JPWO2018174179A1 - Light modulator - Google Patents

Abstract

An optical modulator according to the present invention is at least partially formed on a waveguide substrate having an electro-optic effect, and is opposed to an optical waveguide having two branch optical waveguides so as to sandwich the two branch optical waveguides. The first line, the second line, and the third line, which are arranged, are electromagnetically coupled to each other and have line lengths that substantially resonate with an input high frequency signal for optical modulation. A modulation electrode including a third line conductor. In the optical modulator, the modulation electrode is configured to excite the modulation electrode by inducing a voltage having a different sign in the second line and the third line based on the high frequency signal for optical modulation. It was arranged to be.

Description

  The present invention relates to an optical modulator used for modulating light with an electric signal when transmitting a large amount of information or transmitting a wireless signal through an optical fiber.

  A system called a fiber optic radio system that directly modulates light with a radio signal, transmits the signal to a distant place using an optical fiber, returns the radio signal, and radiates the signal to eliminate radio insensitive zones and deliver radio signals to apartments and offices. Attention has been paid.

  Optical modulators are key devices in such systems. In recent years, as the frequency of radio signals has increased, optical modulators have been required to operate at high frequencies such as the millimeter wave band. However, in general, as the frequency increases, the modulation efficiency increases due to increased loss and structural problems. descend. Therefore, an optical modulator with high efficiency even in a high frequency band such as a millimeter wave band is expected. 2. Description of the Related Art Conventionally, for high-quality optical modulation using a high-frequency signal such as a millimeter wave band, an optical modulator using an electro-optic effect has been used.

  FIG. 8A is a plan view showing a configuration of a general optical modulator according to Conventional Example 1. FIG. FIG. 8B is a longitudinal sectional view of the optical modulator of FIG. 8A along the line E-E '.

  8A and 8B, the optical modulator according to Conventional Example 1 includes a substrate 51, an optical waveguide 52, a buffer layer 53, a modulation electrode 54, and ground electrodes (ground electrodes) 55 and 56. Here, the substrate 51 is a z-cut substrate of lithium niobate having an electro-optical effect. The optical waveguide 52 is formed on the surface of the substrate 51 by thermal diffusion of metallic titanium or the like, and includes an input optical waveguide 52a, a pair of phase modulation waveguides 52b and 52c branched from the input optical waveguide 52a, and an output optical waveguide in which they are merged. 52d, and constitutes a Mach-Zehnder interferometer. In order to utilize the change in the refractive index due to the electric field in the z-direction, the modulation electrode 54 is disposed directly above one phase modulation waveguide (here, the phase modulation waveguide 52b). The buffer layer 53 is a thin film for preventing a part of the light propagating through the optical waveguide 52 from being absorbed by the modulation electrode 54 and the ground electrodes 55 and 56.

  In the optical modulator according to the conventional example 1 configured as described above, when the modulation wave 59 is applied to the modulation electrode 54, as shown by the electric flux lines 151 and 152, the phase modulation waveguides 52b and 52c are vertically Since the electric fields in opposite directions are applied, the directions of the refractive index changes due to the electro-optic effect in the phase modulation waveguides 52b and 52c are opposite to each other. For this reason, the input light 57 to the input optical waveguide 52a is subjected to phase modulation in opposite directions while being split into two and propagating through the phase modulation waveguides 52b and 52c, and this propagating light is transmitted to the output optical waveguide 52d. The light intensity is modulated by interference at the time of merging, and becomes the output light 58.

  FIG. 9 is a plan view showing a configuration of an optical modulator according to Conventional Example 2 disclosed in Patent Document 1.

  9, an optical modulator according to Conventional Example 2 includes an optical path (optical waveguide) 68 having electro-optical effect characteristics, a modulation electrode 61 formed along the optical path 68 for applying an electric field to the optical path, and Common electrodes (ground electrodes) 66 and 67 formed opposite to the modulation electrode 61, and stubs 64 and 65 obliquely connected to substantially the center of the modulation electrode 61 are provided. The feed line including the transformer 63 is connected. Then, by opening the end of the modulation electrode 61, a resonance operation is performed, thereby increasing the modulation efficiency. The stubs 64 and 65 have a function for impedance matching so that the modulated wave is efficiently input to the modulation electrode 61 without being reflected.

  FIG. 10 is a longitudinal sectional view showing a configuration of an optical modulator according to Conventional Example 3 disclosed in Patent Document 2.

  In FIG. 10, the optical modulator according to Conventional Example 3 has an optical waveguide 72 having an electro-optical effect formed on a thin plate 71 having a thickness of about 10 μm, and a modulation electrode arranged so as to sandwich the thin plate 71. The modulation electrode includes a first electrode on the upper surface of the thin plate 71 and a second electrode on the lower surface of the thin plate. The first electrode includes a signal electrode 74 and a ground electrode 75a, and the second electrode functions as a ground electrode 75b. I have. The buffer layers 73a and 73b are thin films for preventing a part of the light propagating through the optical waveguide 72 from being absorbed by the modulation electrode. The thin plate 71 is bonded to a support substrate 77 via an adhesive layer 76. The feature of the optical modulator having this configuration is that, compared to the general optical modulator shown in FIG. 8A, the electric field generated by the voltage between the signal electrode 74 and the ground electrode 73b formed on the back surface of the thin plate 71 is also optical. It can be used for modulation. This improves the modulation efficiency.

