through the control electrode 2406, and a second action portion 2408a (the range indicated by the dashed arrow in the figure) that is within a predetermined distance from an input end 2408d of the electrical signal on the upstream side along the propagation direction. Here, the range of a specified distance refers to the range in which the sign of the phase change of the light generated in each of the parallel waveguides 2404a and 2404b is the same when a DC voltage is applied between the center electrodes 2406a-1 and 2406a-2 and the ground electrodes 2406b, 2406c, and 2406d.

[0103] Furthermore, the action portion 2408 has an intersection portion 2408b where the parallel waveguides 2404a and 2404b intersect with each other between the second action portion 2408a and the first action portion 2408c. As a result, the optical modulation element 2400 is configured so that the phase change generated in the parallel waveguides 2404a and 2404b by the control electrode 2406 in the first action portion 2408c has an opposite sign to the phase change in the second action portion 2408a.

[0104] Fig. 16 is a cross-sectional view taken along the arrows XVI-XVI of the light modulation element 2400 in Fig. 15, i.e., a cross-sectional view of the light modulation element 2400 at the second acting portion 2408a. Fig. 17 is a cross-sectional view taken along the arrows XVII-XVII of the light modulation element 2400 in Fig. 15, i.e., a cross-sectional view of the light modulation element 2400 at the first acting portion 2408c.

[0105] 16, in the second action part 2408a, parallel waveguides 2404a and 2404b are formed in a portion of the substrate 2402 between the center electrode 2406a-1 and the ground electrode 2406c, and in a portion of the substrate 2402 between the center electrode 2406a-2 and the ground electrode 2406d, respectively. In contrast, in the first action part 2408c, as shown in Fig. 17, parallel waveguides 2404b and 2404a are formed in a portion of the substrate 2402 between the center electrode 2406a-1 and the ground electrode 2406c, and in a portion of the substrate 2402 between the center electrode 2406a-2 and the ground electrode 2406d, respectively.

[0106] As a result, for example, in the parallel waveguide 2404a, an electric field (indicated by the dashed-dotted arrow) directed to the right in the figure, i.e., in the -Z direction, is applied to the second action portion 2408a by the center electrode 2406a-1 propagating a modulated signal of signal voltage +Vs as shown in Fig. 16, while in the first action portion 2408c, an electric field directed to the left in the figure, i.e., in the +Z direction, is applied by the center electrode 2406a-2 propagating a modulated signal of signal voltage -Vs as shown in Fig. 17. As a result, in the parallel waveguide 2404a, the phase change occurring in the first action portion 2408c has an opposite sign to the phase change occurring in the second action portion 2408a.

[0107] Similarly, in the parallel waveguide 2404b, an electric field directed to the left in the figure, i.e., in the +Z direction, is applied to the second action portion 2408a by the center electrode 2406a-2 propagating a modulated signal with a signal voltage of -Vs (FIG. 16), while in the first action portion 2408c, an electric field directed to the right in the figure, i.e., in the -Z direction, is applied by the center electrode 2406a-1 propagating a modulated signal with a signal voltage of +Vs (FIG. 17). As a result, in the parallel waveguide 2404b, the phase change occurring in the first action portion 2408c has an opposite sign to the phase change occurring in the second action portion 2408a.

[0108] The optical modulation element 2400 having the above configuration is configured, similarly to the optical modulation element 100, so that the phase change generated in the light waves of the parallel waveguides 2404a, 2404b at the first action part 2408c of the action section 2408 is opposite in sign to the phase change generated at the second action part 2408a. Therefore, based on the same principle as the optical modulation element 100, the operating frequency band is expanded compared to the configuration of the conventional optical modulation element 200 shown in Figure 2.

[0109] Furthermore, in the optical modulation element 2400, the control electrode 2406 and the parallel waveguides 2404a, 2404b that constitute the action portion 2408 are formed to extend linearly, so that no bending loss occurs in the electrical signal propagating through the control electrode 2406 and the optical wave propagating through the parallel waveguides 2404a, 2404b, and efficient optical modulation operation with little loss in the electrical signal and optical wave can be achieved.

[0110] 15, the control electrode 2406 has two center electrodes 2406a-1 and 2406a-2 to which signal voltages of +Vs and -Vs are applied, respectively, but the control electrode configuration is not limited to this. For example, the control electrode may be configured with one center electrode, similar to the control electrode 106 shown in FIG.

[0111] An optical modulation element 2400-1 shown in Fig. 18 is a modified example of the optical modulation element 2400 according to the third embodiment shown in Fig. 15, and includes a control electrode 2506 composed of one center electrode 2506a and two ground electrodes 2506b, 2506c. In Fig. 18, the same components as those in the optical modulation element 2400 shown in Fig. 15 are indicated by the same reference numerals as those in Fig. 15, and the above description of Fig. 15 is to be used.

[0112] Fig. 19 is a cross-sectional view taken along the arrows XIX-XIX of the light modulation element 2400-1 in Fig. 18, i.e., a cross-sectional view of the light modulation element 2400-1 at the second acting portion 2408a. Fig. 20 is a cross-sectional view taken along the arrows XX-XX of the light modulation element 2400-1 in Fig. 18, i.e., a cross-sectional view of the light modulation element 2400 at the first acting portion 2408c.

[0113] In the optical modulation element 2400-1, similar to the optical modulation element 2400, there is an intersection 2408b where the parallel waveguides 2404a and 2404b intersect with each other, so that the phase change generated in the parallel waveguides 2404a and 2404b by the control electrode 2506 in the first action portion 2408c has an opposite sign to the phase change in the second action portion 2408a.

[0114] Specifically, in the parallel waveguide 2404a, in the second action portion 2408a shown in Figure 19, an electric field (indicated by the dashed dotted arrow) is applied between the center electrode 2506a and the ground electrode 2506b to the left in the figure, i.e., in the +Z direction, while in the first action portion 2408c shown in Figure 20, an electric field is applied between the center electrode 2506a and the ground electrode 2506c to the right in the figure, i.e., in the -Z direction.

[0115] On the other hand, in the parallel waveguide 2404b, opposite to the parallel waveguide 2404a, an electric field directed to the right (-Z direction) in the figure is applied to the second action part 2408a shown in Figure 19, while an electric field directed to the left (+Z direction) in the figure is applied to the first action part 2408c shown in Figure 20.

[0116] As a result, in the light modulation element 2400-1, similarly to the light modulation element 2400, the operating frequency band is expanded compared to the configuration of the conventional light modulation element 200 shown in FIG. 2 due to the same principle as the light modulation element 100.

[0117] [Fourth embodiment] Next, a fourth embodiment of the present invention will be described. Similar to the optical modulation element 2400 according to the third embodiment shown in Fig. 18, the optical modulation element according to this embodiment has two parallel waveguides that form an action section and extend linearly without including any folded sections, and a control electrode that is composed of two center electrodes and three ground electrodes is also formed linearly in the action section. However, unlike the optical modulation element 2400 according to the third embodiment, the two parallel waveguides of the optical modulation element according to this embodiment do not include any intersecting sections, but each include two sections where the electric fields applied from the control electrodes are in opposite directions.

[0118] 21 is a diagram showing the configuration of an optical modulation element 2600 according to a fourth embodiment of the present invention. The optical modulation element 2600 includes an optical waveguide 2604 formed on a substrate 2602, and a control electrode 2606 that controls light waves propagating through the optical waveguide 2604. The substrate 2602 is an X-cut substrate made of, for example, LN, similar to the substrate 102 of the optical modulation element 100 shown in FIG. 1, with the Z axis pointing upward in the figure and the Y axis pointing rightward in the figure.

[0119] The optical waveguide 2604 is, for example, a Mach-Zehnder optical waveguide, similar to the optical waveguide 104 of the optical modulation element 100 shown in Fig. 1, and has a pair of parallel waveguides 2604a and 2604b. However, unlike the parallel waveguides 104a and 104b, the parallel waveguides 2604a and 2604b do not have an intersection.

[0120] The control electrode 2606 is composed of two center electrodes 2606a-1 and 2606a-2 and three ground electrodes 2606b, 2606c, and 2606d. The ground electrodes 2606b, 2606c, and 2606d are formed at predetermined distances from the center electrodes 2606a-1 and 2606a-2. As a result, the center electrode 2606a-1, together with the ground electrodes 2606b and 2606c, constitutes a distributed parameter line, and the center electrode 2606a-2, together with the ground electrodes 2606c and 2606d, constitutes a distributed parameter line.

[0121] One end of center electrode 2606a-1 is connected to signal source 2610a that generates a modulation signal, and the other end is terminated by terminator 2612a having a predetermined impedance. Furthermore, one end of center electrode 2606a-2 is connected to signal source 2610b that generates a modulation signal, and the other end is terminated by terminator 2612b having a predetermined impedance. This allows control electrode 2606 to function as a traveling-wave electrode in which an electrical signal (hereinafter also referred to as a modulation signal) propagates in one direction as a traveling wave in each of center electrodes 2606a-1 and 2606a-2. Here, signal sources 2610a and 2610b output the same modulation signal, but the signal voltage of the modulation signal output by signal source 2610b is -Vs, which is the inverse of the signal voltage +Vs of the modulation signal output by signal source 2610a.

