JP6394244B2 - Light control element - Google Patents

Description

  The present invention relates to an optical control element, and more particularly to an integrated optical control element in which a plurality of waveguide optical elements are integrated on the same substrate.

  In recent years, in the fields of optical communication and optical measurement, a waveguide type optical element such as an optical modulator in which an optical waveguide is formed on a substrate having an electro-optic effect is often used. A waveguide-type optical element generally includes a control electrode for controlling a light wave propagating through the optical waveguide together with the optical waveguide.

As such a waveguide type optical element, for example, a Mach-Zehnder type optical modulator using a ferroelectric crystal lithium niobate (LiNbO ₃ ) (also referred to as “LN”) as a substrate is widely used. The Mach-Zehnder type optical modulator includes an incident waveguide for introducing light from the outside, a branching unit for propagating the light introduced by the incident waveguide in two paths, and a branching unit after the branching unit. Mach-Zehnder type optical waveguide composed of two parallel waveguides for propagating each of the generated light and an output waveguide for combining the light propagated through the two parallel waveguides and outputting them to the outside Is provided. In addition, the Mach-Zehnder optical modulator includes a control electrode for applying a voltage to change the phase of the light wave propagating in the parallel waveguide using the electro-optic effect. The control electrode generally includes an RF (high frequency) signal electrode (hereinafter referred to as an “RF electrode”) formed on or near the parallel waveguide, and a ground electrode spaced apart from the RF electrode. It consists of and.

  In a Mach-Zehnder optical modulator using LN as a substrate, the optical output characteristics with respect to the applied voltage shift due to the so-called DC drift phenomenon and temperature drift phenomenon. For example, distortion occurs in the optical modulation waveform output from the modulator. Changes in modulation characteristics (for example, deterioration in waveform quality) may occur.

  As a method for preventing a change in modulation characteristics due to such a drift phenomenon, in addition to an RF electrode and a ground electrode for applying a high-frequency signal voltage, an electrode for applying a DC voltage (hereinafter referred to as “DC bias”). There is known a method in which a voltage shift amount due to a drift phenomenon is compensated by disposing an electrode (referred to as “electrode”) on or near the parallel waveguide (Patent Document 1).

  Further, in the above configuration, in order to detect the voltage shift amount and perform feedback control to the DC voltage applied to the DC bias electrode, the amplitude is small enough not to affect the quality of the modulated light output from the optical modulator, and It is known to superimpose a low-frequency dither signal and control the relative potential difference (hereinafter referred to as “DC bias voltage”) of a DC voltage applied between two parallel waveguides (Patent Document 1). ).

  Further, the DC bias voltage may have a high potential on any parallel waveguide side. For this reason, a push-pull operation may be performed in which one potential of the DC voltage applied to the two parallel waveguides (and thus two DC bias electrodes) is set to a positive potential and the other potential is set to a negative potential. In this case, the DC power supply voltage required for the required DC bias voltage can be reduced.

  Incidentally, in recent years, in the field of optical communication, in order to meet the needs for large-capacity high-speed optical transmission, a multi-level modulation method using phase modulation has been studied and partially commercialized. For example, in an optical modulation system called QPSK (Quadrature Phase Shift Keying) or 16 QAM (Quadrature Amplitude Modulation), a nested type light composed of a plurality of Mach-Zehnder optical modulators is used. A modulator is used.

  In order to reduce the size of such a nested optical modulator, it is conceivable to integrate a plurality of Mach-Zehnder optical modulators on the same substrate. However, when a plurality of Mach-Zehnder optical modulators are close to each other on the same substrate, the DC bias electrode of one Mach-Zehnder optical modulator is close to the DC bias electrode of another Mach-Zehnder optical modulator. In this case, for example, when a potential difference exists between the DC bias electrode of one Mach-Zehnder type optical modulator and the DC bias electrode of another Mach-Zehnder type optical modulator, the electric current passing through the inside of the substrate between the DC bias electrodes. Since the force lines cannot be ignored, an appropriate DC bias voltage is not generated in each Mach-Zehnder type optical modulator, and the modulation operation becomes unstable.

  In addition, the influence of radiation noise through the space between the DC bias electrodes of adjacent Mach-Zehnder optical modulators cannot be ignored.