  FIG. 11A is a plan view showing a configuration of an optical modulator according to Conventional Example 4 disclosed in Patent Document 3. 11B is a longitudinal sectional view taken along the line FF 'when the propagation mode M11 is excited by the parallel coupling line 33 of FIG. 11A. FIG. 11C is a longitudinal sectional view of the propagation mode M12 by the parallel coupling line 33 of FIG. 11A. FIG. 3 is a longitudinal sectional view taken along line FF ′ when is excited.

In FIG. 11A, a surface of a substrate 31 having an electro-optical effect such as a lithium tantalate (LiTaO ₃ ) single crystal or a lithium niobate (LiNbO ₃ ) single crystal is formed by a proton exchange method using benzoic acid or the like. Provided optical waveguide 32 is provided. The optical waveguide 32 is branched into two branched optical waveguides 32a and 32b at two branch points 38a and 38b, and the input light input from the entrance-side optical waveguide 32x is branched at one branch point 38a to form a second branch. After passing through one of the branch optical waveguides 32a and 32b, the other branch point 38b is configured to travel along a common exit side optical waveguide 32y.

  Further, on the substrate 31, there is provided a parallel coupling line 33 composed of three lines 33a, 33b, 33c extending along each of the branch optical waveguides 32a, 32b of the optical waveguide 32. Each inner end of each of the lines 33a and 33b is formed so as to be located immediately above a substantially central portion of each of the branch optical waveguides 32a and 32b. The line 33c is located between the two lines 33a and 33b. Both ends of each of the lines 33a, 33b, 33c are connected to each other via connection lines 36a, 36b. Further, on the substrate 31, there is provided an input line 35 connected to one line 33b of the parallel coupling line 33 for applying an input signal causing resonance in the parallel coupling line 33. Each of the lines 33a to 33c, the connection lines 36a and 36b, and the input line 35 of the parallel coupling line 33 are respectively formed of a metal film such as aluminum or gold formed by a process such as a vacuum deposition method, photolithography, and etching. ing. On the back surface of the substrate 31, a ground plane 34 formed by using a metal film deposition method or the like is provided.

  The input light is introduced from the entrance-side optical waveguide 32x, and undergoes an optical modulation action as described below when passing through the branch optical waveguides 32a and 32b. When a high-frequency signal is input from the input line 35 and resonance occurs in the lines 33a, 33b, and 33c of the parallel coupling line 33, the gaps 37a and 37b are represented by electric lines of force as shown by dotted lines in FIG. 11B. An electric field is generated. Then, due to the electro-optic effect, the refractive index of the material forming the branch optical waveguides 32a and 32b changes according to the electric field intensity. Accordingly, in the exit side optical waveguide 32y, interference between two lights passing through the branch optical waveguides 32a and 32b occurs, and the intensity of the output light changes due to the interference, thereby operating as a light intensity modulator.

  Here, in the parallel coupling line 33 having the three lines 33a to 33c, there are usually three types of propagation modes, and two propagation modes M11 and M12 among these are described with reference to FIGS. 11B and 11C. This will be described below.

  In the propagation mode M11 in FIG. 11B, the electric fields 131 and 132 in the opposite directions are applied to the two branch optical waveguides 32a and 32b, so that a phase difference occurs in the light waves and interference occurs in the exit-side optical waveguide 32y. The light modulation element of the present embodiment functions as a light intensity modulator. On the other hand, in the propagation mode M12 of FIG. 11C, the position of the branch optical waveguide 32b is shifted such that the directions of the electric fields 131 and 133 formed in the branch optical waveguides 32b and 32b are opposite to each other.

  In the propagation mode M11 of the optical modulator configured as shown in FIG. 11B, the coupling line formed by the three lines 33a, 33b, and 33c is used as a modulation electrode, so that the center of the coupling line formed by the three lines 33a, 33b, and 33c is used. In this case, highly efficient optical modulation is performed by using a propagation mode in which the line 33c of the line 33 is 0 and the voltages of the lines on both sides have opposite signs. In the propagation mode M12 of the optical modulator configured as shown in FIG. 11C, the coupling line formed by the three lines 33a, 33b, and 33c is used as the modulation electrode, so that the center of the coupling line formed by the three lines 33a, 33b, and 33c is used. The optical modulation is performed with high efficiency using a propagation mode in which the voltages between the line 33c and the lines 33a and 33b on both sides have opposite signs. In this case, the arrangement relationship between the optical waveguides 32a, 32b and the three lines 33a, 33b, 33c is slightly different from that in FIGS. 11A and 11B. More specifically, the position of the branch optical waveguide 32b is moved so as to extend below the line 33c.

JP 2002-268025 A JP 2008-89936 A JP 2006-285288 A

  However, in the optical modulator according to Conventional Example 1, as can be seen from the vertical cross-sectional view of FIG. 8B, most of the electric field induced around the modulation electrode spreads out of the optical waveguide. Also, in the electric field distribution of the optical modulator according to the conventional example 2 shown in FIG. 9, most of the electric field induced around the modulation electrode spreads out of the optical waveguide due to the structure of the electrodes, as in FIG. 8B.