[0122] The center electrodes 2606a-1 and 2606a-2 are linearly arranged along the extension direction of the two parallel waveguides 2604a and 2604b, and apply an electric field to the parallel waveguides 2604a and 2604b, respectively, to generate a phase change in the light waves propagating through these parallel waveguides 2604a and 2604b. As a result, for example, input light incident on the left end of the optical waveguide 2604 on the substrate 2602 as shown in the figure is modulated, and the modulated light is output from the right end as shown.

[0123] Here, the portion of control electrode 2606 that controls the light waves in parallel waveguides 2604a and 2604b, i.e., the portion that imparts a phase change to the light waves in parallel waveguides 2604a and 2604b, constitutes action portion 2608 (the portion within center electrode 2606a indicated by the dashed arrow in the figure). Action portion 2608 has a first action portion 2608c (the range indicated by the dashed arrow in the figure) that is within a predetermined distance from end 2608e, which is the downstream end along the propagation direction of the traveling wave propagating through control electrode 2606, and a second action portion 2608a (the range indicated by the dashed arrow in the figure) that is within a predetermined distance from input end 2608d of the electrical signal on the upstream side along the propagation direction. Here, the range of a specified distance refers to the range in which the sign of the phase change of the light generated in each of the parallel waveguides 2604a and 2604b is the same when a DC voltage is applied between the center electrodes 2606a-1 and 2606a-2 and the ground electrodes 2606b, 2606c, and 2606d.

[0124] In this embodiment, the two parallel waveguides 2604a and 2604b do not include any intersecting portions, but each includes two portions formed at two positions on the substrate 2602 where the electric fields applied from the control electrode 2606 are in opposite directions.

[0125] Specifically, the parallel waveguide 2604a is formed along the center electrode 2606a-1 to which the signal voltage +Vs is applied, and has a portion where the light propagation direction is changed between the second action portion 2608a and the first action portion 2608c and where the parallel waveguide 2604a intersects with the center electrode 2606a-1. As a result, the parallel waveguide 2604a is composed of a portion formed in the second action portion 2608a between the center electrode 2606a-1 and the ground electrode 2606c to which an electric field is applied downward (in the -Z direction) in the figure, and a portion formed in the first action portion 2608c between the center electrode 2606a-1 and the ground electrode 2606b to which an electric field is applied upward (in the +Z direction) in the figure.

[0126] The parallel waveguide 2604b is formed along the center electrode 2606a-2 to which a signal voltage −Vs is applied, and has a portion where the light propagation direction is changed between the second action portion 2608a and the first action portion 2608c and where the parallel waveguide 2604b intersects with the center electrode 2606a-2. As a result, the parallel waveguide 2604b is composed of a portion formed in the second action portion 2608a between the center electrode 2606a-2 and the ground electrode 2606d to which an electric field is applied upward (+Z direction) in the figure, and a portion formed in the first action portion 2608c between the center electrode 2606a-2 and the ground electrode 2606c to which an electric field is applied downward (−Z direction) in the figure.

[0127] With the above configuration, the optical modulation element 2600 is configured so that the phase change generated in the parallel waveguides 2604a and 2604b by the control electrode 2606 in the first acting portion 2608c has an opposite sign to the phase change in the second acting portion 2608a.

[0128] Fig. 22 is a cross-sectional view taken along the arrows XXII-XXII of the light modulation element 2600 in Fig. 21, i.e., a cross-sectional view of the light modulation element 2600 at the second acting portion 2608a. Fig. 23 is a cross-sectional view taken along the arrows XXIII-XXIII of the light modulation element 2600 in Fig. 21, i.e., a cross-sectional view of the light modulation element 2600 at the first acting portion 2608c.

[0129] In the parallel waveguide 2604a, an electric field is applied to the right (-Z direction) in the second action portion 2608a at a portion between the center electrode 2606a-1 and the ground electrode 2606c as shown in Fig. 22, and an electric field is applied to the left (+Z direction) in the first action portion 2608c at a portion between the center electrode 2606a-1 and the ground electrode 2606b as shown in Fig. 23. As a result, in the parallel waveguide 2604a, the phase change occurring in the first action portion 2608c has an opposite sign to the phase change occurring in the second action portion 2608a.

[0130] On the other hand, in the parallel waveguide 2604b, conversely, in the second action portion 2608a, an electric field is applied to the left in the figure (+Z direction) in the portion between the center electrode 2606a-2 and the ground electrode 2606d as shown in Fig. 22, and in the first action portion 2608c, an electric field is applied to the right in the figure (-Z direction) in the portion between the center electrode 2606a-2 and the ground electrode 2606c as shown in Fig. 23. As a result, in the parallel waveguide 2604b as well, the phase change occurring in the first action portion 2608c has an opposite sign to the phase change occurring in the second action portion 2608a.

[0131] The optical modulation element 2600 having the above configuration is configured, similarly to the optical modulation element 100, so that the phase change occurring in the light waves of the parallel waveguides 2604a, 2604b at the first acting portion 2608c of the acting section 2608 is opposite in sign to the phase change occurring at the second acting portion 2608a. Therefore, based on the same principle as the optical modulation element 100, the operating frequency band is expanded compared to the configuration of the conventional optical modulation element 200 shown in Figure 2.

[0132] Furthermore, in the optical modulation element 2600, similarly to the optical modulation element 2400 according to the third embodiment, the control electrode 2606 and the parallel waveguides 2604a and 2604b constituting the acting portion 2608 are formed to extend linearly, so that no bending loss occurs in the electrical signal propagating through the control electrode 2606 and the optical wave propagating through the parallel waveguides 2604a and 2604b, and efficient optical modulation operation with little loss in the electrical signal and optical wave can be realized. Furthermore, in the optical modulation element, the parallel waveguides 2604a and 2604b do not include any intersections with each other, so that the propagating lights do not interfere with each other, and therefore a high extinction ratio of the modulated light can be maintained, thereby achieving better optical modulation operation.

[0133] [Fifth embodiment] Next, a fifth embodiment of the present invention will be described. Similar to the optical modulation element 2600 according to the fourth embodiment shown in Fig. 21, the optical modulation element according to this embodiment is configured such that the two parallel waveguides constituting the action part extend linearly without including any intersection, and the control electrode, which is composed of two center electrodes and three ground electrodes, is also formed linearly in the action part.

[0134] However, in the optical modulation element according to this embodiment, unlike the optical modulation element 2600 according to the fourth embodiment, the two parallel waveguides do not include a portion that changes the light propagation direction, and the two center electrodes of the control electrode are each formed to change the electrical signal propagation direction between the second action portion and the first action portion and to intersect with one of the parallel waveguides. As a result, in the optical modulation element according to this embodiment, like the optical modulation element 2600 according to the fourth embodiment, the two parallel waveguides are each configured to include two portions where the electric fields applied from the control electrode are in opposite directions to each other.

[0135] Fig. 24 is a diagram showing the configuration of an optical modulation element 2700 according to a fifth embodiment of the present invention. The optical modulation element 2700 includes an optical waveguide 2704 formed on a substrate 2702, and a control electrode 2706 that controls light waves propagating through the optical waveguide 2704. The substrate 2702 is an X-cut substrate made of, for example, LN, similar to the substrate 102 of the optical modulation element 100 shown in Fig. 1, with the Z axis pointing upward in the figure and the Y axis pointing rightward in the figure.

[0136] The optical waveguide 2704 is, for example, a Mach-Zehnder optical waveguide, similar to the optical waveguide 104 of the optical modulation element 100 shown in Fig. 1, and has a pair of parallel waveguides 2704a and 2704b. Unlike the parallel waveguides 104a and 104b, the parallel waveguides 2704a and 2704b do not have an intersection.

[0137] The control electrode 2706 is composed of two center electrodes 2706a-1 and 2706a-2 and three ground electrodes 2706b, 2706c, and 2706d. The ground electrodes 2706b, 2706c, and 2706d are formed at predetermined distances from the center electrodes 2706a-1 and 2706a-2. As a result, the center electrode 2706a-1, together with the ground electrodes 2706b and 2706c, constitutes a distributed parameter line, and the center electrode 2706a-2, together with the ground electrodes 2706c and 2706d, constitutes a distributed parameter line.

[0138] One end of center electrode 2706a-1 is connected to signal source 2710a that generates a modulation signal, and the other end is terminated by terminator 2712a having a predetermined impedance. Furthermore, one end of center electrode 2706a-2 is connected to signal source 2710b that generates a modulation signal, and the other end is terminated by terminator 2712b having a predetermined impedance. This allows control electrode 2706 to function as a traveling-wave electrode in which an electrical signal (hereinafter also referred to as a modulation signal) propagates in one direction as a traveling wave in each of center electrodes 2706a-1 and 2706a-2. Here, signal sources 2710a and 2710b output the same modulation signal, but the signal voltage of the modulation signal output by signal source 2710b is -Vs, which is the inverse of the signal voltage +Vs of the modulation signal output by signal source 2710a.

[0139] The center electrodes 2706a-1 and 2706a-2 are linearly disposed along the extension direction of the two parallel waveguides 2704a and 2704b, and apply an electric field to the parallel waveguides 2704a and 2704b, respectively, to generate a phase change in the light waves propagating through these parallel waveguides 2704a and 2704b. As a result, for example, input light incident on the left end of the optical waveguide 2704 on the substrate 2702 as shown in the figure is modulated, and the modulated light is output from the right end as shown.

