JP2011252942A - Optical control element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical control element that enables low driving voltage and miniaturization, in particular an optical control element that enables cost reduction by using a low-cost drive system component.SOLUTION: In an optical control element having a substrate 1 having an electro-optic effect, plural optical waveguides 2 formed on the substrate, and a control electrode 3 formed on the substrate for controlling the phase of light which propagates through the optical waveguides, the control electrode 3 comprises at least two resonance type electrodes 31 and 32 having the same resonance frequency; and power supply electrodes 41 and 42 that supply a control signal to each resonance type electrode. A shape and a formation position of each resonance type electrode, and power supply positions of the power supply electrodes to each resonance type electrode are provided such that odd mode coupling with each other is enabled. The power supply electrodes have branch wiring parts 41 and 42 that are obtained by branching an input wiring part 40, and each resonance type electrode is supplied with a control signal having an in-phase or prescribed phase difference by the branch wiring parts.

Description

  The present invention relates to a light control element, and more particularly to a light control element provided with a resonant electrode that modulates a light wave propagating through an optical waveguide.

  Resonates with transmitters of optical communication systems such as optical modulators for optical transmission of high-frequency signals of several GHz or more used for radio and pulsar modulators for optical clock generation used with data modulation in long-distance transmission. Light control elements such as type optical modulators are used. A resonant optical modulator uses a substrate material having an electro-optic effect such as lithium niobate, and uses a control electrode having a resonant electrode to change the refractive index of the optical waveguide formed on the substrate, It is configured to modulate the intensity and phase of light propagating through the optical waveguide.

  In the resonance type electrode, when an electric signal having a specific frequency is input from the feeding point, a standing wave of the electric signal is generated at the electrode. As described above, since the resonance type optical modulator uses the resonance phenomenon of the electric signal, it operates particularly efficiently when a specific frequency is inputted, and generally has a unit length longer than that of the traveling wave type optical modulator. Modulation efficiency per unit is good.

  Because of such characteristics, there are many examples in which the length of the electrode of the conventional resonant optical modulator is designed to be shorter than one wavelength of the electric signal. However, in the condition where the speed of light propagating through the optical waveguide and the speed of the control signal propagating through the electrode of the action part are approximately the same, the electrode can be lengthened and affected by the attenuation of the control signal. The driving voltage can be improved according to the length of the electrode.

  Non-Patent Document 1 discloses that the combined use of speed matching and a resonant electrode is effective for improving the efficiency, and Non-Patent Document 2 discloses a resonant electrode type optical modulator using lithium niobate as a substrate. It describes the case where good characteristics were obtained by setting the refractive index (nm) of the electrical signal to approximately 2.2 (the refractive index of lithium niobate with respect to light is approximately 2.2).

  On the other hand, under conditions where the speeds of light and electrical signals do not match, the length of the electrode cannot be made sufficiently long, and even if the modulation efficiency per unit length is high, the overall modulation efficiency can be improved as a result. Can not. For this reason, the half-wave voltage Vπ, which is a parameter representing the efficiency of the modulator, is approximately 10 V or higher, and sufficient operation cannot be obtained unless a very high voltage is applied.

  Further, if a part of the optical waveguide is branched into two paths to form a Mach-Zehnder (MZ) interferometer structure, if the two branched lights are made to interfere with each other, the optical waveguide operates as a light intensity modulator. In a long-distance transmission, a pulsar modulator for generating an optical clock used with data modulation, etc. can be obtained by operating the amount of phase change of the two split lights with the same magnitude and opposite phase change. It is desirable that the wavelength chirp is zero, and a configuration is adopted in which signals of the same magnitude and opposite signs are applied to the respective branch waveguides of the MZ interferometer. In addition, when signals having opposite signs are applied to the two electrodes corresponding to each of the branched waveguides (referred to as “two-electrode type”), there is an effect of lowering the driving voltage.

  From these facts, it is expected that a drastic reduction in driving voltage can be realized if a two-electrode type MZ modulator is configured using a long resonant electrode in which the speeds of light and control signals are matched. . However, since the resonance electrode has high efficiency and a very strong electric field for a signal having a resonance frequency, coupling with a surrounding conductive material (signal crosstalk) is remarkable. In a situation where the signals of both control electrodes are cross-talked, the phase of the control signal is disturbed and the desired light modulation cannot be obtained.

  In particular, signals are likely to be coupled to elements (components) that satisfy the resonance condition for the same frequency. For the two-electrode type MZ optical modulator, an electrode having the same basic structure is adopted as a control electrode corresponding to each branching waveguide. For this reason, when a resonant electrode is configured, each control electrode (resonant electrode) has the same resonant frequency, and coupling (signal crosstalk) becomes significant. In addition, when a long electrode with a speed matching is used, the influence of the coupling between the two electrodes becomes more remarkable because the electrode is long.

  Patent Document 1 discloses a countermeasure for preventing crosstalk of a control signal between a plurality of electrodes in the case of an optical modulator manufactured by forming a coplanar electrode on a substrate having an electro-optic effect such as lithium niobate. There is an example of forming a groove. In general, the strength of the control electric field decreases as the distance from the control electrode increases. This is a phenomenon that depends on the structure of the substrate and the electrode. As shown in FIG. 1, when the waveguide interval is about 150 μm, the electric field strength exerted on the other optical waveguide is about 1%. It is also disclosed that it is about 0.2% for about 300 μm and about 0.1% for about 400 μm.