  Furthermore, the optical modulator according to Conventional Example 3 has been proposed as one of the objects to improve this point. However, also in this configuration, the electric field induced between the signal electrode 74 and the ground electrode 75a is oriented in the horizontal direction, and the electric field is distributed outside the optical waveguide 72, and has nothing to do with light modulation. . Therefore, in the conventional example 3, only a part of the electric field induced by the modulation signal contributes to the light modulation. It is considered that if the electric field distribution spreading outside the optical waveguide can be reduced, the modulation efficiency can be further improved.

  Further, in the conventional examples 2 and 4, the modulation efficiency is improved by using the resonator-type modulation electrode. There is a problem to say. This phenomenon is due to the fact that the substrate operates as a dielectric waveguide, and the modulation signal propagating in the modulation electrode is coupled with the propagation wave in the substrate. In particular, in the case of a resonator-type electrode optical modulator, a modulation signal is resonated to accumulate energy in the modulation electrode to achieve high modulation efficiency. There is a problem that the modulation efficiency is remarkably reduced when the light is radiated.

  This effect becomes more remarkable as the dimension in the cross-sectional direction of the substrate is larger, the dielectric constant of the substrate is higher, and the frequency of the modulation signal is higher. However, it is expected that the future optical modulator will be used in a high frequency band such as a millimeter wave band, and the frequency of a modulated signal cannot be reduced. Lithium niobate, which is an electro-optic crystal used for a substrate, has a very high dielectric constant, but at present, no alternative material is found. Therefore, the deterioration of the modulation characteristic due to the coupling with the surface wave is a serious problem.

  An object of the present invention is to solve the above problems and to provide an optical modulator capable of performing optical modulation with higher efficiency than ultra-high-frequency signals, for example, even in a millimeter-wave band. .

An optical modulator according to one embodiment of the present invention,
An optical waveguide having at least a portion formed on a waveguide substrate having an electro-optic effect and having two branch optical waveguides;
A first line, and a second and a third line, which are arranged opposite to each other so as to sandwich the two branch optical waveguides, and which are electromagnetically coupled to each other and input to the high frequency signal for optical modulation; A modulation electrode having first to third line conductors having a line length that substantially resonates with the optical modulator,
The modulation electrode is arranged such that a voltage having a different sign is induced in the second line and the third line based on the high frequency signal for light modulation, and the modulation electrode is excited. It is characterized by the following.

  Therefore, according to the optical modulator according to the present invention, for example, even for a high-frequency signal such as a millimeter wave band, it is possible to realize an optical modulator having higher efficiency as compared with the related art.

FIG. 2 is a plan view illustrating a configuration example of the optical modulator according to the first embodiment of the present invention. FIG. 1B is a longitudinal sectional view of the optical modulator of FIG. 1A along the line A-A ′. FIG. 2 is a longitudinal sectional view showing the polarities of voltages generated in the line conductors 14, 15a, 15b and the electric field distribution in the waveguide substrate 11 in the propagation mode M1 of the optical modulator of FIGS. 1A and 1B. FIG. 2 is a longitudinal sectional view showing the polarities of voltages generated in the line conductors 14, 15a, 15b and the electric field distribution in the waveguide substrate 11 in the propagation mode M2 of the optical modulator of FIGS. 1A and 1B. FIG. 2 is a longitudinal sectional view showing the polarities of voltages generated in the line conductors 14, 15a, 15b and the electric field distribution in the waveguide substrate 11 in the propagation mode M3 of the optical modulator in FIGS. 1A and 1B. FIG. 9 is a plan view illustrating a configuration example of an optical modulator according to a second embodiment of the present invention. FIG. 3B is a longitudinal sectional view of the optical modulator of FIG. 3A taken along line B-B ′. FIG. 13 is a plan view illustrating a configuration example of an optical modulator according to a modification of the second embodiment of the present invention. FIG. 13 is a plan view illustrating a configuration example of an optical modulator according to a third embodiment of the present invention. FIG. 5B is a longitudinal sectional view of the optical modulator of FIG. 5A along the line C-C ′. FIG. 14 is a plan view illustrating a configuration example of an optical modulator according to a fourth embodiment of the present invention. FIG. 6B is a longitudinal sectional view of the optical modulator of FIG. 6A taken along line D-D ′. FIG. 4 is a graph showing electromagnetic field analysis simulation results for the optical modulators of FIGS. 1A and 1B, showing frequency characteristics of reflection loss. FIG. 9 is a plan view illustrating a configuration of an optical modulator according to Conventional Example 1. FIG. 8B is a longitudinal sectional view of the optical modulator of FIG. 8A along the line E-E ′. FIG. 9 is a plan view illustrating a configuration of an optical modulator according to Conventional Example 2. FIG. 13 is a longitudinal sectional view illustrating a configuration of an optical modulator according to Conventional Example 3. FIG. 14 is a plan view illustrating a configuration of an optical modulator according to Conventional Example 4. FIG. 11B is a longitudinal sectional view along the line F-F ′ of the optical modulator of FIG. 11A in a propagation mode M11. FIG. 11B is a longitudinal sectional view along the line F-F ′ of the optical modulator of FIG. 11A in a propagation mode M12.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiments, the same components are denoted by the same reference numerals.

Embodiment 1 FIG.
FIG. 1A is a plan view illustrating a configuration example of an optical modulator according to Embodiment 1 of the present invention. FIG. 1B is a longitudinal sectional view of the optical modulator of FIG. 1A along the line AA ′.