[0140] Here, the portion of control electrode 2706 that controls the light waves in parallel waveguides 2704 a and 2704 b, i.e., the portion that imparts a phase change to the light waves in parallel waveguides 2704 a and 2704 b, constitutes action portion 2708 (the portion within center electrode 2706 a indicated by the dashed arrow in the drawing). Action portion 2708 has first action portion 2708 c (indicated by the dashed arrow in the drawing) that is within a predetermined distance from end 2708 e, which is the downstream end along the propagation direction of the traveling wave propagating through control electrode 2706, and second action portion 2708 a (indicated by the dashed arrow in the drawing) that is within a predetermined distance from input end 2708 d of the electrical signal on the upstream side along the propagation direction. Here, the range of a predetermined distance refers to the range in which the sign of the phase change of the light generated in each of the parallel waveguides 2704a and 2704b is the same when a DC voltage is applied between the center electrodes 2706a-1 and 2706a-2 and the ground electrodes 2706b, 2706c, and 2706d.

[0141] In this embodiment, unlike the optical modulation element 2600 according to the fourth embodiment, the two parallel waveguides 2704a, 2704b do not include a portion that changes the light propagation direction, and the two center electrodes 2706a-1 and 2706a-2 of the control electrode 2706 are formed so as to change the electrical signal propagation direction between the second action portion 2708a and the first action portion 2708c and intersect with one of the parallel waveguides 2704a and 2704b. As a result, in the optical modulation element 2700 according to this embodiment, similar to the optical modulation element 2600 according to the fourth embodiment, the two parallel waveguides 2704a, 2704b are each configured to include two portions where the electric fields applied from the control electrode 2706 are in opposite directions.

[0142] Specifically, the center electrode 2706a-1, to which the signal voltage +Vs is applied, is formed along the parallel waveguide 2704a. The center electrode 2706a-1 is formed so as to change the propagation direction of the electrical signal and intersect with the parallel waveguide 2704a at the transition portion 2708b between the second action portion 2708a and the first action portion 2708c. As a result, in the parallel waveguide 2704a, an electric field is applied between the center electrode 2706a-1 and the ground electrode 2706b in the second action portion 2708a, in the upward direction (+Z direction) in FIG. 24, and an electric field is applied between the center electrode 2706a-1 and the ground electrode 2706c in the downward direction (-Z direction) in the first action portion 2708c.

[0143] On the other hand, the center electrode 2706a-2 to which the signal voltage −Vs is applied is formed along the parallel waveguide 2704b and is formed so as to change the propagation direction of the electrical signal at the transition portion 2708b and intersect with the parallel waveguide 2704b. As a result, in the parallel waveguide 2704b, an electric field is applied downward in the figure (−Z direction) between the center electrode 2706a-2 and the ground electrode 2706c at the second action portion 2708a, and an electric field is applied upward in the figure (+Z direction) between the center electrode 2706a-2 and the ground electrode 2706d at the first action portion 2708c.

[0144] As a result, the optical modulation element 2700 is configured so that the phase change induced in the parallel waveguides 2704a, 2704b by the control electrode 2706 in the first acting portion 2708c has an opposite sign to the phase change in the second acting portion 2708a.

[0145] Fig. 25 is a cross-sectional view taken along the arrows XXV-XXV of the light modulation element 2700 in Fig. 24, i.e., a cross-sectional view of the light modulation element 2700 at the second acting portion 2708a. Fig. 26 is a cross-sectional view taken along the arrows XXVI-XXVI of the light modulation element 2700 in Fig. 24, i.e., a cross-sectional view of the light modulation element 2700 at the first acting portion 2708c.

[0146] In the parallel waveguide 2704a, an electric field is applied to the left in the figure (+Z direction) between the center electrode 2706a-1 and the ground electrode 2706b in the second action portion 2708a as shown in Fig. 25, and an electric field is applied to the right in the figure (-Z direction) in the first action portion 2708c between the center electrode 2706a-1 and the ground electrode 2706c as shown in Fig. 26. As a result, in the parallel waveguide 2704a, the phase change occurring in the first action portion 2708c has an opposite sign to the phase change occurring in the second action portion 2708a.

[0147] On the other hand, in the parallel waveguide 2704b, conversely, in the second action portion 2708a, an electric field is applied to the right in the figure (-Z direction) at a portion between the center electrode 2706a-2 and the ground electrode 2706c as shown in Fig. 25, and in the first action portion 2708c, an electric field is applied to the left in the figure (+Z direction) at a portion between the center electrode 2706a-2 and the ground electrode 2706d as shown in Fig. 26. As a result, in the parallel waveguide 2704b, the phase change occurring in the first action portion 2708c has an opposite sign to the phase change occurring in the second action portion 2708a.

[0148] The optical modulation element 2700 having the above configuration is configured, similarly to the optical modulation element 100, so that the phase change generated in the light waves of the parallel waveguides 2704a, 2704b at the first acting portion 2708c of the acting section 2708 has an opposite sign to the phase change generated at the second acting portion 2708a. Therefore, based on the same principle as the optical modulation element 100, the operating frequency band is expanded compared to the configuration of the conventional optical modulation element 200 shown in Figure 2.

[0149] Furthermore, in the optical modulation element 2700, similar to the optical modulation element 2400 according to the fourth embodiment, the parallel waveguides 2704a and 2704b do not include any intersecting portions, and the propagating lights do not interfere with each other, so that the extinction ratio of the modulated light can be maintained high, thereby achieving better optical modulation operation.

[0150] [Sixth embodiment] Next, a sixth embodiment of the present invention will be described. In the optical modulation element according to this embodiment, two parallel waveguides constituting the action section are configured to extend linearly without including any folded sections. Furthermore, each of the two parallel waveguides is configured to include two sections formed so as to pass through two sections on the substrate whose polarization directions are opposite to each other.

[0151] Fig. 27 is a diagram showing the configuration of a light modulation element 2800 according to a sixth embodiment of the present invention, and Fig. 28 is a cross-sectional view taken along arrows XXVIII-XXVIII of the light modulation element 2800 in Fig. 27, i.e., a cross-sectional view of the light modulation element 2800 at a second acting portion 2808a. Also, Fig. 29 is a cross-sectional view taken along arrows XXIX-XXIX of the light modulation element 2800 in Fig. 27, i.e., a cross-sectional view of the light modulation element 2800 at a first acting portion 2808c.

[0152] The optical modulation element 2800 includes an optical waveguide 2804 formed on a substrate 2802, and a control electrode 2806 that controls the light wave propagating through the optical waveguide 2804. The substrate 2802 is an X-cut substrate made of, for example, LN, similar to the substrate 102 of the optical modulation element 100 shown in Fig. 1, with the Z axis pointing upward in Fig. 27 and the Y axis pointing rightward in the figure.

[0153] Furthermore, the substrate 2802 is a thin plate substrate processed to a thickness of 10 μm or less (e.g., 1 μm), and the optical waveguide 2804 is a convex optical waveguide, such as a rib-type optical waveguide or a ridge-type optical waveguide, which is composed of a convex portion extending on the plane of the substrate 2802.

[0154] The optical waveguide 2804 is, for example, a Mach-Zehnder optical waveguide, similar to the optical waveguide 104 of the optical modulation element 100 shown in Fig. 1, and has a pair of parallel waveguides 2804a and 2804b. The parallel waveguides 2804a and 2804b are formed linearly and do not have any crossing or turning portions.

[0155] Control electrode 2806 is composed of center electrode 2806a and two ground electrodes 2806b and 2806c. Ground electrodes 2806b and 2806c are formed at a predetermined distance from center electrode 2806a. As a result, center electrode 2806a, together with ground electrodes 2806b and 2806c, constitute a distributed constant line.

[0156] One end of the central electrode 2806a is connected to a signal source 2810 that generates a modulation signal Vs, and the other end is terminated by a terminator 2812 having a predetermined impedance. This configures the control electrode 2806 as a traveling-wave electrode in which an electrical signal (hereinafter also referred to as a modulation signal) propagates in one direction as a traveling wave.

[0157] Center electrode 2806a is provided linearly along the extension direction of two parallel waveguides 2804a and 2804b, and applies an electric field to parallel waveguides 2804a and 2804b, respectively, to generate a phase change in the light waves propagating through these parallel waveguides 2804a and 2804b. As a result, for example, input light incident on the left end of optical waveguide 2804 on substrate 2802 as shown in the figure is modulated, and the modulated light is output from the right end as shown.

[0158] Here, the portion of control electrode 2806 that controls the light waves in parallel waveguides 2804a and 2804b, i.e., the portion that imparts a phase change to the light waves in parallel waveguides 2804a and 2804b, constitutes action portion 2808 (the portion within center electrode 2806a indicated by the dashed arrow in the drawing). Action portion 2808 has a first action portion 2808c (the range indicated by the dashed arrow in the drawing) that is within a predetermined distance from end 2808e, which is the downstream end along the propagation direction of the traveling wave propagating through control electrode 2806, and a second action portion 2808a (the range indicated by the dashed arrow in the drawing) that is within a predetermined distance from input end 2808d of the electrical signal on the upstream side along the propagation direction. Here, the range of a predetermined distance refers to the range in which the sign of the phase change of the light generated in each of the parallel waveguides 2804a and 2804b is the same when a DC voltage is applied between the center electrodes 2806a-1 and 2806a-2 and the ground electrodes 2806b, 2806c, and 2806d.