  Thus, it is disadvantageous in terms of device size and cost to increase the distance between the two branch optical waveguides of the MZ interferometer. In addition, although the method of forming a groove shown in Patent Document 1 can be expected to improve, it is disadvantageous in terms of device manufacturing costs such as processing of an additional structure for prevention.

  In order to solve such a problem, the present applicant has proposed the light control element shown in FIG. 2 in another application. Light control including a substrate 1 having an electro-optic effect, a plurality of optical waveguides 2 formed on the substrate, and a control electrode 3 provided on the substrate for controlling the phase of light propagating through the optical waveguide. In the element, the control electrode includes at least resonance electrodes 31 and 32 having the same resonance frequency, and power supply electrodes 41 and 42 for supplying a control signal to each of the resonance electrodes, and the shape of each resonance electrode and The formation position and the feeding position by the feeding electrode to each resonance type electrode are set so that the odd-mode coupling is possible with each other, and the control signal of the same phase is fed to each resonance type electrode by the feeding electrode. It is characterized by.

  In FIG. 2, the optical waveguide 2 constitutes an MZ interferometer having branching waveguides 21 and 22. The control electrode has one signal electrode to be the resonance type electrodes 31 and 32 and a ground electrode surrounding the signal electrode, and both ends of the signal electrode (resonance type electrode) are open from the ground electrode. Can be short-circuited to the ground electrode, or one end can be short-circuited to the ground electrode. The length of the resonant electrode is indicated by L.

  Further, in FIG. 2, the shape and formation position of each resonance electrode 31, 32 and the power supply position by the power supply electrodes 41, 42 to each resonance electrode are point-symmetric with respect to the two resonance electrodes (fixed point O). Is set to be point-symmetric with respect to the center.

  FIG. 3 shows the relationship between the feeding position and the impedance and the electric field at a specific timing when the length L of the resonant electrode (signal electrode) in the light control element of FIG. 2 is half wavelength λ / 2 (λ: signal wavelength). It is a figure explaining the mode of a vector (arrow). As is apparent from the graph showing the relationship between the power feeding position and impedance in FIG. 3, the length of the signal electrode in the case where both ends of the signal electrodes (resonance type electrodes) 31 and 32 are open from the ground electrode 33 is shown. However, when the signal wavelength is a half wavelength, there are two feeding positions where the impedance is 50Ω.

  In addition, if the power supply position is different even if an in-phase control signal is input, the electric field vector at a specific timing, such as the electric field vectors shown in the upper resonance electrode and the lower resonance electrode in FIG. Are opposite to each other. In such a state, even if crosstalk (coupling) occurs between resonant electrodes, electric field energy is always transferred in the same phase, so that stable operation can be performed in the same manner as when there is no coupling. Become. Such a state is called an “odd (symmetric) mode coupling” state.

  By using the light control element shown in FIG. 2 or FIG. 3, it is possible to expect an epoch-making reduction in driving voltage and miniaturization in an optical modulator such as a pulsar modulator. However, a problem with such a light control element is that it is necessary to feed two series of control signals in phase and in the same magnitude. For this reason, in order to drive the light control element, it is necessary to use a differential driver and an external phase shifter, and the entire apparatus becomes expensive.

JP 2009-53444 A

Mark Yu and Anand Gopinath, "Velocity Matched Resonant Slow-Wave Structure for Optical Modulator", Proceedings of Integrated Photonics Research (IPR), ITuH7-1, pp.365-369, Palm Springs, California, March 22, 1993 Roger Krahenbuhl and M. M. Howerton, "Investigations on Short-Path-Length High-Speed Optical Modulators in LiNbO3 With Resonant-Type Electrodes", JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL.19, No. 9, pp.1287-1297, SEPTEMBER 2001

  The problem to be solved by the present invention is to solve the above-mentioned problems and to provide a light control element that can be reduced in driving voltage and reduced in size, particularly by using low-cost driving system components. An object of the present invention is to provide a light control element capable of reducing the cost.

  In order to solve the above problems, an invention according to claim 1 is directed to a substrate having an electro-optic effect, a plurality of optical waveguides formed on the substrate, and a phase of light that is provided on the substrate and propagates through the optical waveguide. The control electrode includes a control electrode for controlling the at least two resonance-type electrodes having the same resonance frequency, and a power supply electrode for supplying a control signal to each of the resonance-type electrodes. The shape and formation position of each resonance electrode, and the power supply position by the power supply electrode to each resonance electrode are set so that odd mode coupling is possible, and the power supply electrode includes a plurality of input wiring portions. The resonance wiring electrode is supplied with a control signal having the same phase or a predetermined phase difference from each of the resonance type electrodes.

  According to a second aspect of the present invention, in the optical control element according to the first aspect, the optical waveguide constitutes one or a plurality of Mach-Zehnder interferometers, and the two resonant electrodes are the Mach-Zehnder interferometers. It is arranged corresponding to two branching waveguides constituting

  The invention according to claim 3 is the light control element according to claim 1 or 2, wherein the predetermined phase difference is an integral multiple of 2π with respect to a control signal having a predetermined frequency.