  In the case where a plurality of Mach-Zehnder type optical modulators are formed on the same substrate, as a method for preventing unstable phenomenon of modulation operation due to interference between DC bias electrodes of adjacent Mach-Zehnder type optical modulators, conventionally adjacent There are known a method of forming the DC bias electrodes at a predetermined distance or more, a method of making the frequency of the dither signal of each Mach-Zehnder type optical modulator different, and a method of reducing the amplitude of the dither signal as much as possible.

  As another method for preventing the unstable phenomenon of the modulation operation due to the interference between the DC bias electrodes, conventionally, a method of providing a groove in the substrate portion between the DC bias electrodes of adjacent Mach-Zehnder optical modulators (Patent Document) 2) and a method of providing a ground pattern (Patent Document 3) are known.

  However, in the method of forming adjacent Mach-Zehnder type optical modulators with DC bias electrodes separated by a predetermined distance or more, the size of the light control element in which the plurality of Mach-Zehnder type optical modulators are integrated is increased. In addition, with the increase in size, the number of light control elements that can be manufactured on one substrate wafer is reduced, resulting in an increase in cost. Further, the method of limiting the frequency and amplitude of the dither signal has a problem that the control circuit for driving the optical modulator becomes complicated and the control accuracy becomes severe.

  In addition, the method of providing a groove in the substrate portion between the optical modulators and the method of providing a ground pattern can alleviate interference through electric lines of force that pass through the substrate, but by electromagnetic radiation through the space described above. The interference cannot be sufficiently suppressed. In particular, in applications requiring high modulation accuracy, such as multi-level modulation such as xPSK and QAM, analog modulation, and high-purity signal generation for measurement applications, interference of the bias control signal is a serious problem. For example, when an optical two-tone signal is generated using a modulator having a nested Mach-Zehnder interferometer (Patent Document 4), the bias voltage of the main Mach-Zehnder modulator is 0.01% from the optimum state due to interference of the DC bias signal ( Even if the half-wave voltage ratio is changed, an unnecessary optical carrier component is increased by 6 dB or more.

JP-A-5-224163 JP 2009-053444 A JP 2011-028014 A International Publication No. 2013/047829 (A1) Pamphlet

  Based on the above background, in a light control element in which a plurality of waveguide type optical elements (for example, optical modulators) equipped with DC bias electrodes are integrated, interference between adjacent DC bias electrodes is effectively prevented and the degree of integration is improved. Realization of a configuration that can be performed is desired.

One aspect of the present invention is a control element having a substrate having an electro-optic effect and a plurality of optical waveguides formed on the substrate, and an RF electrode for controlling a light wave propagating through the optical waveguide And a first ground electrode spaced apart from the RF electrode, and a DC bias electrode for applying a DC voltage, the second and third sides on both sides of the DC bias electrode in the extending direction. The second and third ground electrodes are connected so as to cover the DC bias electrode above the substrate through a space.
According to another aspect of the present invention, at least one of the plurality of waveguide type optical elements formed on the substrate is an optical modulator including a Mach-Zehnder type optical waveguide.
According to another aspect of the invention, the substrate is X-cut or Z-cut lithium niobate.
According to another aspect of the present invention, the substrate has a thickness of 50 μm or less, and a surface of the substrate that faces the surface on which the waveguide optical element is formed has a lower dielectric constant than the substrate. Configured to contact material or air with
According to another aspect of the present invention, the first, second, and third ground electrodes are formed in the same step as the step of forming the RF electrode, and are formed by a plating method.
According to another aspect of the present invention, the second and third ground electrodes are formed so as to straddle the DC bias electrodes by sequentially laminating a plurality of metal films having openings having different widths on the substrate. Formed.
According to another aspect of the present invention, the second and third ground electrodes are provided on the substrate so as to run in parallel with the DC bias electrodes at a predetermined distance from the DC bias electrodes. And a substantially plate-like conductive member.
In another aspect of the invention, the joining is performed by flip chip bonding.
According to another aspect of the present invention, the conductive member has a V-groove formed so as to straddle the DC bias electrode.
According to another aspect of the invention, the conductive member is a plate of semiconductor material.
According to another aspect of the present invention, the size of the portion where the second and third ground electrodes straddle each DC bias electrode is the same as the portion where the second and third ground electrodes contact the substrate and the DC. It is determined so that creeping discharge does not occur on the surface of the substrate between the bias electrode.
According to another aspect of the present invention, the dimension is further set such that the light propagating through the optical waveguide is not absorbed by the portion where the second and third ground electrodes are in contact with the substrate to cause loss. Determined.