  1A and 1B, the optical modulator according to the first embodiment includes a waveguide substrate 11, an optical waveguide 12 provided on the waveguide substrate 11, and a buffer layer 13 provided on the upper surface of the waveguide substrate 11. , A line conductor 14, two line conductors 15 a and 15 b disposed on the lower surface of the waveguide substrate 11 with a gap 16 interposed therebetween, and a waveguide substrate 11 disposed via the line conductor 15 b and the gap 18. It comprises a line conductor 19c provided on the upper surface, a support substrate 20 which is a dielectric substrate supporting the waveguide substrate 11, and a ground conductor 21 arranged on the back surface of the support substrate 20.

  Here, each of the line conductors 14, 15a, 15b is a microstrip line having a common ground conductor 21 made of a metal thin film or the like, and the three line conductors 14, 15a, 15b are close to each other so as to be electromagnetically coupled to each other. , And thus constitutes three coupled lines that are electromagnetically coupled to each other and simultaneously operates as a modulation electrode. The line conductor 19c also forms the feed line 19, which is a microstrip line having the ground conductor 21 in common. The line conductor 19c disposed close to the line conductor 15b via the gap 18 is capacitively coupled to the line conductor 15b.

  The optical waveguide 12 is disposed at two opposing portions 17 sandwiched between both sides of the line conductor 14 of the waveguide substrate 11 and one side of the line conductor 15, and the two line conductors 15a and 15b have both ends thereof. The short-circuit portions 22 are short-circuited to each other.

The waveguide substrate 11 is made of a z-cut substrate of lithium niobate (LiNbO ₃ ) having an electro-optic effect (the z-axis in crystal engineering points in a direction perpendicular to the waveguide substrate 11). The optical waveguide 12 is formed on the surface of the waveguide substrate 11 by thermal diffusion of titanium metal or the like. The input light 25 is introduced into the optical waveguide 12, splits into two at the splitting portion 23, and after each passing through the opposing portion 17, merges at the multiplexing portion 24 and forms a Mach-Zehnder interferometer that becomes the output light 26. Make up.

  In addition, a buffer (buffer) layer 13 made of silicon oxide or the like is formed on the upper surface of the waveguide substrate 11 to suppress attenuation of light waves in the optical waveguide 12, and a metal thin film or the like is formed on the buffer layer 13. One line conductor 14 is formed. On the lower surface of the waveguide substrate 11, two line conductors 15a and 15b made of a metal thin film or the like are arranged in parallel with the line conductor 14 with a gap 16 interposed therebetween. , The inner portions (on the side of the gap 16) of the line conductors 15a and 15b are opposed to each other with the waveguide substrate 11 interposed therebetween. The waveguide substrate 11 is fixed on a support substrate 20 made of sapphire (single crystal alumina) or the like.

  In the optical modulator configured as described above, the three lines coupled as described above have three propagation modes M1, M2, and M3 in which the following voltages are generated, and propagate independently.

  FIG. 2A is a longitudinal sectional view showing the polarities of voltages generated in the line conductors 14, 15a, 15b and the electric field distribution in the waveguide substrate 11 in the propagation mode M1 of the optical modulator of FIGS. 1A and 1B. FIG. 2B is a longitudinal sectional view showing the polarities of voltages generated in the line conductors 14, 15a and 15b and the electric field distribution in the waveguide substrate 11 in the propagation mode M2 of the optical modulator of FIGS. 1A and 1B. FIG. 2C is a longitudinal sectional view showing the polarities of voltages generated in the line conductors 14, 15a, and 15b and the electric field distribution in the waveguide substrate 11 in the propagation mode M3 of the optical modulator in FIGS. 1A and 1B. 2A to 2C, illustration of the line conductor 19c is omitted.

  When the propagation mode M2 in FIG. 2B is excited, the voltage of the line conductor 14 is 0, and voltages of opposite signs are generated in the two line conductors 15a and 15b. Therefore, in the electric field distribution, strong electric fields 101 and 102 concentrated on the opposing portion 17 are generated, and the two opposing portions 17 are in opposite directions. Since the electric fields 101 and 102 are applied to the optical waveguide 12 having the electro-optic effect, the two optical waveguides 12 passing through the facing portion 17 change in refractive index in directions opposite to each other. The input light 25 to the optical waveguide 12 is split into two and propagates in the respective different optical waveguides 12, where they undergo phase modulation in opposite directions, interfere with each other in the multiplexing unit 24, modulate the light intensity, and output light. 26. The modulation signal input to the feed line 19 excites the propagation mode M2, and the propagation mode M2 repeats reflection at both ends of the modulation electrode, so that resonance occurs at a frequency determined by the length of the line conductor. Due to the resonance, the energy of the modulation signal is accumulated in the modulation electrode, the voltage amplitude of the propagation mode M2 increases, and a large voltage is induced. Thereby, highly efficient light modulation is realized.