  According to a fourth aspect of the present invention, in the light control element according to any one of the first to third aspects, the power supply electrode has a branch wiring portion that branches one input wiring portion into two, and each branch The impedance of the power supply electrode in the wiring part and the impedance of the resonance electrode at the power supply position are set to be approximately twice the impedance of the power supply electrode in the input wiring part.

  The invention according to claim 5 is the light control element according to any one of claims 1 to 4, wherein the resonance electrode has one signal electrode, and the length of the signal electrode has a predetermined frequency. The control signal is longer than the wavelength formed on the signal electrode.

  According to a sixth aspect of the present invention, in the light control element according to any one of the first to fifth aspects, the feeding position is set at a position closest to the center of the resonant electrode.

  In order to solve the above-described problem, according to the first aspect of the present invention, a substrate having an electro-optic effect, a plurality of optical waveguides formed on the substrate, and a phase of light provided on the substrate and propagating through the optical waveguide The control electrode includes a control electrode for controlling the at least two resonance-type electrodes having the same resonance frequency, and a power supply electrode for supplying a control signal to each of the resonance-type electrodes. The shape and formation position of each resonance electrode, and the power supply position by the power supply electrode to each resonance electrode are set so that odd mode coupling is possible, and the power supply electrode includes a plurality of input wiring portions. Since each branching wiring section is fed with a control signal having the same phase or a predetermined phase difference by the branching wiring section, there is a crosstalk (coupling) between the resonance type electrodes. Even if it occurs To receive the same amount of electric field energy and minute bound in a square electrode in phase, it acts as if binding is not, it is possible to stable optical modulation operation. In addition, this is the same regardless of the magnitude of the coupling between both electrodes. Therefore, it is possible to provide an optical control element that can be reduced in driving voltage and reduced in size. In addition, the supply of the control signal to each resonance type electrode uses a branch wiring portion obtained by branching one input wiring portion into a plurality of parts, and therefore has a predetermined phase difference (including the same phase) and the same magnitude. The control signal can be formed very easily, and the use of expensive equipment such as a differential driver and an external phase shifter is not required, and it is possible to provide a light control element that is reduced in cost.

  According to the invention of claim 2, the optical waveguide constitutes one or a plurality of Mach-Zehnder interferometers, and the two resonant electrodes are arranged corresponding to the two branch waveguides constituting the Mach-Zehnder interferometer. Therefore, it is possible to provide a light control element with a lower driving voltage, such as a two-electrode optical modulator using a resonant electrode.

  According to the third aspect of the present invention, the predetermined phase difference is an integer multiple of 2π with respect to the control signal having the predetermined frequency, so that the same operation as when the in-phase control signal is supplied to the resonant electrode can be easily performed. It can be realized.

  According to the invention of claim 4, the power supply electrode has a branch wiring portion that branches one input wiring portion into two, the impedance of the power supply electrode in each branch wiring portion, and the power supply of each resonance electrode The impedance at the position is set to be approximately twice the impedance of the power feeding electrode in the input wiring portion. Therefore, the control signal supplied to the input wiring portion is caused by impedance mismatching, etc. Reflection can be suppressed, and the modulation efficiency by the control signal can be increased, and a further lower driving voltage can be realized.

  According to the invention of claim 5, the resonant electrode has one signal electrode, and the length of the signal electrode is longer than the wavelength that a control signal having a predetermined frequency forms on the signal electrode. An optical control element that can be driven at a lower voltage can be provided.

  According to the invention of claim 6, since the power feeding position is set at a position closest to the center of the resonant electrode, variation in characteristics based on manufacturing errors of the electrode is suppressed, and furthermore, each resonant electrode is optically connected. Since the electric field intensity distribution exerted on the waveguide can be made substantially the same, wavelength chirp can be suppressed.

10 is a graph showing the relationship between electric field strength and distance (interval of optical waveguide) disclosed in Patent Document 1. It is a figure explaining an example (what opened both ends of the signal electrode from the ground electrode) of the light control element which the present applicant proposed in other applications. When the length of the signal electrode in the light control element of FIG. 2 is a half-wavelength λ / 2 (λ: signal wavelength), it is a diagram for explaining the relationship between the feeding position and the impedance and the state of the electric field vector at a specific timing. It is a figure explaining the Example which concerns on the light control element of this invention. It is a figure explaining the other Example which concerns on the light control element of this invention. When the length of the resonance type signal electrode (with both ends of the signal electrode open from the ground electrode) is 3λ / 2 (λ: signal wavelength), the relationship between the feeding position and the impedance, and the electric field vector at a specific timing It is a figure explaining a mode. It is a figure explaining the structure which made resonance type electrode unevenly distributed in a part of MZ interferometer. It is a figure explaining the structure which uses only a part of resonance type electrode as an action part which acts on an optical waveguide. It is a figure which shows a mode that the drive circuit was connected to the light control element of this invention.