It is a top view which shows the structure of the light control element which concerns on the 1st Embodiment of this invention. It is an AA cross-sectional arrow view of the light control element shown in FIG. It is sectional drawing which shows the example of the ground electrode formed by laminating | stacking a metal layer. It is sectional drawing which shows the example of the ground electrode formed by joining of a conductive member. It is sectional drawing which shows the example of the ground electrode using the silicon plate in which V groove | channel was formed. It is a top view which shows the structure of the light control element which concerns on the 2nd Embodiment of this invention. FIG. 7 is a BB cross-sectional view of the light control element shown in FIG. 6.

  Embodiments of the present invention will be described below with reference to the drawings.

[First Embodiment]
FIG. 1 is a plan view showing the configuration of the light control element according to the first embodiment of the present invention.
The present light control element 100 is, for example, an integrated optical modulator, and two Mach-Zehnder optical modulators 110a and 110b (parts shown by thin broken lines) as a plurality of waveguide optical elements on the same substrate 102. Is formed.

  The light control element 100 performs, for example, QPSK modulation, and lights having the same wavelength and different phases by 90 degrees with respect to each other are incident on the optical modulators 110a and 110b, respectively. Modulated and the combined light of these modulated lights is output as QPSK modulated light.

  The optical modulator 110a includes an incident waveguide 120a for introducing light from the outside, two parallel waveguides 122a and 124a for propagating the light introduced by the incident waveguide 120a in two paths, And an output waveguide 126a for combining and outputting the light propagated through the parallel waveguides 122a and 124a (each waveguide is indicated by a thick broken line). In addition, the optical modulator 110a includes bias electrodes 130a and 132a (both indicated by dotted lines) provided over a predetermined length in adjacent portions of the parallel waveguides 122a and 124a. Furthermore, the optical modulator 110a includes an RF electrode 134a provided over a predetermined length at a position on the parallel waveguide 122a at a predetermined distance from the bias electrode 130a.

  That is, the bias electrodes 130a and 132a and the RF electrode 134a constitute a control electrode for controlling light propagating through the waveguide (particularly the parallel waveguides 122a and 124a) included in the optical modulator 110a. The lights controlled in the parallel waveguides 122a and 124a by these control electrodes are combined in the output waveguide 126a to become modulated light. In FIG. 1, the bias electrodes 130a and 132a and the RF electrode 134a are connected to lead lines drawn in the vertical direction in the drawing, and are connected to an external control circuit via the lead lines.

  Similarly, the optical modulator 110b includes an incident waveguide 120b for introducing light from the outside, two parallel waveguides 122b for propagating the light introduced by the incident waveguide 120b in two paths, and 124b, and an output waveguide 126b for combining and outputting the light propagated through the parallel waveguides 122b and 124b. In addition, the optical modulator 110b includes bias electrodes 130b and 132b (both indicated by dotted lines) provided over a predetermined length in adjacent portions of the parallel waveguides 122b and 124b. Further, the optical modulator 110b includes an RF electrode 134b provided over a predetermined length on the parallel waveguide 122b at a position away from the bias electrode 130b by a predetermined distance.

  That is, the bias electrodes 130b and 132b and the RF electrode 134b constitute a control electrode that controls light propagating through the waveguide portion (particularly, the parallel waveguides 122b and 124b) of the optical modulator 110b. The lights controlled by the electrodes in the parallel waveguides 122b and 124b are combined in the output waveguide 126b to become modulated light. In FIG. 1, the bias electrodes 130b and 132b and the RF electrode 134b are connected to lead lines drawn in the vertical direction in the drawing, and are connected to an external control circuit via the lead lines.

  In addition, the light control element 100 combines the modulated light from the output waveguide 126a of the optical modulator 110a and the modulated light from the output waveguide 126b of the optical modulator 110b and outputs the output to the outside. A waveguide 128 is provided.

The substrate 102 is, for example, an LN (lithium niobate, LiNbO ₃ ) substrate, and each of the waveguides 120a to 126a, 120b to 126b, and 128 has a desired waveguide pattern on the substrate 102 by a technique such as photolithography and sputtering. The metal titanium (Ti) deposited to form the film can be formed by thermally diffusing into the substrate 102. The formation of the waveguide on the substrate 102 using LN is not limited to the Ti thermal diffusion method described above, and can be performed by other known methods such as a proton exchange method. In addition to the above LN, the substrate 102 has an electro-optic constant capable of inducing a required refractive index change by applying an electric field, such as lithium tantalate, PLZT (lead lanthanum zirconate titanate), and an electro-optic polymer. It can be used as a material.