[0159] In this embodiment, the parallel waveguide 2804a is located between the center electrode 2806a and the ground electrode 2806b over the entire range of the action portion 2808, and an electric field is applied in the upward direction in the figure. Also, the parallel waveguide 2804b is located between the center electrode 2806a and the ground electrode 2806c over the entire range of the action portion 2808, and an electric field is applied in the downward direction in the figure.

[0160] However, in this embodiment, the two parallel waveguides 2804a and 2804b each include two portions formed to pass through two portions of the substrate 2802 whose polarization directions are opposite to each other.

[0161] Specifically, two polarization inversion parts 2820a and 2820b (shaded parts surrounded by two dotted lines in FIGS. 27 and 29) whose polarization direction (Z-axis direction) is reversed relative to the other parts of the substrate are formed within the range of the first acting part 2808c in the substrate 2802. That is, in the parts of the substrate 2802 other than the polarization inversion parts 2820a and 2820b, the +Z direction which is the polarization direction is the upward direction in the figure, as shown in the upper part of FIG. 27, whereas within the range of the polarization inversion parts 2820a and 2820b, the polarization direction (+Z direction) is the downward direction in the figure.

[0162] In addition, in Figure 29, the thick arrows shown in the polarization inversion sections 2820a and 2820b indicate the polarization direction of substrate 2802 in polarization inversion sections 2820a and 2820b, and the thick arrows shown in parts other than polarization inversion sections 2820a and 2820b indicate the polarization direction in parts of substrate 2802 other than polarization inversion sections 2820a and 2820b.

[0163] With the above configuration, in the parallel waveguide 2804a, the electric field applied upward in the figure from the center electrode 2806a to the ground electrode 2806b becomes an electric field in the +Z direction within the range of the second action portion 2808a, and the electric field applied upward in the figure becomes an electric field in the -Z direction within the range of the first action portion 2808c where the polarization inversion portion 2820a is located. Therefore, in the parallel waveguide 2804a, the refractive index change caused in the first action portion 2808c by the control electrode 2806 is opposite in sign to the refractive index change caused in the second action portion 2808a. As a result, in the parallel waveguide 2804a, the phase change caused in the first action portion 2808c is opposite in sign to the phase change caused in the second action portion 2808a.

[0164] On the other hand, in the parallel waveguide 2804b, in the range of the second action portion 2808a, the applied electric field directed downward in the figure from the center electrode 2806a to the ground electrode 2806c becomes an electric field in the -Z direction, and in the range of the first action portion 2808c where the polarization inversion portion 2820b is located, the applied electric field directed downward in the figure becomes an electric field in the +Z direction. Therefore, in the parallel waveguide 2804b as well, the refractive index change caused in the first action portion 2808c by the control electrode 2806 is opposite in sign to the refractive index change caused in the second action portion 2808a. As a result, in the parallel waveguide 2804b as well, the phase change caused in the first action portion 2808c is opposite in sign to the phase change caused in the second action portion 2808a.

[0165] Therefore, in the optical modulation element 2800, as in the optical modulation element 100, the phase change that occurs in the light waves of the parallel waveguides 2804a, 2804b at the first acting portion 2808c of the acting section 2808 has an opposite sign to the phase change that occurs at the second acting portion 2808a, and based on the same principle as the optical modulation element 100, the operating frequency band is expanded compared to the configuration of the conventional optical modulation element 200 shown in Figure 2.

[0166] In particular, since the substrate 2802 of the optical modulation element 2800 is formed from a thin plate of 10 μm or less, an electric field generated by applying a voltage to an electrode formed on the substrate 2802 tends to concentrate within the substrate 2802. For this reason, in the optical modulation element 2800, for example, the polarization inversion portions 2820a and 2820b can be formed relatively easily by forming a pair of temporary electrodes on the substrate 2802 and applying a high voltage to the temporary electrodes before forming the control electrode 2806. Such a pair of temporary electrodes can be provided, for example, at positions on the substrate 2802 that sandwich the portions where the polarization inversion portions 2820a and 2820b are to be formed. In this case, the control electrode 2806 can be formed at a desired position, for example, after etching away the temporary electrodes on the substrate 2802.

[0167] [Seventh embodiment] Next, a seventh embodiment of the present invention will be described. The optical modulation element according to this embodiment differs from the optical modulation element 100 according to the first embodiment shown in Fig. 1 in that the action portion does not include a folded portion and extends linearly. Two parallel waveguides forming a pair intersect in the middle of the action portion, so that the phase changes occurring in the two parallel waveguides on either side of the intersecting point have opposite signs.

[0168] 30 is a diagram showing the configuration of an optical modulation element 1500 according to a seventh embodiment of the present invention. The optical modulation element 1500 includes an optical waveguide 1504 formed on a substrate 1502, and a control electrode 1506 that controls light waves propagating through the optical waveguide 1504. The substrate 1502 is a substrate that has an electro-optic effect. For example, the substrate 1502 is an LN substrate similar to the substrate 102 of the optical modulation element 100 shown in FIG. 1, but is a Z-cut substrate with a different crystal axis direction from that of the substrate 102, with the X-axis pointing downward in the figure and the Y-axis pointing rightward in the figure.

[0169] The optical waveguide 1504 is, for example, a Mach-Zehnder optical waveguide, similar to the optical waveguide 104 of the optical modulation element 100 shown in Fig. 1, and has a pair of parallel waveguides 1504a and 1504b. Unlike the parallel waveguides 104a and 104b, the parallel waveguides 1504a and 1504b do not have any folded portions and extend linearly in the Y-axis direction.

[0170] The control electrode 1506 is composed of center electrodes 1506a-1 and 1506a-2 and ground electrodes 1506b, 1506c, and 1506d. The ground electrodes 1506b, 1506c, and 1506d are formed at predetermined distances from the center electrodes 1506a-1 and 1506a-2. As a result, the center electrode 1506a-1, together with the ground electrodes 1506b and 1506c, constitutes a distributed parameter line, and the center electrode 1506a-2, together with the ground electrodes 1506c and 1506d, constitutes a distributed parameter line.

[0171] One end of center electrode 1506a-1 is connected to signal source 1510a that generates a modulation signal, and the other end is terminated by terminator 1512a having a predetermined impedance. Furthermore, one end of center electrode 1506a-2 is connected to signal source 1510b that generates a modulation signal, and the other end is terminated by terminator 1512b having a predetermined impedance. This allows control electrode 1506 to function as a traveling-wave electrode in which an electrical signal (hereinafter also referred to as a modulation signal) propagates in one direction as a traveling wave in each of center electrodes 1506a-1 and 1506a-2. Here, signal sources 1510a and 1510b output the same modulation signal, but the signal voltage of the modulation signal output by signal source 1510b is -Vs, which is the inverse of the signal voltage +Vs of the modulation signal output by signal source 1510a.

[0172] The center electrodes 1506a-1 and 1506a-2 are provided along the extension direction of the two parallel waveguides 1504a and 1504b, and apply an electric field to the parallel waveguides 1504a and 1504b, thereby generating a phase change in the light waves propagating through these parallel waveguides 1504a and 1504b. As a result, for example, input light incident on the left end of the optical waveguide 1504 on the substrate 1502 as shown in the figure is modulated, and the modulated light is emitted from the right end as shown.

[0173] Here, the portion of control electrode 1506 that controls the light waves in parallel waveguides 1504a and 1504b, i.e., the portion that imparts a phase change to the light waves in parallel waveguides 1504a and 1504b, constitutes action portion 1508 (the portion within center electrode 1506a indicated by the dashed arrow in the figure). Action portion 1508 has first action portion 1508c (the range indicated by the dashed arrow in the figure) that is within a predetermined distance from end 1508e, which is the downstream end along the propagation direction of the traveling wave propagating through control electrode 1506, and second action portion 1508a (the range indicated by the dashed arrow in the figure) that is within a predetermined distance from input end 1508d of the electrical signal on the upstream side along the propagation direction. Here, the range of a predetermined distance refers to the range in which the sign of the phase change of the light generated in each of the parallel waveguides 1504a and 1504b is the same when a DC voltage is applied between the center electrodes 1506a-1 and 1506a-2 and the ground electrodes 1506b, 1506c, and 1506d.

[0174] Furthermore, the action portion 1508 has an intersection 1508b between the second action portion 1508a and the first action portion 1508c where the parallel waveguides 1504a and 1504b intersect with each other. As a result, the optical modulation element 1500 is configured so that the phase change generated in the parallel waveguides 1504a and 1504b by the control electrode 1506 in the first action portion 1508c has an opposite sign to the phase change in the second action portion 1508a.

[0175] Fig. 31 is a cross-sectional view taken along the arrows XXXI-XXXI of the light modulation element 1500 in Fig. 30, i.e., a cross-sectional view of the light modulation element 1500 at the second acting portion 1508a. Fig. 32 is a cross-sectional view taken along the arrows XXXII-XXXII of the light modulation element 1500 in Fig. 30, i.e., a cross-sectional view of the light modulation element 1500 at the first acting portion 1508c.

[0176] As shown in Fig. 31, in the second acting portion 1508a, parallel waveguides 1504a and 1540b are formed below the center electrodes 1506a-1 and 1506a-2, respectively. In contrast, in the first acting portion 1508c, as shown in Fig. 32, parallel waveguides 1504b and 1540a are formed below the center electrodes 1506a-1 and 1506a-2, respectively.