  The electric field generated by the propagation mode M2 is generated only in a limited area of the facing portion 17, as shown in FIG. 2B. Therefore, since the energy of the input modulation signal is efficiently used for inducing the electric field, improvement in the modulation efficiency can be expected as compared with the conventional example 1 of FIG. 8B. In addition, by using the thin waveguide substrate 11, the distance between the line conductors above and below the opposing portion 17 can be reduced, so that a strong electric field is generated even at the same voltage, and the modulation efficiency can be further improved. Also, by using the thin waveguide substrate 11 and the supporting substrate 20 made of sapphire having a relatively small dielectric constant, the equivalent dielectric constant of the substrate viewed from the propagation wave in the modulation electrode can be greatly reduced. Therefore, it is possible to effectively prevent the modulated signal from leaking to the surface wave. This is particularly effective when the modulation electrode is used as a resonator as in the present embodiment.

  Further, as an advantage of using such a thin waveguide substrate 11, there is velocity matching between the propagation wave in the modulation electrode and the light wave in the optical waveguide 12. Since the dielectric constant of high frequency signals of lithium niobate, which is an electro-optic crystal, is 28 or more, the speed of the propagating wave in the modulating electrode is much slower than that of the light wave in the optical waveguide, thereby deteriorating the modulation efficiency. Was. In the optical modulator having this configuration, as described above, since the equivalent permittivity of the substrate viewed from the propagating wave in the modulation electrode can be greatly reduced, the speed between the propagating wave and the light wave can be reduced. The difference can be reduced, thereby also improving the modulation efficiency.

  Here, power supply of the modulation signal will be described. Depending on the positional relationship between the line conductor 19c and the line conductors 14, 15a, 15b, the degree of coupling between the high-frequency signal input to the feed line 19 of the line conductor 19c and the resonance mode in which the modulation electrode resonates can be adjusted. Then, by selecting a positional relationship corresponding to the optimum coupling degree, all the energy of the high-frequency signal input to the feed line 19 is converted into the resonance mode, and the efficiency is further improved. In addition, since the three propagation modes M1 to M3 have different propagation speeds, the resonance frequencies are also different. Therefore, if the length of the line conductor is adjusted so that the propagation mode M2 resonates at a desired frequency, the resonance mode of the propagation mode M2 can be selectively excited.

  Next, the positional relationship between the power supply line 19 and the modulation electrode and the degree of coupling will be described below. Here, as for the angle at which the line conductor 19c intersects with the line conductors 14, 15a, 15b, in order to obtain a practical coupling degree, as shown in FIG. Are desirably substantially orthogonal. As for the relationship between the positional relationship between the tip of the line conductor 19c and the line conductors 14, 15a, 15b and the degree of coupling, the tip of the line conductor 19c is extended so that the gap 18 is formed in the line conductor 15b on the side of the line conductor 19c. The degree of bonding can be increased by arranging them so as to cover them deeply. As for the relationship between the position of the line conductor 19c and the degree of coupling with respect to the position of the line conductor 15b in the longitudinal direction, the line conductor 19c does not necessarily have to be arranged at the center of the line conductor 15b, as shown in FIG. 1A. When the conductor 19c is arranged at the center of the line conductor 15b, the degree of coupling is the largest, and the degree of coupling can be reduced by shifting to the end.

FIG. 7 is a graph showing a result of an electromagnetic field analysis simulation with respect to the optical modulators of FIGS. 1A and 1B, showing a frequency characteristic of a reflection loss (S ₁₁ ). In FIG. 7, the horizontal axis represents the frequency (GHz) of the input high-frequency signal, and the vertical axis represents the reflection loss (dB). As is apparent from FIG. 7, the reflection is greatly reduced at the assumed resonance frequency (around 10 GHz), and almost all input signal power is coupled to the modulation electrode. Since the resonance frequency can be adjusted by the length of the line conductors 15a and 15b, it is possible to resonate at a desired frequency.

In this embodiment, a resonator structure in which both ends of the line conductors 15a and 15b are short-circuited by the short-circuit portion 22 is used as the modulation electrode, but the ends of the line conductors 15a and 15b are opened without being short-circuited, or By short-circuiting one and opening the other, a resonator structure can be formed, which is similarly effective. Further, a traveling wave type structure in which a propagation mode M2 excited by a high frequency signal without causing resonance propagates in the same direction as the light wave in the optical waveguide 12 is also effective.
On the other hand, in the propagation mode M3 shown in FIG. 2C, the potentials of the line conductors 15a and 15b are in phase with the line conductor 14, so that the electric fields 101 and 103 generated in the opposing portion 17 are also in phase as shown. Therefore, the two optical waveguides 12 undergo a change in the refractive index in the same direction, so that light cannot be modulated.

Embodiment 2. FIG.
FIG. 3A is a plan view illustrating a configuration example of an optical modulator according to Embodiment 2 of the present invention. FIG. 3B is a longitudinal sectional view of the optical modulator of FIG. 3A along the line BB ′. 3A and 3B, the same or corresponding components as those in FIGS. 1A and 1B (Embodiment 1) are denoted by the same reference numerals as those in FIGS. 1A and 1B. Descriptions of those components will be omitted unless necessary.

3A and 3B, the optical modulator according to the second embodiment is different from the optical modulator according to the first embodiment in FIGS. 1A and 1B in the following points.
(1) The line conductors 15a and 15b are provided on the upper surface of the waveguide substrate 11 and the line conductor 14 is provided on the lower surface of the waveguide substrate 11 in place of the arrangement of the line conductors 14, 15a and 15b in FIGS. 1A and 1B.
(2) On the upper surface of the waveguide substrate 11, the tip of the line conductor 19c is arranged close to the side of the line conductor 15b via the gap 18, and the line conductor 19c is capacitively coupled to the line conductor 15b. To excite the resonance mode.
Other configurations are the same as those of the optical modulator according to the first embodiment.