  The bias electrodes 130a, 132a, 130b, and 132b and the RF electrodes 134a and 134b can be metal electrodes formed of a metal such as Au or Al with Ti or Cr as a base, for example.

In this embodiment, the LN substrate used as the substrate 102 is a Z-cut substrate, and it is necessary to apply an electric field in the substrate depth direction to the waveguide. Therefore, the bias electrodes 130a and 132a, the RF electrode 134a, and The bias electrodes 130b and 132b and the RF electrode 134b are formed directly above the corresponding parallel waveguides 122a, 124a, 122b, or 124b via the buffer layer 104 (see FIG. 2). Buffer layer 104, the light propagating through the waveguide (122a, etc.) is provided for preventing the receiving loss is absorbed by the metal of the upper electrode (130a, etc.), for example a dielectric such as SiO ₂ Consists of the body. In addition, in order to reduce bias drift (so-called DC drift phenomenon) that gradually occurs in the long term, the SiO _{2 of the} buffer layer 104 is usually doped with impurities such as In, Ti, Sn, etc. to reduce the electric resistance value. (For example, refer to Japanese Patent No. 3001027). Further, in order to suppress a bias drift (so-called temperature drift phenomenon) that occurs with a temperature change, a semiconductive thin film such as silicon having a charge dispersion effect is usually formed on the surface layer of the buffer layer 104. (For example, see Japanese Patent Publication No. 4-22485).

  Also, ground electrodes 140, 142, and 144 are formed on the buffer layer 104 on the substrate 102 so as to sandwich the RF electrodes 134a and 134b, respectively (see FIG. 1). Accordingly, the RF 134a constitutes a coplanar electrode together with the ground electrodes 140 and 142, and the RF 134b constitutes a coplanar electrode together with the ground electrodes 142 and 144.

  Further, on the buffer layer 104 on the substrate 102, a ground electrode 146 is provided so as to cover the bias electrodes 130a, 132a, 130b, and 132b and to straddle the bias electrodes.

  FIG. 2 is an arrow view of the AA cross section of the light control element 100 shown in FIG. The ground electrode 146 is formed on the substrate 102 via the buffer layer 104. The ground electrode 146 is formed so as to cover the bias electrodes 130a, 132a, 130b, 132b formed on the parallel waveguides 122a, 124a, 122b, 124 of the optical modulators 110a, 110b, and these bias electrodes. Bridges 150a, 152a, 150b, and 152b are formed so as to straddle each.

  As a result, the light control element 100 not only prevents interference due to electric lines of force passing through the substrate 102 between the bias electrodes between the adjacent light modulators 110a and 110b, but also electromagnetic radiation propagating in space. Interference due to the above is effectively prevented.

  In other words, the interference due to the electric lines of force passing through the inside of the substrate 102 is caused by the electric lines of force having one end of each bias electrode 130a, 132a, 130b, 132b as the ground electrode 146 adjacent to each bias electrode and the substrate 102 (more accurately This is prevented by being terminated at the contact portion with the buffer layer 104). At the same time, the interference caused by electromagnetic radiation propagating in the space is caused by the bridges 150a, 152a, 150b formed so that the electromagnetic radiation from each bias electrode 130a, 132a, 130b, 132b straddles each bias electrode. And 152b are prevented by being shielded respectively.

  In FIG. 2, the shapes of the bridges 150a, 152a, 150b, and 152b are shown as semicircles. However, the shape of these bridges is not limited to this, and may be any shape. It can be a long semi-ellipse or an arbitrary polygon.

  The ground electrode 146 can be realized, for example, by a process similar to the process used to form an air connection wiring (air bridge) between circuit elements in a manufacturing process of MMIC (Monolithic Microwave Integrated Circuits). In this case, as a material for the sacrificial layer (portion that is removed after metal deposition to form the bridge) for forming the bridge portion, a photoresist or a metal such as nickel, chromium, or nichrome can be used.