Hereinafter, the light control element of the present invention will be described in detail.
As shown in FIG. 4, the light control element of the present invention includes a substrate 1 having an electro-optic effect, a plurality of optical waveguides 2 formed on the substrate, and light that is provided on the substrate and propagates through the optical waveguide. In the light control element having the control electrode 3 for controlling the phase of the control electrode 3, the control electrode 3 transmits at least two resonance electrodes 31 and 32 having the same resonance frequency and a control signal to each of the resonance electrodes. Power supply electrodes 41 and 42 to supply power, the shape and formation position of each resonance electrode 31, 32, and the power supply position by the power supply electrode to each resonance electrode is set so that odd mode coupling is possible, The power supply electrode has branch wiring portions 41 and 42 that are branched from one input wiring portion 40. A control signal having the same phase or a predetermined phase difference is supplied to each resonance type electrode by the branch wiring portion. It is characterized by being

  As the substrate 1 having an electro-optic effect, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), quartz-based materials, and combinations thereof can be used. In particular, lithium niobate (LN) or lithium tantalate (LT) crystals having a high electro-optic effect are preferably used. The light control element of the present invention has a two-electrode structure as shown in FIG. 4, and a configuration in which a resonant electrode is disposed on an optical waveguide can expect the most effective modulation. Therefore, a Z-cut substrate is preferable. .

  The optical waveguide can be formed by a method of forming a ridge on the substrate, a method of adjusting the refractive index of a part of the substrate, or a method of combining both. In the ridge type waveguide, other portions are removed by mechanical cutting or chemical etching so as to leave a substrate portion to be an optical waveguide. It is also possible to form grooves on both sides of the optical waveguide. In the method of adjusting the refractive index, the refractive index of a part of the substrate surface corresponding to the optical waveguide becomes higher than the refractive index of the substrate itself by using a thermal diffusion method such as Ti or a proton exchange method. Configure as follows.

Control electrodes such as signal electrodes and ground electrodes can be formed by forming a Ti / Au electrode pattern, a gold plating method, or the like. Also, each electrode is optionally arranged via a buffer layer such as SiO ₂ film between a substrate. The buffer layer has an effect of preventing light waves propagating through the optical waveguide from being absorbed or scattered by the control electrode. Moreover, as a structure of a buffer layer, in order to relieve the pyroelectric effect of a thin plate, a Si film or the like can be incorporated as necessary.

The light control element of the present invention employs a configuration in which at least two resonance electrodes are formed on the control electrode, and the modulation efficiency of the control signal (modulation signal) is not affected even if both resonance electrodes cross-talk. is doing. For this purpose, the following two requirements are required.
(1) Both resonance-type electrodes have basically the same shape and are in a condition of being coupled with odd (symmetric) modes.
(2) An in-phase signal is supplied to both resonant electrodes.

  The resonant electrode is mainly composed of one signal electrode and a ground electrode surrounding it. As a combination of the two resonance-type electrodes, as described later, both ends of the signal electrode are both open from the ground electrode, or both ends of the signal electrode are short-circuited to the ground electrode. The combination of “short-circuited at both ends—short-circuited at both ends” is one of the preferred embodiments, but the light control element of the present invention is not limited to this, “open at both ends—short-circuited at both ends”, “open at both ends—one short-circuited and other open” It goes without saying that various combinations such as “short-circuited at both ends—one short-circuited and other open” are possible.

  Since the two resonance-type electrodes are basically the same shape electrodes, the resonance frequency is the same, and it is easy to combine them. Normally, when this electrode is arranged so as to be symmetric with respect to the central axis (light propagation direction) of the MZ interferometer, it is supplied to the signal electrode by the position on one signal electrode constituting the resonance electrode. Since the state (the direction of the electric field vector) between the electric field generated by the control signal and the electric field generated by the crosstalk between the electric field formed by the other resonant electrode is different, the control signal interferes in a complicated manner, and the resonant electrode (particularly, The normal propagation of the control signal on one signal electrode) is hindered.

  On the other hand, if both resonant electrodes are placed under the condition that odd-mode coupling occurs even when coupled, and a large signal with the same phase is fed to each other, the same amount of electric field energy as that coupled to the other is fed to the same phase. It works in the same way as when there is no connection. This is the same regardless of the strength of the coupling between both electrodes.

  In the light control element of the present invention, as shown in FIG. 4, the control signal is bifurcated, and the phases are matched to supply power to the resonant electrodes 31 and 32. The paths after branching are arranged on the same substrate by adjusting the refractive index of the branch wiring so that each feeding point is supplied with a control signal in a frequency band that resonates in phase. This eliminates the need for expensive components such as differential electrodes and external phase adjusters. The frequency band refers to a band within 6 dB.

When the impedance of the input wiring portion 40 fed from the outside of the light control element (chip) is Z ₀ , the impedance of each of the branch wiring portions 41 and 42 that are bifurcated equally without branching loss is as shown in FIG. order to matching, as large as 2Z _0.
In the present invention, “substantially double” is indicated. This means that the double relationship can most reduce the branch loss, but the impedance value is within a practical range where the effects of the present invention can be expected. Even if it is somewhat different from this double, it means that the present invention is acceptable. In addition, it is desirable that a preferable allowable range is within about ± 20% with respect to twice and reflection is within about ± 10%.

  The branched control signal is supplied to the resonance type electrodes 31 and 32. The resonance type electrodes have different impedances depending on the feeding points (feeding positions) by the feeder electrodes (branch wiring portions) 41 and 42. Therefore, it has an impedance from 0Ω to almost infinite. For this reason, there is no reflection loss and proper power feeding can be performed by selecting an appropriate power feeding position regardless of the impedance of the branch wiring portion. This is a technology that can be realized because the power supply destination is a resonant electrode.