  Also in the optical modulator configured as described above, the mode corresponding to the propagation mode M2 in FIG. 2B in which the voltage of the line conductor 14 is 0 and the voltages of the line conductors 15a and 15b on both sides have the opposite sign is excited. As a result, an electric field whose electric lines of force are represented by 101 and 102 is generated. As a result, strong electric fields 101 and 102 in opposite directions are generated in the two opposed portions 17, and efficient light modulation can be performed based on the same principle as in the first embodiment. In this configuration, since the line conductor 19c and the line conductor 15b are on the same plane, the positional relationship between the line conductors and the interval between the gaps 18 can be created with high accuracy. Control with high accuracy.

Modification of the second embodiment.
FIG. 4 is a plan view showing a configuration example of an optical modulator according to a modification of the second embodiment of the present invention. In FIG. 4, a modification of the second embodiment is different from the second embodiment in FIG. 3A in that the gap 18 is eliminated and the line conductor 19c and the line conductor 15b are directly connected to each other, so that inductive coupling is achieved. It is characterized by using the method. In this case, the degree of coupling is determined by the position in the longitudinal direction on the line conductor 15b to which the line conductor 19c is connected. However, a higher degree of coupling can be easily obtained as compared with the capacitive coupling by the gap 18.

Embodiment 3 FIG.
FIG. 5A is a plan view illustrating a configuration example of an optical modulator according to Embodiment 3 of the present invention. FIG. 5B is a longitudinal sectional view of the optical modulator of FIG. 5A along the line CC ′. 5A and 5B, the same or corresponding components as those in Embodiment 1 of FIGS. 1A and 1B are denoted by the same reference numerals as those of FIGS. 1A and 1B. Descriptions of those components will be omitted unless necessary.

  5A and 5B, the third embodiment is different from the first embodiment in FIGS. 1A and 1B in that a hollow portion 27 is provided on a portion of the upper surface of the support substrate 20 where the line conductors 15a and 15b are provided. It is characterized by: Other than that is the same as the first embodiment. Hereinafter, the difference will be described in detail.

  As described above, by using the thin waveguide substrate 11 and the support substrate 20 having a relatively small dielectric constant, it is possible to greatly reduce the equivalent dielectric constant of the substrate viewed from the propagation wave in the modulation electrode. However, in the optical modulator according to the third embodiment, this effect can be further enhanced. That is, since the line conductors 15a and 15b come into contact with air in the hollow portion 27, the equivalent dielectric constant of the substrate as viewed from the propagating wave in the modulation electrode is further reduced, thereby further increasing the surface of the modulation signal. Suppression of wave leakage and speed matching between the propagating wave and the light wave in the modulation electrode are realized, and the modulation efficiency is further improved. If the depth of the hollow portion 27 is small, it is effective. However, if the depth is not less than the width of the gap portion 16, a great effect is exhibited.

Embodiment 4. FIG.
FIG. 6A is a plan view illustrating a configuration example of an optical modulator according to Embodiment 4 of the present invention. FIG. 6B is a longitudinal sectional view of the optical modulator of FIG. 6A along the line DD ′. 6A and 6B, the same or corresponding components as those in Embodiment 3 in FIGS. 5A and 5B are denoted by the same reference numerals as those in FIGS. 5A and 5B. Descriptions of those components will be omitted unless necessary.

  6A and 6B, the fourth embodiment is characterized in that a conductive film 28 is formed on the bottom surface of the hollow portion 27 of the third embodiment as compared with the third embodiment of FIGS. 5A and 5B. Other than that is the same as the third embodiment. Hereinafter, the difference will be described in detail.

  When the conductor film 28 is formed on the bottom surface of the hollow portion 27, the voltage difference between the line conductors 15a and 15b and the conductor film 28 causes the optical modulator to operate inside the hollow portion 27 when the optical modulator operates in the propagation mode M2. An electric field is induced. Therefore, the dielectric constant felt by the propagating wave is strongly influenced by the dielectric constant of the air in the hollow portion 27 and is reduced. Therefore, in the fourth embodiment, the propagation speed of the propagating wave in the modulation electrode is further increased as compared with the case of the third embodiment, and the speed with the light wave in the optical waveguide 12 is further matched. Matching is also possible.

  Although the electric field generated in the cavity 27 does not contribute to the light modulation, the dielectric constant of air in the cavity 27 is much smaller than that of a general waveguide substrate such as lithium niobate. The energy of the electric field generated in 27 is much smaller than the electric field energy generated in the waveguide substrate 11. Therefore, the decrease in the modulation efficiency due to the electric field generated in the hollow portion 27 is very small, and the effect of the improvement in the modulation efficiency due to the speed matching effect is greater.

  First, an example based on the configuration of the fourth embodiment will be described below.