  In addition, the bias electrodes 130a, 132a, 130b, and 132b do not handle signal frequencies extending to several tens of GHz unlike the RF electrodes 134a and 134b, and therefore do not need to be tall electrodes with a large aspect ratio. For example, a thickness of about 500 to 5 μm is sufficient. For this reason, the height of the bridges 150a, 152a, 150b, and 152b can be lowered accordingly, and as a result, the thickness of the ground electrode 146 may be about 50 to 100 μm, for example.

  This thickness is equivalent to the deposition thickness of the RF electrode material formed as a tall center strip as described above, and therefore the material deposition of the ground electrode 146 is the same plating process as used for the deposition of the RF electrode material ( For example, in the same step as the step of depositing the RF electrode material, a plating method can be used. For example, after depositing the sacrificial layer (photoresist etc.) to be the bridge portion on the buffer layer 104, when depositing the material of the RF electrodes 134a, 134b, the ground electrode 146 is also deposited by the same material, Thereafter, the sacrificial layer is removed by etching, whereby the bridges 150a, 152a, 150b, and 152b can be easily formed. In this case, although the material of the ground electrode 146 and the material used for the sacrificial layer are limited, the ground electrode 146 can be formed with a small number of steps, which is advantageous in terms of manufacturing cost.

  The ground electrode 146 is formed by stacking a plurality of metal films including at least one metal layer having an opening having a predetermined width on the substrate 102 so as to straddle the bias electrodes 130a, 132a, 130b, and 132b. It can also be configured as follows.

  FIG. 3 is a cross-sectional view showing an example of a ground electrode formed by laminating metal layers. The illustrated ground electrode 346 can be used in the light control element 100 in place of the ground electrode 146 of FIG. The ground electrode 346 has two metal layers 360a and 360b having openings for forming the bridges 350a, 352a, 350b, and 352b straddling the bias electrodes 130a, 132a, 130b, and 132b, respectively, and has no openings. The metal layer 360 c is formed by being stacked on the substrate 102 with the buffer layer 104 interposed therebetween. Note that the number of metal layers to be stacked is not limited to three, and four or more metal layers can be stacked in accordance with a desired shape for the bridges 350a, 352a, 350b, and 352b.

  Further, in FIGS. 1 and 2, the ground electrode 146 may be a conductive material formed so as to straddle the bias electrode by the bridges 150 a, 152 a, 150 b, and 152 b, and the entirety thereof is not necessarily formed on the buffer layer 104. It need not be formed by material deposition. Therefore, for example, a substantially plate-like or four-groove conductive member formed so as to straddle the bias electrodes 130a, 132a, 130b, and 132b is replaced with each bias electrode 130a, 132a, 130b, and 132b. The ground electrode 146 may be configured by bonding to a metal film provided so as to run in parallel at a predetermined distance from, for example, by flip chip bonding. In this case, unlike the plating process, it is not necessary to use a sacrificial layer.

  FIG. 4 is a cross-sectional view showing an example of a ground electrode formed by joining conductive members. The ground electrode 446 shown in FIG. 4 can be used for the light control element 100 in place of the ground electrode 146 shown in FIG. The ground electrode 446 is a substantially plate-like conductive member 460 (for example, aluminum) provided with four grooves for constituting the bridges 450a, 452a, 450b, and 452b straddling the bias electrodes 130a, 132a, 130b, and 132b, respectively. Plate) and metal films 462a, 462b, 462c, 462d, and 462e provided so as to run parallel to each bias electrode 130a, 132a, 130b, and 132b by a predetermined distance. Solder bumps 464a, 464b, 464c, 464d, and 464e are provided on portions of the conductive member 460 that face the metal films 462a, 462b, 462c, 462d, and 462e. Using these solder bumps, the conductive member is provided. 460 is bonded to the metal films 462a, 462b, 462c, 462d, and 462e by flip chip bonding and fixed to the substrate 102.

  As another alternative example of the ground electrode 146, the bridges 150a, 152a, 150b, and 152b are formed in an inverted V shape, and are configured to straddle the bias electrodes 130a, 132a, 130b, and 132b, respectively. A grounding electrode 146 is formed by bonding a conductive member (for example, a silicon plate) having a V-groove to a metal film provided so as to run in parallel across the bias electrodes 130a, 132a, 130b, and 132b. Can be.