[0177] As a result, for example, in the parallel waveguide 1504a, as shown in Fig. 31, an electric field (indicated by the dashed-dotted arrow) directed downward, i.e., in the -Z direction, is applied to the second action portion 1508a by the center electrode 1506a-1 propagating a modulated signal with a signal voltage of +Vs, while, as shown in Fig. 32, an electric field directed upward, i.e., in the +Z direction, is applied to the first action portion 1508c by the center electrode 1506a-2 propagating a modulated signal with a signal voltage of -Vs. As a result, in the parallel waveguide 1504a, the phase change occurring in the first action portion 1508c has an opposite sign to the phase change occurring in the second action portion 1508a.

[0178] Similarly, in the parallel waveguide 1504b, the second action section 1508a is applied with an electric field directed upward, i.e., in the +Z direction, by the center electrode 1506a-2, which propagates a modulated signal with a signal voltage of -Vs (FIG. 31). Meanwhile, the first action section 1508c is applied with an electric field directed downward, i.e., in the -Z direction, by the center electrode 1506a-1, which propagates a modulated signal with a signal voltage of +Vs. As a result, in the parallel waveguide 1504b, the phase change occurring in the first action section 1508c is opposite in sign to the phase change occurring in the second action section 1508a. Even without a ground electrode in 1506c, a similar effect can be achieved by performing differential operation to invert the polarity of the signal voltages input to the center electrodes 1506a-1 and 1506a-2, i.e., +Vs and -Vs, respectively. The same approach can be taken when using a Z-cut substrate.

[0179] The optical modulation element 1500 having the above configuration is configured, similarly to the optical modulation element 100, so that the phase change occurring in the light waves of the parallel waveguides 1504a, 1504b at the first acting portion 1508c of the acting section 1508 is opposite in sign to the phase change occurring at the second acting portion 1508a. Therefore, based on the same principle as the optical modulation element 100, the operating frequency band is expanded compared to the configuration of the conventional optical modulation element 200 shown in Figure 2.

[0180] Furthermore, in the optical modulation element 1500, the control electrode 1506 and the parallel waveguides 1504a and 1504b that constitute the action portion 1508 are formed to extend linearly, so that no bending loss occurs in the electrical signal propagating through the control electrode 1506 and the optical wave propagating through the parallel waveguides 1504a and 1504b, and efficient optical modulation operation with little loss in the electrical signal and optical wave can be achieved.

[0181] [Eighth embodiment] Next, an eighth embodiment of the present invention will be described. The optical modulation element according to this embodiment has a similar configuration to the optical modulation element 1500 according to the eleventh embodiment shown in Fig. 30, but the route of the parallel waveguides in the plane of the substrate 1502 is different.

[0182] Fig. 33 is a diagram showing the configuration of an optical modulation element 1800 according to the eighth embodiment of the present invention. In Fig. 33, the same components as those shown in Fig. 30 are denoted by the same reference numerals as those in Fig. 30, and the above description of Fig. 30 is used.

[0183] 30, but differs in that it has an optical waveguide 1804 instead of the optical waveguide 1504. The optical waveguide 1804 has a configuration similar to the optical waveguide 1504, but differs in that it has parallel waveguides 1804a and 1804b instead of the parallel waveguides 1504a and 1504b.

[0184] The portion of the control electrode 1506 that controls the optical waves in the parallel waveguides 1804a and 1804b, i.e., the portion that imparts a phase change to the optical waves in the parallel waveguides 1804a and 1804b, constitutes an action portion 1808 (the portion within the range indicated by the dashed arrow in the drawing in the ground electrode 1506c). The action portion 1808 has a first action portion 1808c (the range indicated by the dashed arrow in the drawing) that is within a predetermined distance from an end 1808e, which is the downstream end along the propagation direction of the traveling wave propagating through the control electrode 1506, and a second action portion 1808a (the range indicated by the dashed arrow in the drawing) that is within a predetermined distance from an input end 1808d of the electrical signal on the upstream side along the propagation direction. Here, the range of a predetermined distance refers to the range in which the sign of the phase change of the light generated in each of the parallel waveguides 1804a and 1804b is the same when a DC voltage is applied between the center electrodes 1506a-1 and 1506a-2 and the ground electrodes 1506b, 1506c, and 1506d.

[0185] 30, but there is no intersection where the parallel waveguides 1804a and 1804b intersect with each other between the second acting portion 1808a and the first acting portion 1808c. That is, the parallel waveguides 1804a and 1804b have the same configuration as the parallel waveguides 1504a and 1504b, but there is no intersection and the routes within the plane of the substrate 1502 are different.

[0186] Specifically, in the second action portion 1808a, the parallel waveguides 1804a and 1804b are formed below the center electrodes 1506a-1 and 1506a-2, similar to the optical modulation element 1500 shown in Fig. 30. However, in the first action portion 1808c, the parallel waveguides 1804a and 1804b are formed so as to pass below the ground electrode 1506b adjacent to the center electrode 1506a-1 and below the ground electrode 1506d adjacent to the center electrode 1506a-2, respectively.

[0187] As a result, the optical modulation element 1800 is configured so that the phase change occurring in the parallel waveguides 1804a and 1804b in the first acting portion 1808c has an opposite sign to the phase change occurring in the second acting portion 1808a.

[0188] The optical modulation element 1800 having the above configuration is configured so that the phase change occurring in the first acting portion 1808c of the acting part 1808 has an opposite sign to the phase change occurring in the second acting portion 1808a, and therefore, based on the same principle as the optical modulation element 100, the operating frequency band is expanded compared to the configuration of the conventional optical modulation element 200 shown in Figure 2.

[0189] Furthermore, in the optical modulation element 1800, the parallel waveguides 1804a and 1804b do not have any intersections with each other, so that optical modulation operation with even less optical loss than the optical modulation elements 100 and 1500 can be realized.

[0190] [Ninth embodiment] Next, a ninth embodiment of the present invention will be described. The optical modulation element according to this embodiment has a similar configuration to the optical modulation element 1500 according to the eleventh embodiment shown in Fig. 30, but the two parallel waveguides extend linearly without any intersection, and the shape of the control electrode within the substrate surface is different.

[0191] Fig. 34 is a diagram showing the configuration of an optical modulation element 1900 according to the ninth embodiment of the present invention. In Fig. 34, the same components as those shown in Fig. 30 are designated by the same reference numerals as in Fig. 30, and the above description of Fig. 30 is to be cited.

[0192] 30, but differs in that it has an optical waveguide 1904 and a control electrode 1906 instead of the optical waveguide 1504 and the control electrode 1506. The optical waveguide 1904 has a configuration similar to that of the optical waveguide 1504, but instead of the parallel waveguides 1504a and 1504b having an intersection, it has parallel waveguides 1904a and 1904b that each extend linearly without an intersection.

[0193] The control electrode 1906 has a configuration similar to that of the control electrode 1506, but the pattern shapes of the center electrodes 1906a-1, 1906a-1 and the ground electrodes 1906b, 1906c, 1906d within the surface of the substrate 1502 are different from those of the center electrodes 1506a-1, 1506a-1 and the ground electrodes 1506b, 1506c, 1506d of the control electrode 1506.

[0194] Here, the portion of control electrode 1906 that controls the light waves in parallel waveguides 1904a and 1904b, i.e., the portion that imparts a phase change to the light waves in parallel waveguides 1904a and 1904b, constitutes action portion 1908 (the portion within center electrode 1906a indicated by the dashed arrow in the figure). Action portion 1908 has a first action portion 1908c (the range indicated by the dashed arrow in the figure) that is within a predetermined distance from end 1908e, which is the downstream end along the propagation direction of the traveling wave propagating through control electrode 1906, and a second action portion 1908a (the range indicated by the dashed arrow in the figure) that is within a predetermined distance from input end 1908d of the electrical signal on the upstream side along the propagation direction. Here, the range of a predetermined distance refers to the range in which the sign of the phase change of the light generated in each of the parallel waveguides 1904a and 1904b is the same when a DC voltage is applied between the center electrodes 1906a-1 and 1906a-2 and the ground electrodes 1906b, 1906c, and 1906d.

[0195] In the second action portion 1908a, the center electrodes 1906a-1 and 1906a-2 are formed above the parallel waveguides 1904a and 1904b, similar to the optical modulation element 1500 shown in Figures 30 and 31. However, in this embodiment in particular, in the first action portion 1908c, the width of the ground electrode 1906c is expanded in the vertical direction in the figure, so that the ground electrode 1906c is formed above the parallel waveguides 1904a and 1904b.

[0196] As a result, the optical modulation element 1900 is configured such that the phase change generated in the parallel waveguides 1904a and 1904b by the control electrode 1906 in the first action portion 1908c has an opposite sign to the phase change in the second action portion 1908a.

[0197] The optical modulation element 1500 having the above configuration is configured, similarly to the optical modulation element 100, such that the phase change occurring in the light waves of the parallel waveguides 1504a, 1504b at the first acting portion 1508c of the acting section 1508 has an opposite sign to the phase change occurring at the second acting portion 1508a. Therefore, based on the same principle as the optical modulation element 100, the operating frequency band is expanded compared to the configuration of the conventional optical modulation element 200 shown in Figure 2.

[0198] Furthermore, in the optical modulation element 1500, the control electrode 1506 and the parallel waveguides 1504a and 1504b that constitute the acting portion 1508 are formed in a straight line, so that no bending loss occurs in the light waves propagating through the parallel waveguides 1904a and 1904b. Therefore, it is possible to realize an optical modulation operation with even less optical loss than the optical modulation elements 1500 and 1800.