  In a resonance type electrode whose electrode length is longer than the signal wavelength, there are a plurality of excitation points that perform the same resonance operation. Therefore, with respect to the power feeding position, it is possible to use the point where the resonance of the same condition is excited as the power feeding point, and there are many variations in the adjustment combination of the phase difference.

  When adopting the shape of an MZ interferometer in the optical waveguide, conventionally, in order to reduce the influence of control signal interference, the distance between the branched optical waveguides is increased so that the distance between electrodes (hot electrodes) is 400 μm or more. It was necessary to take this, but this is not necessary in the present invention. It may be 100 μm or less at which crosstalk becomes significant. For this reason, size reduction of a light control element is realizable.

  The resonant electrode has a coplanar type (CPW) structure in which the ground electrode is arranged so as to sandwich or surround the signal electrode, and the speed of the optical signal propagating through the optical waveguide and the speed of the control signal propagating through the electrode are approximately Equal production conditions are used.

  When the speeds are approximately equal (when the speed matching condition is substantially satisfied), the length of the electrode can be made longer than the wavelength of the resonance frequency of the control signal, which is advantageous in reducing the driving voltage. Here, for the purpose of further reducing the driving voltage, a ridge-type optical waveguide in which a control signal is effectively applied to the optical waveguide portion is used. Naturally, any electrode type or optical waveguide may be used as long as the velocity matching is substantially satisfied, regardless of whether the electrode has a non-CPW structure or a non-ridge type waveguide.

  In the light control element shown in FIG. 4, a shape in which both ends of one signal electrode (resonance type electrode) are opened from the ground electrode is used as the resonance type electrode. As shown in FIG. 3, the feeding position is not centered on the resonant electrode, but is asymmetrical on the upper and lower resonant electrodes in consideration of the impedance of the resonant electrode and the intensity waveform of the electric field vector formed on the resonant electrode. It is provided at a position (not symmetrical with respect to the horizontal straight line in the drawing).

  In FIG. 3, the position closest to the center of the resonant electrode among the positions where the impedance is 50Ω on the resonant electrode is used as the feeding point. However, as described above, the feeding electrode branches off the input wiring portion. When two branch wiring portions are formed, since impedance matching is normally performed with an external device at 50Ω, the feeding point of the resonant electrode is mainly at a position where the impedance is 100Ω, and the resonant type It is preferable that a position closest to the center of the electrode is a feeding point. In the present invention, the drive circuit outside the light control element can directly supply the control signal without using the impedance matching circuit.

  In addition, when the position closest to the center of the resonant electrode is used as the feed point, there is a problem in the reproducibility of the manufacturing process compared to the case where power is fed toward the end of the electrode. There is also an advantage that there is little change in the characteristics. Further, since the electric field intensity distribution exerted on the optical waveguide by each resonant electrode can be made substantially the same, wavelength chirp can be suppressed.

  As shown in FIG. 4, the two resonance electrodes have the same shape and are located on a plane equidistant from the action part of the MZ interferometer (the part where the electric field formed by each resonance electrode acts on the optical waveguide). An arbitrary point to be rotated is set at a position that is 180 ° rotationally symmetric (point symmetry, see fixed point O in FIG. 4). A control signal having the same phase and the same magnitude is fed to the feeding point (feeding position) of one signal electrode (31, 32) of each resonance type electrode using the feeding electrodes 41,.

  Further, in a resonant electrode whose electrode length is longer than the signal wavelength, there are a plurality of excitation points that perform the same resonant operation. In the example of FIG. 4, the point near the center of the electrode is the feeding position. However, as shown in FIG. 5, the position where the same excitation action as this position is obtained is any of the feeding points (any of a1 to a3 and any of b1 to b3). Can be selected). Therefore, in the case of a long resonant electrode, there are various combinations for selecting each feeding point.

  FIG. 6 is a diagram showing a resonance type electrode having one signal electrode when the length of the signal electrode (with both ends of the signal electrode opened from the ground electrode) is 3λ / 2 (λ: signal wavelength). It is a figure explaining the relationship with an impedance, and the mode of the electric field vector in specific timing. The impedance of the resonance-type electrode varies depending on the position, and as shown in the upper graph of FIG.

  In FIG. 6, there are three feeding positions for realizing an impedance of 100Ω, which are three places in one electric field vector pattern in consideration of the direction of the electric field vector. Then, the feeding positions of the resonance electrodes are set so that the electric field vectors formed by the two resonance electrodes are opposite to each other as shown in FIG.

  In this way, impedance matching can be realized by selecting an appropriate power feeding position regardless of the magnitude of the impedance of the branch wiring, and appropriate power feeding can be performed without reflection loss. In addition, the product of the length of each wiring and the refractive index with respect to the control signal of wiring in each branch wiring part is set so that it may become equal, and it is comprised so that it may input into a resonance type electrode in an in-phase state.

  Here, the structure of the power supply electrode is CPW (configuration in which the ground electrode is arranged so as to sandwich the signal electrode), but CPS (configuration in which the ground electrode is arranged on one side of the signal electrode), G-CPW (of the substrate) It may be a configuration in which CPW is formed on the front surface and a ground electrode is disposed on the rear surface, a strip line, or a combination thereof. Further, in order to suppress the loss of the control signal, the impedance is set to be constant when the electrode structure is changed in the middle.