  FIG. 5 is a cross-sectional view showing an example of a ground electrode using a silicon plate in which a V-groove is formed. The ground electrode 546 shown in FIG. 5 can be used for the light control element 100 in place of the ground electrode 146 shown in FIG. The ground electrode 546 includes a silicon plate 560 that is a conductive member provided with four V-groove portions for constituting the bridges 550a, 552a, 550b, and 552b over the bias electrodes 130a, 132a, 130b, and 132b, respectively. Each of the bias electrodes 130a, 132a, 130b, and 132b includes metal films 562a, 562b, 562c, 562d, and 562e provided so as to run in parallel at a predetermined distance. For example, a thin film such as Au is deposited on the flat portion of the silicon plate 560 where the V-groove is not formed, and the flat portion and the metal films 562a, 562b, 562c, 562d, 562e (with solder or the like). By bonding, the silicon plate 560 is fixed to the substrate 102. Note that a solder bump may be provided on the silicon plate 560, and the silicon plate 560 and the metal films 562a, 562b, 562c, 562d, and 562e may be joined by flip chip bonding.

  In the configuration in which the ground electrode 146 is formed using the multilayer metal film or the conductive member described above, the ground electrode 146 is formed using a process different from the material deposition for the RF electrodes 134a and 134b. The material of the electrode 146 does not need to be the same as that of the RF electrodes 134a and 134b, and the degree of freedom in material selection is increased.

  The thickness of the substrate 102 of the light control element 100 according to the present embodiment is not limited. For example, the thickness of the substrate 102 is 50 μm or less, and the surface on which the light control element 100 is formed (that is, the waveguide type). If the surface opposite to the surface on which the optical modulators 110a and 110b, which are optical elements) are formed so as to be in contact with a material having a dielectric constant lower than that of the substrate 102 or air, the adjacent bias electrodes 132a and 130b are adjacent to each other. The electric lines of force passing through the substrate 102 generated between the optical modulators 110a and 110b through the electric lines of force can be further reduced.

[Second Embodiment]
The light control element according to the second embodiment of the present invention is different from the light control element 100 according to the first embodiment in that X-cut LN is used as a substrate.
FIG. 6 is a plan view showing the configuration of the light control element according to the second embodiment of the present invention. FIG. 7 is a cross-sectional view of the light control element 600 shown in FIG.

  Of the components of the light control element 600 shown in FIGS. 6 and 7, the same reference numerals as those in FIGS. 1 and 2 are used for the same components as those of the light control element 100 shown in FIGS. The above description of the component is incorporated herein by reference.

  The light control element 600 uses X-cut LN as the substrate 602. For this reason, since it is necessary to apply an electric field in parallel to the substrate surface with respect to the waveguide, the bias electrodes 630a, 632a, 630b, 632b (all shown with dotted lines) and the RF electrodes 634a, 634b Rather than directly above 122a, 124a, 122b, 124b, they run in parallel over a predetermined distance on the side of the corresponding parallel waveguides 122a, 124a, 122b, or 124b (positions shifted in the vertical direction in the figure). Is formed.

Since no electrode is formed immediately above the waveguide, the light propagating through the parallel waveguides 122a, 124a, 122b, 124 is not absorbed by the bias electrodes 130a, 132a, 130b, 132b and the RF electrodes 634a, 634b. A buffer layer is not provided on 602. However, a buffer layer such as SiO ₂ may be provided on the substrate 602. When the buffer layer is provided, for example, it is possible to prevent light propagating through the parallel waveguide 122a from being absorbed by the portion of the bias electrode 632a intersecting with the parallel waveguide 122a and receiving loss. Generation of light loss can be avoided.

  On the substrate 602, ground electrodes 640, 642, and 644 are formed so as to sandwich the RF electrodes 634a and 634b, respectively (see FIG. 3). As a result, the RF 634a constitutes a coplanar electrode together with the ground electrodes 640 and 642, and the RF 634b constitutes a coplanar electrode together with the ground electrodes 642 and 644.

  Furthermore, a ground electrode 646 is provided on the substrate 602 so as to cover the bias electrodes 630a, 632a, 630b, and 632b and to straddle the bias electrodes.

  As shown in FIG. 7, the ground electrode 646 is formed on the substrate 602. The ground electrode 646 is formed so as to cover the bias electrodes 630a, 632a, 630b, and 632b formed in the vicinity of the parallel waveguides 122a, 124a, 122b, and 124 of the optical modulators 610a and 610b. Bridges 650a, 652a, 650b, and 652b are formed so as to straddle the electrodes, respectively.