  The interval between the optical waveguides 12 constituting the Mach-Zehnder interferometer is 50 μm, the thickness of the support substrate 20 made of sapphire single crystal is 0.5 mm, the thickness of the waveguide substrate 11 made of z-cut lithium niobate is 10 μm, The width of the line conductor 14 is 55 μm, the width of the line conductors 15 a and 15 b is 225 μm, the width of the gap 16 is 45 μm, the length of the line conductors 15 a and 15 b is 6.9 mm, and the overlap length of the gap 18. When the length of the overlap between the line conductor 15b and the tip of the line conductor 19c in the plan view is 20 μm, the depth of the hollow portion 27 is 30 μm, and the line width of the line conductor 19c is 0.1 mm, 10 GHz Near the modulation signal from the signal source having an internal impedance of 50Ω, resonance occurs in the propagation mode M2, and the input modulation signal almost completely resonates. Coupled to the mode, and also provides perfect modulation of the propagating and light waves at the modulating electrode.

  Next, desirable dimensions of each part will be described. As for the thickness of the waveguide substrate 11, a thinner one is more advantageous for improving the modulation efficiency. However, in reality, when using lithium niobate as the material of the waveguide substrate 11 and modulating light having a wavelength of 1.5 μm, the mechanical strength in the fourth embodiment and the mechanical strength in the fourth embodiment are reduced. In order to maintain the thickness, a range of 5 μm to 100 μm is highly effective, and a thickness of about 10 μm is most desirable.

  The interval between the optical waveguides 12 constituting the Mach-Zehnder interferometer is usually about 20 to 500 μm. It is desirable that the width of the facing portion 17 be about five times or less the thickness of the waveguide substrate 11. From these dimensions, the width of the gap 16 is necessarily determined. The narrower the line width of the line conductors 15a and 15b, the higher the characteristic impedance of the propagating wave and a higher electric field can be created, but on the contrary, there is an effect that the propagation loss increases. Therefore, although it depends on the thickness and material of the line conductors 15a and 15b, there is no general optimum value. If the thickness is set within a range smaller than the combined length, the effect of the present invention is exhibited.

  In the fourth embodiment, the height (depth) of the hollow portion 27 is effective if the depth is small, but a great effect is exhibited particularly if the depth is about the width of the gap portion 16. Further, in the fourth embodiment, in order to realize velocity matching between the propagation wave and the light wave in the modulation electrode, the dielectric constant and the refractive index of the waveguide substrate 11, the dielectric constant of the support substrate 20, the line conductors 14, 15a, Although it is affected by many factors such as the size of the 15b and the frequency of the modulation signal, it is effective if the width of the gap 16 is 1/10 or more.

  Further, the buffer layer 13 is formed to suppress the attenuation of the light wave in the optical waveguide 12, and is preferably a material having a lower refractive index than the waveguide substrate 11, and may be a material other than silicon oxide. The thickness of the buffer layer 13 is desirably about 1/50 to 1/10 of the wavelength of the light wave to be used, but is not necessarily required for the light modulation operation, and need not be formed.

  As a material of the waveguide substrate 11, lithium niobate or lithium tantalate, which is an electro-optic crystal and has mechanical strength, is desirable, but is not necessarily limited to these materials. Further, since only the optical waveguide 12 needs to have the electro-optic effect, a thin plate is made of another material having mechanical strength, and the optical waveguide having the electro-optic effect is partially formed therein. May be. In such a method, an organic material or a ceramic material having an electro-optic effect can be used. The support substrate 20 may be made of any material that can support the waveguide substrate 11, and is not necessarily sapphire. However, as described above, a material having a low dielectric constant is desirable, and a material having a small dielectric loss with respect to a modulation signal is desirable.

  Although the example of the fourth embodiment has been described above, when the conductive film 28 is not formed, the same can be applied to the third embodiment.

  In the first to third embodiments, it is more preferable that the support substrate 20 be made of a material having a lower dielectric constant than the waveguide substrate 11. Further, depending on circumstances, it is necessary to interpose an adhesive for bonding the support substrate 20 and the waveguide substrate 11 therebetween.

  Next, the modulation between the optical modulator having the structure of FIG. 16 (FIGS. 11A to 11C of the present application) of Patent Document 3 according to Conventional Example 4 and the optical modulator according to the fourth embodiment (FIGS. 6A and 6B). The comparison of the efficiency will be described below.

  The operating frequency (corresponding to the resonance frequency of the resonance electrode) is 10 GHz, the electrode spacing for applying an electric field to the optical waveguide is 10 μm, and the maximum electric field strength between the electrodes and the electrode length when a signal of the same input power is input. The result of estimating the modulation efficiency by the multiplied value (corresponding to the interaction length between light and modulated wave) is shown below.

  Here, a comparison of the modulation efficiency when operating between the two optical modulators was performed using commercially available electromagnetic field analysis software HFSS (manufactured by Ansys Japan). The analysis conditions were as follows: the operating frequency (corresponding to the resonance frequency) was 10 GHz; each conductor was a thick gold film having a thickness of 10 μm; and the distance between the line electrodes at the position where the waveguide was present was 10 μm. The shape is as described above in the optical modulator according to the fourth embodiment. In the optical modulator according to Conventional Example 4, the substrate 31 is assumed to be a 0.5 mm thick z-cut lithium niobate crystal, and the line widths of the lines 33a, 33b, and 33c are set to 50 μm, 50 μm, and 30 μm, respectively. . The gaps 37a and 37b are both 10 μm in order to match the conditions with the optical modulator according to the fourth embodiment. Further, the input line 35 of the fourth conventional example is connected to a position (0.8 mm from the center of the resonator electrode) where the second resonance mode can be matched at a frequency of 10 GHz.