  Similarly, any configuration of CPW, CPS, and G-CPW may be adopted for the resonant electrode. However, in order to facilitate the connection between the resonance type electrode and the power feeding electrode, it is preferable that both adopt the same electrode configuration. The branch wiring portion of the power supply electrode is possible as long as it is electrically continuous even when the power supply wire such as a coupler or a hybrid is discontinuous.

  In the electrode arrangement condition and the control signal power supply condition adopted by the light control element of the present invention, even if coupling occurs between both electrodes, the mode becomes an odd symmetry mode, so the control signal on each resonant electrode is disturbed. Never happen.

  Accordingly, the amount of phase change of light in each branch optical waveguide has the same reverse sign, and an ON / OFF pulse optical signal without wavelength chirp is generated at the optical output unit of the MZ interferometer. This ON / OFF pulse optical signal without wavelength chirp is the most desirable characteristic as an optical clock.

  When the optical control element of the present invention is used as an optical pulsar (optical clock generator) instead of a resonant modulator, the phase difference to the feeding point of each resonant electrode is not zero (in phase) but 2π. Needless to say, it can be an integral multiple of. In addition, as described above, the combination of the selection of the feeding point with substantially equivalent effects and the phase difference satisfying an integer multiple of 2π increases the degree of freedom of the shape and arrangement of the control electrode (resonance type electrode and feeding electrode), Product design is also easy.

  Note that the length of the wiring necessary for shifting the phase by 2π is the same as the interval between the points having the same excitation effect on the resonant electrode shown in FIG. 6 when the refractive index of the feeding electrode is the same as that of the resonant electrode. become. Therefore, if the power supply electrode has substantially the same configuration as the resonance electrode, it is convenient for the arrangement of the power supply electrode.

  In the above description, the linear signal electrode (resonance type electrode) has been mainly described. However, the present invention is not limited to this, and when the branched waveguide is curved or bent, it resonates according to the waveguide. The electrode may be bent or bent. In addition, if the operational efficiency applied to the branched optical waveguides is the same and the requirements to be arranged at positions where they are odd-mode coupled to each other are satisfied, each signal electrode (resonance type electrode) is not limited to a linear type but a ring type. There is no problem.

  Further, as described above, the position of the resonance electrode with respect to the optical waveguide constituting the MZ interferometer is not limited to the arrangement of the two resonance electrodes so as to coincide with the center of the MZ interferometer. 7, the resonance type electrodes 31 and 32 are configured to be unevenly distributed in a part of the branching waveguide of the MZ interferometer, or only part of the resonance type electrode is branched as shown in FIG. It is also possible to arrange so that it overlaps with the waveguide, and the resonance type electrode is constituted by an action part (range S) acting on the optical waveguide and a non-action part. In the case of FIG. 8, the shape and arrangement of the resonance electrode of the non-acting portion increases the degree of design freedom compared to the case where the entire resonance electrode described above becomes the action portion.

  In order to drive the light control element of the present invention, as shown in FIG. 9, it can be constituted by one drive circuit. As an example of a drive circuit, a signal having a predetermined frequency from a signal source is input to a driver, amplified to a predetermined signal voltage, and then input to the input wiring section 40 of the light control element through a bandpass filter that removes noise. . Unlike the present invention, an optical control element that feeds two series of control signals with the same phase and the same magnitude requires a differential driver, an external phase shifter, and the like. However, the optical control element of the present invention has a single drive circuit. It becomes possible to drive only by this, and the whole apparatus can be reduced in cost.

Further, when the light control element of the present invention is applied to a pulser, the following effects can be expected.
・ Realization of high-speed, ultra-low voltage, small pulsar ・ Innovative reduction of power consumption ・ Reduction of user cost by using low-cost drive system ・ Size reduction including peripheral circuits, improvement of integration ・ Device by size reduction Cost reduction by increasing the number

  As described above, according to the present invention, there is provided a light control element capable of reducing the driving voltage and reducing the size, and in particular, the light capable of reducing the cost by using low-cost driving system components. A control element can be provided.

DESCRIPTION OF SYMBOLS 1 Substrate having electro-optic effect 2 Optical waveguide 21, 22 Branched waveguide 3 Control electrode 31, 32 Signal electrode (resonance type electrode)
33 Ground electrode 41, 42 Feed electrode

  The present invention relates to a light control element, and more particularly to a light control element provided with a resonant electrode that modulates a light wave propagating through an optical waveguide.

  Resonates with transmitters of optical communication systems such as optical modulators for optical transmission of high-frequency signals of several GHz or more used for radio and pulsar modulators for optical clock generation used with data modulation in long-distance transmission. Light control elements such as type optical modulators are used. A resonant optical modulator uses a substrate material having an electro-optic effect such as lithium niobate, and uses a control electrode having a resonant electrode to change the refractive index of the optical waveguide formed on the substrate, It is configured to modulate the intensity and phase of light propagating through the optical waveguide.

  In the resonance type electrode, when an electric signal having a specific frequency is input from the feeding point, a standing wave of the electric signal is generated at the electrode. As described above, since the resonance type optical modulator uses the resonance phenomenon of the electric signal, it operates particularly efficiently when a specific frequency is inputted, and generally has a unit length longer than that of the traveling wave type optical modulator. Modulation efficiency per unit is good.