  Thereby, the light control element 600 is caused by the lines of electric force passing through the substrate 602 between the bias electrodes between the adjacent light modulators 610a and 610b, similarly to the light control element 100 according to the first embodiment. This not only prevents interference, but also effectively prevents interference due to electromagnetic radiation propagating in space.

  That is, the interference due to the electric lines of force passing through the inside of the substrate 602 is caused by the contact portion between the ground electrode 646 adjacent to each bias electrode and the substrate 602 with each electric line of force having each bias electrode 630a, 632a, 630b, 632b as one end. It is prevented by terminating by. At the same time, interference caused by electromagnetic radiation propagating in the space is caused by the bridges 650a, 652a, 650b formed so that the electromagnetic radiation from each bias electrode 630a, 632a, 630b, 632b straddles each bias electrode. And 652b are prevented by being shielded respectively.

  In addition, the manufacturing method (process) of each waveguide 120a-126a, 120b-126b, 128 to the board | substrate 602, the manufacturing method of electrode 630a-634a, 630b-634b, and the manufacturing method of the ground electrodes 640-646 are mentioned above. This is the same as the manufacturing method of each corresponding waveguide, each electrode, and each ground electrode in the light control element according to the embodiment.

  In addition, regarding the dimensions of the bridges 650a, 652a, 650b, and 650b of the ground electrode 646, the substrate between the bias electrodes 630a, 632a, 630b, and 632b and the bridges 650a, 652a, 650b, and 650b adjacent to each bias electrode is used. In addition to the dimension capable of avoiding creeping discharge along the surface of 602, it is necessary to have a dimension that does not cause absorption loss due to the ground electrode 646 for light propagating through each parallel waveguide 122a, 124a, 122b, 124b. is there. For example, when the field diameter of light passing through the parallel waveguides 122a, 124a, 122b, and 124b is 10 μm, the edge of the light field, the adjacent bridge (650a, 652a, 650b, or 650b), and the substrate 602 are used. It is necessary that the distance to the contact portion is at least about 2.5 μm (the distance between the edge of the optical field and the adjacent bias electrodes 630a, 632a, 630b, or 632b is similarly It is necessary to secure at least about 2.5 μm).

  When X-cut LN is used as the substrate, a buffer layer and a charge dispersion film are not required, unlike when Z-cut is used. Since the substrate 302 made of LN crystal is a high-resistance insulator and there is no film of a material with low electrical resistance, the electrical resistance value between the bias electrodes 130a, 132a, 130b, 132b and the ground electrode 646 is Z-cut. It is much larger than when LN is used. Depending on the distance between the electrodes and the shape of the electrodes, in the case of a bias electrode having a length of about 10 mm, the resistance between the bias electrodes of the modulator using the Z-cut LN is several tens to several hundreds Ω. The resistance between the bias electrodes of the modulator using X-cut LN without a buffer layer is several M (mega) Ω or more. This also means that a large voltage is applied between the bias electrodes 630a, 632a, 630b, and 632b and the ground electrode 646 in the LN using the X cut, even if the interference noise is small power.

  When a thin substrate is used, the ratio of the electric lines of force that pass through the space on the side of the bias electrode is larger than the ratio of electric lines of force that pass through the substrate. It is effective to suppress this. In the case of an optical modulator using LN as a substrate, in general, grounding is strengthened in order to increase noise resistance of the bias application electrode, and the ground electrode portion has a width of 50 μm or more. If a ground electrode exists between the bias application electrodes and the width is equal to or greater than the thickness of the substrate, the influence of the electric lines of force generated from the bias electrode on the optical waveguides and bias electrodes of other optical modulators via the substrate The effect of relaxing is particularly high. When the thickness of the substrate is 1/5 or less of the width of the ground electrode existing between the bias application electrodes, that is, when the thickness of the substrate is 10 μm or less, the electric lines of force generated from the bias electrode pass through the substrate. The influence on the optical waveguide and bias electrode of other optical modulators is −50 dB or less and can be ignored. In other words, if a substrate having a thickness of 10 μm or less is used and the ground electrode is formed so as to straddle the bias electrode, the interference wave of the bias signal can be suppressed almost completely.

  As described above, the light control element of this embodiment covers each of the bias electrodes individually so as to cover the whole of the plurality of bias electrodes respectively included in two adjacent light modulators formed on the same substrate. A ground electrode is provided so as to straddle. As a result, in the present light control element, the contact portion between the ground electrode and the substrate is provided in the gap portion between the adjacent bias electrodes, and each bias electrode is shielded by the ground electrode. Interference due to electromagnetic radiation generated from each bias electrode as well as interference between the bias electrodes between the two optical modulators as well as electric field lines passing through the inside of the substrate is effectively prevented. Accordingly, it is possible to improve the degree of integration by reducing the distance between the optical modulators on the substrate.