  Table 1 below summarizes the maximum electric field strength between the electrodes where the optical waveguide is located and the resonator length when a signal of the resonance frequency is input.

  As is clear from Table 1, in the optical modulator according to the fourth embodiment, an electric field having an intensity three times or more between the electrodes is induced as compared with the conventional example 4. Further, in the optical modulator according to the fourth embodiment, the provision of the hollow portion 27 can realize speed matching between the propagation wave and the light wave in the modulation electrode, and the electrode length becomes longer than that in the conventional example 4. Here, since the modulation efficiency is proportional to the value obtained by multiplying the electric field strength and the electrode length, the theoretical modulation efficiency was estimated to be about 5.7 times that of Conventional Example 4. This indicates that the optical modulator according to the fourth embodiment has much higher efficiency than the fourth conventional example.

  Table 2 below summarizes the operating speed, modulation voltage, size, and advantages of the optical modulators according to the comparative example and the embodiment.

  In the above first to fourth embodiments, at least the following configuration is required. The line conductor 14 and the line conductors 15a and 15b are formed close to each other with the buffer layer 13, the optical waveguides 12 and 12 and the waveguide substrate 11 interposed therebetween and electromagnetically coupled to each other. A modulation electrode for modulating an optical signal propagating through the optical waveguides 12 according to a high-frequency signal input through the modulator 19 is formed. Here, the line conductors 15a and 15b are formed so as to face the two branched optical waveguides 12 and 12, respectively. Further, the line conductors 14, 15a, 15b have a line length that substantially resonates with an input high-frequency signal. Further, as described above, in order to obtain a higher modulation efficiency as compared with the related art, as shown in FIG. 2B, voltages having different signs are applied to the line conductors 15a and 15b based on the high frequency signal for optical modulation. The modulation electrode needs to be excited in the propagation mode M2 to be induced. In this case, the line conductor 14 becomes a floating electrode and is usually at 0V.

  The waveguide substrate 11 has an upper surface and a lower surface that are substantially parallel to each other. In the first, third, and fourth embodiments, the line conductor 14 is formed on the upper surface of the waveguide substrate 11 having the two branch optical waveguides 12, 12 via the buffer layer 13, while the line conductors 15a, 15b are conductive. It is formed on the lower surface of the waveguide substrate 11. In the second embodiment, the line conductors 15a and 15b are formed on the upper surface of the waveguide substrate 11 having the two branch optical waveguides 12 and 12 via the buffer layer 13, while the line conductor 14 is formed on the waveguide substrate. 11 is formed on the lower surface. Further, the line conductor 19c may be formed so as to be electromagnetically, capacitively, or directly coupled to any of the line conductors 14, 15a, 15b. The conductor film 28 may be formed so as to face the line conductors 15a and 15b at least in a part of the hollow portion 27.

  As described in detail, according to the optical modulator according to the present invention, for example, even for a high-frequency signal such as a millimeter-wave band, an electro-optical optical modulator having higher efficiency than the related art can be realized.

11: waveguide substrate,
12 ... optical waveguide,
13 ... buffer layer,
14, 15a, 15b ... line conductor,
16 gaps,
17 ... facing part,
18 gaps,
19 ... feeding line,
19c ... line conductor,
20 ... Support substrate,
21 ... Ground conductor,
22 ... short circuit part,
23 ... branch part,
24 ... multiplexing part,
25 ... input light,
26 output light,
27 ... hollow part,
28 ... conductor film,
101, 102, 103 ... electric field,
M1, M2, M3... Propagation modes.

Claims (8)

An optical waveguide having at least a portion formed on a waveguide substrate having an electro-optic effect and having two branch optical waveguides;
A first line, and a second and a third line, which are arranged opposite to each other so as to sandwich the two branch optical waveguides, and which are electromagnetically coupled to each other and input to the high frequency signal for optical modulation; A modulation electrode having first to third line conductors having a line length that substantially resonates with the optical modulator,
The modulation electrode is arranged such that a voltage having a different sign is induced in the second line and the third line based on the high frequency signal for light modulation, and the modulation electrode is excited. An optical modulator, comprising: The waveguide substrate has first and second surfaces;
The first line is formed on a first surface of the waveguide substrate;
The optical modulator according to claim 1, wherein the second and third lines are formed on a second surface of the waveguide substrate.   The optical modulator according to claim 1, further comprising a support substrate that supports the waveguide substrate and the modulation electrode. Forming a ground conductor on a surface of the support substrate opposite to a surface facing the first to third lines;
The optical modulator according to claim 3, wherein the first to third lines are configured as microstrip lines.   The optical modulator according to claim 3, wherein the support substrate is disposed on a second surface of the waveguide substrate.   The optical modulator according to claim 5, further comprising a hollow portion formed in the support substrate so as to include the second surface of the waveguide substrate.   7. The optical modulator according to claim 6, further comprising a conductor film formed in a part of the hollow portion so as to face the second and third lines.   A power supply line for inputting the optical modulation high-frequency signal, the power supply line being formed to be electromagnetically, capacitively, or directly coupled to any of the first to third lines; The optical modulator according to any one of claims 1 to 7, comprising:

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