  Because of such characteristics, there are many examples in which the length of the electrode of the conventional resonant optical modulator is designed to be shorter than one wavelength of the electric signal. However, in the condition where the speed of light propagating through the optical waveguide and the speed of the control signal propagating through the electrode of the action part are approximately the same, the electrode can be lengthened and affected by the attenuation of the control signal. The driving voltage can be improved according to the length of the electrode.

  Non-Patent Document 1 discloses that the combined use of speed matching and a resonant electrode is effective for improving the efficiency, and Non-Patent Document 2 discloses a resonant electrode type optical modulator using lithium niobate as a substrate. It describes the case where good characteristics were obtained by setting the refractive index (nm) of the electrical signal to approximately 2.2 (the refractive index of lithium niobate with respect to light is approximately 2.2).

  On the other hand, under conditions where the speeds of light and electrical signals do not match, the length of the electrode cannot be made sufficiently long, and even if the modulation efficiency per unit length is high, the overall modulation efficiency can be improved as a result. Can not. For this reason, the half-wave voltage Vπ, which is a parameter representing the efficiency of the modulator, is approximately 10 V or higher, and sufficient operation cannot be obtained unless a very high voltage is applied.

  Further, if a part of the optical waveguide is branched into two paths to form a Mach-Zehnder (MZ) interferometer structure, if the two branched lights are made to interfere with each other, the optical waveguide operates as a light intensity modulator. In a long-distance transmission, a pulsar modulator for generating an optical clock used with data modulation, etc. can be obtained by operating the amount of phase change of the two split lights with the same magnitude and opposite phase change. It is desirable that the wavelength chirp is zero, and a configuration is adopted in which signals of the same magnitude and opposite signs are applied to the respective branch waveguides of the MZ interferometer. In addition, when signals having opposite signs are applied to the two electrodes corresponding to each of the branched waveguides (referred to as “two-electrode type”), there is an effect of lowering the driving voltage.

  From these facts, it is expected that a drastic reduction in driving voltage can be realized if a two-electrode type MZ modulator is configured using a long resonant electrode in which the speeds of light and control signals are matched. . However, since the resonance electrode has high efficiency and a very strong electric field for a signal having a resonance frequency, coupling with a surrounding conductive material (signal crosstalk) is remarkable. In a situation where the signals of both control electrodes are cross-talked, the phase of the control signal is disturbed and the desired light modulation cannot be obtained.

  In particular, signals are likely to be coupled to elements (components) that satisfy the resonance condition for the same frequency. For the two-electrode type MZ optical modulator, an electrode having the same basic structure is adopted as a control electrode corresponding to each branching waveguide. For this reason, when a resonant electrode is configured, each control electrode (resonant electrode) has the same resonant frequency, and coupling (signal crosstalk) becomes significant. In addition, when a long electrode with a speed matching is used, the influence of the coupling between the two electrodes becomes more remarkable because the electrode is long.

  Patent Document 1 discloses a countermeasure for preventing crosstalk of a control signal between a plurality of electrodes in the case of an optical modulator manufactured by forming a coplanar electrode on a substrate having an electro-optic effect such as lithium niobate. There is an example of forming a groove. In general, the strength of the control electric field decreases as the distance from the control electrode increases. This is a phenomenon that depends on the structure of the substrate and the electrode. As shown in FIG. 1, when the waveguide interval is about 150 μm, the electric field strength exerted on the other optical waveguide is about 1%. It is also disclosed that it is about 0.2% for about 300 μm and about 0.1% for about 400 μm.

  Thus, it is disadvantageous in terms of device size and cost to increase the distance between the two branch optical waveguides of the MZ interferometer. In addition, although the method of forming a groove shown in Patent Document 1 can be expected to improve, it is disadvantageous in terms of device manufacturing costs such as processing of an additional structure for prevention.

  In order to solve such a problem, the present applicant has proposed the light control element shown in FIG. 2 in another application. Light control including a substrate 1 having an electro-optic effect, a plurality of optical waveguides 2 formed on the substrate, and a control electrode 3 provided on the substrate for controlling the phase of light propagating through the optical waveguide. In the element, the control electrode includes at least resonance electrodes 31 and 32 having the same resonance frequency, and power supply electrodes 41 and 42 for supplying a control signal to each of the resonance electrodes, and the shape of each resonance electrode and The formation position and the feeding position by the feeding electrode to each resonance type electrode are set so that the odd-mode coupling is possible with each other, and the control signal of the same phase is fed to each resonance type electrode by the feeding electrode. It is characterized by.

  In FIG. 2, the optical waveguide 2 constitutes an MZ interferometer having branching waveguides 21 and 22. The control electrode has one signal electrode to be the resonance type electrodes 31 and 32 and a ground electrode surrounding the signal electrode, and both ends of the signal electrode (resonance type electrode) are open from the ground electrode. Can be short-circuited to the ground electrode, or one end can be short-circuited to the ground electrode. The length of the resonant electrode is indicated by L.

  Further, in FIG. 2, the shape and formation position of each resonance electrode 31, 32 and the power supply position by the power supply electrodes 41, 42 to each resonance electrode are point-symmetric with respect to the two resonance electrodes (fixed point O). Is set to be point-symmetric with respect to the center.