[0199] [Tenth embodiment] Next, a tenth embodiment of the present invention will be described. An optical modulation element 2000 according to this embodiment differs from the optical modulation element 100 and the like described above in that the optical waveguide and the control electrode are formed on a semiconductor substrate.

[0200] FIG. 35 is a plan view showing the configuration of an optical modulation element 2000 according to this embodiment, and FIG. 36 is a cross-sectional view taken along the line XXXVI-XXXVI of the optical modulation element 2000 shown in FIG.

[0201] The optical modulation element 2000 includes an optical waveguide 2004 formed on a substrate 2002, and a control electrode 2006 that controls the light wave propagating through the optical waveguide 2004. The optical waveguide 2004 is, for example, a Mach-Zehnder optical waveguide, and includes a pair of parallel waveguides 2004a and 2004b.

[0202] Substrate 2002 is a substrate having an electro-optic effect and is made of a semiconductor such as InP, GaAs, or Si. Specifically, as shown in FIG. 36 , an n-layer 2120 and an i-layer 2124 are formed on substrate 2002, and i-layer 2124 is divided into left and right halves by trench 2126. P-type channels 2128a and 2128b are formed in the upper portions of each of the two i-layers 2124 divided into left and right halves. As a result, parallel waveguides 2004a and 2004b are formed in the two i-layers 2124 on the left and right sides of the figure, which are configured as intermediate layers of a so-called pin structure. N-layer 2120, i-layer 2124, and p-type channels 2128a and 2128b are formed so as to be embedded in cladding portion 2130 provided on substrate 2002. This cladding portion 2130 is made of a material having a lower refractive index than the i-layer 2124 in which the parallel waveguides 2004a and 2004b are formed, such as BCB (benzocyclobutene).

[0203] The semiconductor junction structure that constitutes the parallel waveguides 2004a and 2004b is not limited to the pin structure, but may be an npin structure, an nipn structure, or a pnin structure.

[0204] Control electrode 2006 includes first electrode 2006a and second electrode 2006b, which are two conductor patterns formed along parallel waveguides 2004a and 2004b, respectively. First electrode 2006a and second electrode 2006b form a distributed constant line, and the right end thereof, which faces the left end to which signal source 2010 is connected, is terminated by terminator 2012 having a predetermined impedance. As a result, an electrical signal (modulation signal) input from signal source 2010 propagates as a traveling wave on first electrode 2006a and second electrode 2006b, generating a phase change in the light waves propagating through parallel waveguides 2004a and 2004b, respectively.

[0205] As a result, for example, input light input from the left end of the optical waveguide 2004 in the drawing is modulated, and the modulated output light (modulated light) is output from the right end in the drawing.

[0206] The first electrode 2006a includes a strip-shaped first main conductor portion 2006a-1 (the portion surrounded by a dash-dotted rectangle in the figure) extending left and right along the parallel waveguide 2004a in the figure, and a plurality of T-shaped first protruding conductor portions 2006a-2 extending downward in the figure from the first main conductor portion 2006a-1 toward the parallel waveguide 2004a. Similarly, the second electrode 2006b includes a strip-shaped second main conductor portion 2006b-1 (the portion surrounded by a dash-dotted rectangle in the figure) extending left and right along the parallel waveguide 2004b in the figure, and a plurality of T-shaped second protruding conductor portions 2006b-2 extending downward in the figure from the second main conductor portion 2006b-1 toward the parallel waveguide 2004b.

[0207] Here, in Figure 35, in order to simplify the drawing and make it easier to understand, only one protruding conductor extending from each of the first main conductor portion 2006a-1 and the second main conductor portion 2006b-1 is labeled with a symbol; however, it should be understood that the entire portion protruding in a similar T-shape from the first main conductor portion 2006a-1 is the first protruding conductor portion 2006a-2, and the entire portion protruding in a similar T-shape from the second main conductor portion 2006b-1 is the second protruding conductor portion 2006b-2.

[0208] With the above configuration, the control electrode 2006 propagates an electrical signal as a traveling wave through the first main conductor portion 2006a-1 and the second main conductor portion 2006b-1. The potentials at the first main conductor portion 2006a-1 and the second main conductor portion 2006b-1 are applied to the parallel waveguides 2004a and 2004b via the multiple first protruding conductor portions 2006a-2 and the second protruding conductor portions 2006b-2, respectively, thereby generating a phase change in the light waves propagating through the parallel waveguides 2004a and 2004b. Such a control electrode configuration is described, for example, in Japanese Patent Application Laid-Open No. 2006-65085.

[0209] Here, the portion where the control electrode 2006 controls the light waves in the parallel waveguides 2004a and 2004b, i.e., the portion where it imparts a phase change to the light waves in the parallel waveguides 2004a and 2004b, constitutes an action portion 2008 (the portion indicated by the dashed arrow in the figure, shown between the parallel waveguides 2004a and 2004b in FIG. 35 ). The action portion 2008 has a first action portion 2008c (the range indicated by the dashed arrow in the figure shown on the second main conductor portion 2006b-1) that is within a predetermined distance from an end 2008e, which is the downstream end along the propagation direction of the traveling wave propagating through the control electrode 2006, and a second action portion 2008a (the range indicated by the dashed arrow in the figure) that is within a predetermined distance from an input end 2008d of the electrical signal on the upstream side along the propagation direction. Here, the range of a predetermined distance refers to the range in which the sign of the phase change of the light generated in each of the parallel waveguides 2004a and 2004b is the same when a DC voltage is applied between the first electrode 2006a and the second electrode 2006b.

[0210] In particular, in this embodiment, an intersection 2008b where the parallel waveguides 2004a and 2004b intersect with each other is provided between the second action portion 2008a and the first action portion 2008c. As a result, the control electrode 2006 and the optical waveguide 2004 are configured so that the phase change generated in the light waves of the parallel waveguides 2004a and 2004b at the first action portion 2008c of the action portion 2008 has an opposite sign to the phase change generated at the second action portion 2008a.

[0211] Furthermore, the lengths of the first acting portion 2008c and the second acting portion 2008a measured along the extension direction of the acting section 2008 are shorter for the first acting portion 2008c than for the second acting portion 2008a. That is, the length of the portion of the first acting portion 2008c that acts on the parallel waveguides 2004a and 2004b is shorter than the length of the portion of the second acting portion 2008a that acts on the parallel waveguides 2004a and 2004b.

[0212] 1, the optical modulation element 2000 having the above configuration is configured such that the control electrode 2006 and the parallel waveguides 2004a, 2004b are configured such that the phase change generated in the first action portion 2008c in the action portion 2008 has an opposite sign to the phase change generated in the second action portion 2008a. Therefore, in the optical modulation element 2000, similar to the optical modulation element 100, the amount of phase change generated in the second action portion 2008a due to conductor loss, etc., of the amount of phase change that is offset by the first action portion 2008c has frequency characteristics, and as a result, the operating frequency band realized by the entire action portion 2008 is expanded.

[0213] [Eleventh embodiment] Next, an eleventh embodiment of the present invention will be described. This embodiment is an optical waveguide device configured using any of the optical waveguide elements such as the optical modulation element 100 described above. Fig. 37 is a diagram showing the configuration of an optical modulation device 2200, which is an optical waveguide device according to this embodiment.

[0214] The optical modulation device 2200 includes an optical modulation element 2202, a housing 2204 that houses the optical modulation element 2202, an input optical fiber 2206 that introduces input light to the optical modulation element 2202 into the housing 2204, and an output optical fiber 2208 that guides output light from the optical modulation element 2202 out of the housing 2204.

[0215] Here, the light modulation element 2202 can be any of the above-mentioned light modulation elements 100, 100-1, 100-2, 100-3, 1400, 1500, 1800, 1900, and 2000 (hereinafter also referred to as light modulation element 100, etc.).

[0216] The housing 2204 is provided with a connector 2210 for inputting an electrical signal to be applied to a control electrode (not shown), which is a traveling wave electrode provided in the optical modulation element 2202. The electrical signal input from the connector 2210 is input to one end of the control electrode via a relay substrate 2212 by wire bonding or the like (not shown), and the other end of the control electrode is connected to a terminator 2214 having a predetermined impedance by wire bonding or the like (not shown).

[0217] The optical modulation device 2200 having the above configuration can achieve a modulation operation with a wider band than conventional ones by using the optical modulation element 100 and the like.

[0218] [Twelfth embodiment] Next, a twelfth embodiment of the present invention will be described. This embodiment is an optical transmission apparatus configured using an optical modulation device 2200 according to the eleventh embodiment, which uses any of the optical modulation elements 100 described above.

[0219] 38 is a diagram showing the configuration of an optical transmitting device 2300 according to this embodiment. The optical transmitting device 2300 includes an optical modulation device 2200, a light source 2302 that inputs light to the optical modulation device 2200, a modulation signal generating unit 2304, and a modulation data generating unit 2306.

[0220] The light source 2302 is, for example, a semiconductor laser. The modulation data generation unit 2306 receives transmission data provided from the outside, generates modulation data for transmitting the transmission data (for example, data obtained by converting or processing the transmission data into a predetermined data format), and outputs the generated modulation data to the modulation signal generation unit 2304.