  In the above-described embodiment, the configuration in which two light modulators are provided on the same substrate as the light control element has been described. However, the present invention is not limited to this configuration, and is provided on the same substrate. Even if a ground electrode straddling each bias electrode is provided for the entire plurality of bias electrodes of any number of optical modulators, the same effect as described above can be obtained.

  In the present embodiment, a Mach-Zehnder type optical modulator is shown as a plurality of waveguide type optical elements provided on the same substrate. However, the present invention is not limited to this, and the present invention uses an operating point voltage as a reference voltage for optical wave control. Other types of waveguide-type optical modulators and other waveguide-type optical functional elements (optical switches, optical frequency shifters, optical phase shifters, etc.) in which bias electrodes for controlling are provided separately from RF electrodes, Alternatively, the present invention can be similarly applied to a light control element in which any combination thereof is integrated.

  DESCRIPTION OF SYMBOLS 100,300 ... Light control element, 102, 302 ... Board | substrate, 104 ... Buffer layer, 110a, 110b, 610a, 610b ... Optical modulator, 120a, 120b ... Incident waveguide, 122a 122b, 124a, 124b ... Parallel waveguide, 126a, 126b ... Outgoing waveguide, 128 ... Output waveguide, 130a, 130b, 132a, 132b, 630a, 630b, 632a, 632b ... Bias Electrode, 134a, 134b, 634a, 634b ... RF electrode, 140, 142, 144, 146, 346, 446, 546, 640, 642, 644, 646 ... Ground electrode, 150a, 150b, 152a, 152b, 350a, 350b, 352a, 352b, 450a, 450b, 452a, 452b, 55 a, 550b, 552a, 552b, 650a, 650b, 652a, 652b ... bridge, 360a, 360b, 360c ... metal layer, 460 ... conductive member, 462a, 462b, 462c, 462d, 462e, 562a , 562b, 562c, 562d, 562e... Metal film, 464a, 464b, 464c, 464d, 464e,.

Claims (11)

A substrate having an electro-optic effect;
A plurality of optical waveguides formed on the substrate;
An RF electrode for controlling a light wave propagating through the optical waveguide;
A first ground electrode spaced apart from the RF electrode;
A DC bias electrode for applying a DC voltage;
With
There are second and third ground electrodes on both sides in the extending direction of the DC bias electrode, and the second and third ground electrodes cover the DC bias electrode with a space above the substrate. Connected so that
Light control element.   The light control element according to claim 1, wherein at least one of the plurality of waveguide type optical elements formed on the substrate is an optical modulator including a Mach-Zehnder type optical waveguide.   The light control element according to claim 1, wherein the substrate is X-cut or Z-cut lithium niobate. The thickness of the substrate is 50 μm or less,
Of the substrate, a surface facing the surface on which the waveguide optical element is formed is configured to contact a material having a dielectric constant lower than that of the substrate or air.
The light control element according to claim 3.   The second and third ground electrodes are formed so as to straddle the DC bias electrodes by sequentially laminating a plurality of metal films having openings with different widths on the substrate. 5. The light control element according to any one of 4 above.   The second and third ground electrodes include a metal film provided on the substrate so as to run in parallel with the DC bias electrodes at a predetermined distance from the DC bias electrodes, and a substantially plate-like conductive member. The light control element according to claim 1, wherein the light control element is formed by bonding together. The light control element according to claim 6 , wherein the bonding is performed by flip chip bonding. The conductive member has a formed V-shaped groove so as to straddle the DC bias electrode, the light control device according to claim 6 or 7. The light control element according to claim 8 , wherein the conductive member is a plate of a semiconductor material. The dimension of the portion where the second and third ground electrodes straddle each DC bias electrode is the surface of the substrate between the portion where the second and third ground electrodes are in contact with the substrate and the DC bias electrode. creeping discharge is determined so as not to cause the light control device according to any one of claims 1 to 7. The dimensions, further, the portion where the second and third ground electrode is in contact with the substrate, the light propagating through the optical waveguide is determined so as not to cause loss is absorbed, according to claim 10 Light control element.

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