[0221] The modulation signal generation unit 2304 is an electronic circuit (drive circuit) that outputs an electrical signal for causing the optical modulation device 2200 to perform a modulation operation, and based on the modulation data output by the modulation data generation unit 2306, generates a modulation signal, which is an electrical signal for causing the optical modulation device 2200 to perform an optical modulation operation in accordance with the modulation data, and inputs the modulation signal to the optical modulation device 2200. Here, the modulation signal generation unit 2304 corresponds to an electronic circuit that outputs an electrical signal for causing the optical waveguide device to perform a modulation operation.

[0222] The optical transmitting apparatus 2300 having the above configuration includes an optical modulation device 2200 that realizes modulation operations over a wider bandwidth than conventional ones using the optical modulation element 100 or the like, and therefore can achieve a larger transmission capacity at low cost.

[0223] The present invention is not limited to the above-described embodiment and its modified examples, and can be embodied in various forms without departing from the spirit of the present invention.

[0224] 1, 3, 6, 8, 10, 14, 15, 18, 21, 24, 27, 30, 33, 34, and 35, which show the configurations of the optical modulation elements according to the respective embodiments, the corresponding control electrodes 106 etc. are depicted as forming a distributed constant line (e.g., a coplanar transmission line (such as a GSG, GSSG, or GSGSG configuration), a microstrip line transmission line, or the like) consisting of a center electrode and a ground electrode only in the portion along the parallel waveguide, but the configuration of the control electrodes 106 etc. is not limited to this. The control electrodes 106 etc. may form a distributed constant line together with the ground electrode over the entire center electrode formed on the surface of a substrate such as the substrate 102 according to conventional technology.

[0225] In addition, in the above-described embodiments, the optical modulation element 100 using a Mach-Zehnder optical waveguide is shown as an example of an optical waveguide element, but the optical waveguide element according to the present invention is not limited to an optical modulation element. The optical waveguide element may be an element having any function that operates by generating a phase change in an optical wave propagating in an optical waveguide using a traveling-wave electrode. Such an optical waveguide element may be an optical switch element, a polarization rotation element, etc.

[0226] In the above-described embodiments, the optical waveguides 104, 1404, 1504, 1804, 1904, and 2004 may be formed by metal diffusion or impurity diffusion into the substrates 102, 1402, 1502, and 2002, respectively. However, the method for forming the optical waveguides in these substrates is not limited to metal diffusion or impurity diffusion. For example, these optical waveguides may be rib-type or ridge-type waveguides formed by protrusions formed on the surface of the substrate.

[0227] In the above-described embodiment, an LN substrate (e.g., substrate 102) or a semiconductor substrate (substrate 2002) such as InP, GaAs, or Si is used as the substrate constituting the optical waveguide element, but the material of the substrate is not limited to these. The substrate constituting the optical waveguide element according to the present invention can be made of any material, such as a ferroelectric, semiconductor, or polymer material, as long as it has an electro-optic effect. Such substrates can be, for example, inorganic crystals such as LiNbO3 (LN), Mg-doped LiNbO3 (Mg:LN), Er-doped LiNbO3 (Er:LN), LiTaO3 (LT), Mg-doped LiTaO3 (Mg:LT), Er-doped LiTaO3 (Er:LT), KTiOPO4 (KTP), KH2PO4 (KDP), NH4H2PO4 (ADP), BaTiO3 (BT or BTO), (Pb,La)(Zr,Ti)O3 (PLZT), or organic materials such as polymers containing polarized molecules, which have the Pockels effect.

[0228] Furthermore, the substrate can be made of a material having a carrier plasma effect, such as Si or SiGe, or a material having a quantum confined Stark effect, such as InGaAsP / InP or InAlGaAs / InGaAs. The substrate is not limited to the materials listed above, as long as it can achieve the above-mentioned effects, and may be made of a material in which some or all of the constituent elements are replaced with elements of the same group in the periodic table, a material in which a group IV element is replaced with an element in groups III-V, or a material doped with a trace amount of other element as an additive.

[0229] Furthermore, in the above-described embodiments, the optical waveguides 104, 1404, 1504, 1804, 1904, and 2004 are formed inside the substrates 102, 1402, 1502, and 2002, respectively, as part of the substrates. However, these optical waveguides are not limited to those formed inside the substrates as part of the substrates. For example, these optical waveguides may be made of a polymer material having an electro-optic effect provided on the surface of a substrate that does not have an electro-optic effect. An optical modulation element configured with such an optical waveguide may be a so-called silicon-organic hybrid optical modulator (see, for example, Stefan Wolf et al., "DAC-Less Amplifier-Less Generation and Transmission of QAM Signals Using Sub-Volt Silicon-Organic Hybrid Modulators," JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 33, NO. 7, APRIL 1, 2015, and JP 2016-71214 A).

[0230] Furthermore, in the above-described embodiment, as a modified example of the optical modulation element 100, the length of the first action part of the action section, the ratio m of the voltage attenuation coefficient of the control electrode in the first action part and the second action part, or the ratio g of the gap between the control electrode and the parallel waveguide in the first action part and the second action part are changed, but the length of the first action part, the ratio m of the voltage attenuation coefficient, or the ratio g of the gap can be similarly changed in the optical modulation elements 1400, 1500, 1800, 1900, and 2000 to adjust the peaking in the frequency response.

[0231] Furthermore, the characteristic configurations of the light modulation element 100 etc. according to the above-described embodiment and modified examples can be mutually applied. For example, the configuration of the control electrode 2006 having a T-shaped protrusion in the light modulation element 2000 configured with the semiconductor substrate 2002 can also be applied to the light modulation elements 100 and 1500 configured with the LN substrates 102 and 1502.

[0232] As described above, an optical waveguide element, such as optical modulator 100, includes substrate 102, which is an LN substrate having an electro-optic effect, and optical waveguide 104, which is, for example, a Mach-Zehnder optical waveguide, provided on substrate 102. Optical waveguide 104 may be disposed inside substrate 102 or on the substrate surface of substrate 102. Substrate 102 also includes control electrode 106, which is provided along parallel waveguides 104a and 104b that constitute optical waveguide 104 and acts on these parallel waveguides to generate a phase change in the light wave propagating through the parallel waveguide. Here, control electrode 106 is a traveling-wave electrode in which an electric signal propagates in one direction as a traveling wave. The control electrode 106 and the parallel waveguides 104a, 104b are configured such that, in the action section 108 where the light wave is controlled by the control electrode 106, the phase change occurring in the first action section 108c, which is within a predetermined distance from the downstream end 108e along the propagation direction of the traveling wave, has an opposite sign to the phase change occurring in the second action section 108a, which is within a predetermined distance from the upstream input end 108d of the electrical signal along the propagation direction.

[0233] According to this configuration, in an optical waveguide element using an optical waveguide formed on a substrate, the frequency characteristics can be improved easily and at low cost even for substrates having various crystal orientations.

[0234] Furthermore, the length of the portion of the control electrode 106 that acts on the parallel waveguides 104a and 104b in the first acting portion 108c is shorter than the length of the portion of the control electrode 106 that acts on the parallel waveguides 104a and 104b in the second acting portion 108a. This configuration makes it possible to expand the operating frequency band while ensuring modulation operation.

[0235] Furthermore, for example, the first acting portion 108c is configured so that the electric field applied to the parallel waveguides 104a and 104b is reversed relative to the second acting portion 108a. That is, for example, in the optical modulation element 100, the operating frequency band can be easily expanded by configuring the control electrodes 106, 1506 and the optical waveguides 104, 1504 so that the electric field applied to the parallel waveguides 104a and 104b is reversed without changing the polarization direction of the substrate 102, etc.

[0236] In the optical modulation element 100-2 described above, the control electrode 106-2 is configured so that the voltage attenuation coefficient α2 of the electrical signal in the first acting portion 108-2c is different from the voltage attenuation coefficient α1 in the second acting portion 108-2a. With this configuration, the intensity of peaking in the frequency response characteristics can be adjusted by adjusting the ratio of α1 to α2.

[0237] Furthermore, for example, the optical waveguide 104 of the optical modulation element 100 is a Mach-Zehnder optical waveguide including two paired parallel waveguides 104a and 104b, and the action section 108 is composed of the two paired parallel waveguides 104a and 104b. With this configuration, the operating frequency band of an optical waveguide element, such as an optical modulation element, composed of a Mach-Zehnder optical waveguide can be easily and inexpensively improved regardless of the crystal orientation of the substrate.

[0238] Furthermore, action section 108 has turn-back section 108b, which is a first turn-back section where the propagation direction of light is reversed, and first action section 108c is configured as the region between turn-back section 108b and end section 108e. With this configuration, by turning back at turn-back section 108b the extension direction of action section 108, which is configured of control electrode 106 and parallel waveguides 104a, 104b, in the plane of substrate 102, it is possible to easily reverse the direction of the electric field applied from control electrode 106 to parallel waveguides 104a, 104b along the plane of substrate 102.

[0239] Furthermore, in the optical modulation element 1400 described above, the second acting portion 1408a of the acting portion 1408 has a second turning portion 1408f where the propagation direction of light is reversed, and the parallel waveguides 1404a and 1404b intersect with each other at the turning portion 1408f. With this configuration, even when the second acting portion 1408a is turned back, the direction of the electric field applied to the parallel waveguides 1404a and 1404b in the second acting portion 1408a, and therefore the phase change occurring in the parallel waveguides 1404a and 1404b, can be easily maintained without being reversed. Therefore, the operating frequency band can be expanded based on the same principle as the optical modulation element 100, and the size of the optical modulation element 1400 in the longitudinal direction can be reduced by turning back the second acting portion 1408a.

[0240] Furthermore, in the optical modulation elements 1500 and 2000, the pairs of parallel waveguides 1504a, 1504b and 2004a, 2004b have intersections where they intersect with each other. The first action portions 1508c, 2008c are formed as regions between the intersections and the ends 1508e, 2008e, respectively. This configuration allows the control electrodes 1506, 2006 to be formed in a simple linear shape, thereby reducing radiation loss of electrical signals in the control electrodes 1506, 2006 and widening the operating frequency band, thereby achieving efficient wideband modulation operation.

[0241] In addition, for example, in the optical modulation element 1800, the control electrode 1806 is composed of center electrodes 1506a-1 and 1506a-2 and ground electrodes 1506b, 1506c, and 1506d formed along these center electrodes at a predetermined distance. The action portion 1808 composed of the pair of parallel waveguides 1804a and 1804b has the center electrodes 1506a-1 and 1506a-2 formed above the parallel waveguides 1804a and 1804b that form the second action portion 1808a, and the ground electrodes 1506b and 1506d formed above the parallel waveguides 1804a and 1804b that form the first action portion 1808c, respectively. According to this configuration, for example, when a Z-cut LN substrate 1502 is used, the electric field applied to the parallel waveguides 1804a and 1804b in the thickness direction of the substrate 1502 can be easily inverted between the second acting portion 1808a and the first acting portion 1808c, so that the phase change generated in the parallel waveguides 1804a and 1804b is reversed between the second acting portion 1808a and the first acting portion 1808c.

[0242] Furthermore, for example, the substrate 102 of the optical modulation element 100 is made of lithium niobate (LN) crystal, and the control electrode 106 causes the optical waveguide 104 to perform optical modulation. With this configuration, in an optical modulation element made of a substrate using LN crystal, the frequency characteristics can be improved easily and at low cost without being restricted by the crystal orientation of the substrate.

[0243] Furthermore, the substrate 102 may be made of a ferroelectric, semiconductor, or polymer material, and the optical waveguide 104 may be formed as part of the substrate 102 or may be formed of a polymer material disposed on the surface of the substrate 102. The control electrode 106 causes the optical waveguide 104 to perform optical modulation. With this configuration, the optical modulation element 100 can be formed of a variety of materials.

[0244] Moreover, the optical modulation device 2200, which is an optical waveguide device, includes any one of the optical modulation elements 100, which are optical waveguide elements, and a housing that houses the optical modulation element. With this configuration, an optical waveguide device that can achieve modulation operation over a wider band than conventional devices can be realized at low cost using an optical waveguide element that can easily and inexpensively improve frequency characteristics even on substrates with various crystal orientations.

[0245] The optical transmitter 2300 also includes an optical modulation device 2200, which is an optical waveguide device. According to this configuration, an optical transmitter with a larger transmission capacity can be realized at low cost. [Explanation of symbols]

[0246] 100, 100-1, 100-2, 100-3, 200, 1400, 1500, 1800, 1900, 2000, 2202, 2400, 2 400-1, 2600, 2700, 2800...light adjustment element, 102, 1402, 1502, 2002, 2402, 2602, 2702, 28 02…substrate, 104, 1404, 1504, 1804, 1904, 2004, 2404, 2604, 2704, 2804…optical waveguide, 104a, 104b, 1404a, 1404b, 1504a, 1504b, 1804a, 1804b, 1904a, 1904b, 2004a, 2 004b, 2404a, 2404b, 2604a, 2604b, 2704a, 2704b, 2804a, 2804b… parallel waveguides, 106, 106-1, 106-2, 106-3, 1406, 1506, 1906, 2006, 2406, 2506, 2606, 2706, 2806 …control electrodes, 106a, 106-1a, 106-2a, 106-3a, 1406a, 1506a-1, 1506a-2, 1906a-1, 1906a-2, 2406a-1, 2406a-2, 2506a, 2606a-1, 2606a-2, 2706a-1, 2706a-2, 2 806a…center electrode, 106b, 106c, 106-1b, 106-1c, 106-3b, 106-3c, 1406b, 1406c, 1506b, 1506c, 1506d, 1906b, 1906c, 1906d, 2406b, 2406c, 2406d, 2506b, 2506c 2606b, 2606c, 2606d, 2706b, 2706c, 2706d, 2806b, 2806c… Grand electrodes, 108, 108-1, 108-2, 108-3, 1408, 1508, 1808, 1908, 2008, 2408, 2608, 2708, 2808… Functions Part, 108a, 108-1a, 108-2a, 108-3a, 1408a, 1508a, 1808a, 1908a, 2008a, 2408a, 2608a, 2708a, 2808a… Second functional part, 108b, 108-1b, 108-2b, 108-3b, 1408b, 1408 f… (returning part), 108c, 108-1c, 108-2c, 108-3c, 1408c, 1508c, 1808c, 1908c, 2008c, 2408c, 2608c, 2708c, 2808c… (first function part), 108d, 108-1d, 108-2d, 108-3d, 1408d…1508d, 1808d, 1908d, 2008d, 2408d, 2608d, 2708d, 2808d...Input end, 108e, 108-1e, 108-2e, 108 -3e, 1408e, 1508e, 1808e, 1908e, 2008e, 2408e, 2608e, 2708e, 2808e...end, 110, 210, 1410, 1 510a, 1510b, 2010, 2410a, 2410b, 2610a, 2610b, 2710a, 2710b, 2810...Signal source, 112, 212, 1412, 1512a, 1512b, 2012, 2214, 2412a, 2412b, 2612a, 2612b, 2712a, 2712b, 2812...terminator, 1508b, 20 2008b...intersection, 2006a...first electrode, 2006b...second electrode, 2006a-1...first main conductor portion, 2006a-2...first protruding conductor portion, 2006b-1...second main conductor portion, 2006b-2...second protruding conductor portion, 2120...n layer, 2124...i layer, 2126...trench, 2128a, 2128b...p-type channel, 2130...cladding portion, 2 200...optical modulation device, 2204...housing, 2206...input optical fiber, 2208...output optical fiber, 2210...connector, 2212...relay board, 2300...optical transmitter, 2302...light source, 2304...modulation signal generation unit, 2306...modulation data generation unit, 2408b...intersection, 2708b...transition unit, 2820a, 2820b...polarization inversion unit.

Claims

1. A substrate; an optical waveguide disposed in or on the substrate; an electrode provided along the optical waveguide, which acts on the optical waveguide to generate a phase change in a light wave propagating through the optical waveguide; Equipped with the electrode is a traveling wave electrode, the electrode and the optical waveguide are configured such that, in an action portion where the light wave is controlled by the electrode, the phase change occurring in a first action portion within a predetermined distance from a downstream end along the propagation direction of a traveling wave of the electrical signal propagating through the electrode has an opposite sign to the phase change occurring in a second action portion within a predetermined distance from an input end of the electrical signal on the upstream side along the propagation direction; The length of the portion of the first action portion that acts on the optical waveguide is shorter than the length of the portion of the second action portion that acts on the optical waveguide, and the electrodes are composed of two center electrodes and a plurality of ground electrodes formed on the surface of the substrate so as to sandwich the two center electrodes at predetermined distances from the center electrodes, the optical waveguide is a Mach-Zehnder optical waveguide including two parallel paired waveguides, the action portion formed on the substrate and composed of the two parallel waveguides has the center electrode formed on an upper portion of each of the parallel waveguides constituting the second action portion, and the ground electrode formed on an upper portion of each of the parallel waveguides constituting the first action portion; Optical waveguide element.

2. each of the central electrodes is linear across the active portion of the parallel waveguide; the first action portion and the second action portion of each of the parallel waveguides are linear and extend along the extension direction of the center electrode, the first action portion of each of the parallel waveguides does not overlap with the center electrode in the extension direction and is spaced apart from the second action portion in a direction perpendicular to the extension direction; The optical waveguide element according to claim 1 .

3. each of the parallel waveguides is linear throughout the working portion; a portion of each of the central electrodes along the first active portion of the parallel waveguide is linear and is spaced apart from a portion of the central electrode located above the second active portion in a direction perpendicular to the extension direction of the parallel waveguide; The optical waveguide element according to claim 1 .

4. the electrodes are configured such that a voltage attenuation coefficient of the electrical signal at the first working portion is different from a voltage attenuation coefficient at the second working portion; The optical waveguide element according to claim 1 .

5. The ratio of the length of the first working portion to the length of the second working portion is r: (1+r), r=2, 1.5, 1.2, 1.0, or 0.8 have a relationship of The optical waveguide element according to claim 1 or 4.

6. The ratio of the length of the first working portion to the length of the second working portion is 1:2, When the length of the first active portion is L and the voltage attenuation coefficient of the electrical signal at the electrode is α, A = exp(-αL) The value A given by 0.12<A≦0.88 is in the range of The optical waveguide element according to claim 1 or 4.

7. The ratio of the length of the first working portion to the length of the second working portion is 1:2, When the length of the first active portion is L and the voltage attenuation coefficient of the electrical signal at the electrode is α, A = exp(-αL) The value A given by 0.27<A≦0.83 is in the range of The optical waveguide element according to claim 1 or 4.

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