  FIG. 3 shows the relationship between the feeding position and the impedance and the electric field at a specific timing when the length L of the resonant electrode (signal electrode) in the light control element of FIG. 2 is half wavelength λ / 2 (λ: signal wavelength). It is a figure explaining the mode of a vector (arrow). As is apparent from the graph showing the relationship between the power feeding position and impedance in FIG. 3, the length of the signal electrode in the case where both ends of the signal electrodes (resonance type electrodes) 31 and 32 are open from the ground electrode 33 is shown. However, when the signal wavelength is a half wavelength, there are two feeding positions where the impedance is 50Ω.

  In addition, if the power supply position is different even if an in-phase control signal is input, the electric field vector at a specific timing, such as the electric field vectors shown in the upper resonance electrode and the lower resonance electrode in FIG. Are opposite to each other. In such a state, even if crosstalk (coupling) occurs between resonant electrodes, electric field energy is always transferred in the same phase, so that stable operation can be performed in the same manner as when there is no coupling. Become. Such a state is called an “odd (symmetric) mode coupling” state.

  By using the light control element shown in FIG. 2 or FIG. 3, it is possible to expect an epoch-making reduction in driving voltage and miniaturization in an optical modulator such as a pulsar modulator. However, a problem with such a light control element is that it is necessary to feed two series of control signals in phase and in the same magnitude. For this reason, in order to drive the light control element, it is necessary to use a differential driver and an external phase shifter, and the entire apparatus becomes expensive.

JP 2009-53444 A

Mark Yu and Anand Gopinath, "Velocity Matched ResonantSlow-Wave Structure for Optical Modulator", Proceedings of Integrated Photonics Research (IPR), ITuH7-1, pp.365-369, Palm Springs, California, March22, 1993 Roger Krahenbuhl and M. M. Howerton, "Investigations on Short-Path-Length High-Speed Optical Modulators in LiNbO3 With Resonant-Type Electrodes", JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL.19, No. 9, pp.1287-1297, SEPTEMBER 2001

  The problem to be solved by the present invention is to solve the above-mentioned problems and to provide a light control element that can be reduced in driving voltage and reduced in size, particularly by using low-cost driving system components. An object of the present invention is to provide a light control element capable of reducing the cost.

In order to solve the above problems, an invention according to claim 1 is directed to a substrate having an electrooptic effect, a plurality of optical waveguides formed on the substrate, and a phase of light provided on the substrate and propagating through the optical waveguide. The control electrode includes a control electrode for controlling the at least two resonance-type electrodes having the same resonance frequency, and a power supply electrode for supplying a control signal to each of the resonance-type electrodes. The optical waveguide constitutes one or a plurality of Mach-Zehnder interferometers, and the two resonant electrodes are arranged corresponding to the two branch waveguides constituting the Mach-Zehnder interferometer, and each resonant type The shape and formation position of the electrodes, and the power feeding position by the power feeding electrode to each resonance type electrode are set so that odd mode coupling is possible with each other, and the power feeding electrode is branched by dividing one input wiring portion into a plurality of Wiring part And, each resonant electrode, a control signal having an in-phase or a predetermined phase difference by the branch wiring section is powered, other than the end portion of the position the resonant electrodes of the resonant electrodes, the control signal fed a position, in a state where the control signal is fed, the resonant electrode is characterized that you bond odd mode together.

The invention according to claim 2 is the light control element according to claim 1 , wherein the predetermined phase difference is an integral multiple of 2π with respect to a control signal having a predetermined frequency.

According to a third aspect of the present invention, in the light control element according to any one of the first to second aspects, the power supply electrode has a branch wiring portion that branches one input wiring portion into two, and each branch The impedance of the power supply electrode in the wiring part and the impedance of the resonance electrode at the power supply position are set to be approximately twice the impedance of the power supply electrode in the input wiring part.

According to a fourth aspect of the present invention, in the light control element according to any one of the first to third aspects, the resonant electrode has one signal electrode, and the length of the signal electrode has a predetermined frequency. The control signal is longer than the wavelength formed on the signal electrode.

According to a fifth aspect of the present invention, in the light control element according to any one of the first to fourth aspects, the feeding position is set to a position closest to the center of the resonant electrode.

In order to solve the above-described problem, according to the first aspect of the present invention, a substrate having an electro-optic effect, a plurality of optical waveguides formed on the substrate, and a phase of light provided on the substrate and propagating through the optical waveguide The control electrode includes a control electrode for controlling the at least two resonance-type electrodes having the same resonance frequency, and a power supply electrode for supplying a control signal to each of the resonance-type electrodes. The optical waveguide constitutes one or a plurality of Mach-Zehnder interferometers, and the two resonant electrodes are arranged corresponding to the two branch waveguides constituting the Mach-Zehnder interferometer, and each resonant type The shape and formation position of the electrodes, and the power feeding position by the power feeding electrode to each resonance type electrode are set so that odd mode coupling is possible with each other, and the power feeding electrode is branched by dividing one input wiring portion into a plurality of wiring Having, each resonant electrode, a control signal having an in-phase or a predetermined phase difference by the branch wiring section is powered, of the resonant electrodes, the end position is the resonant electrode control signal is fed a position other than parts, in a state where the control signal is fed, because the resonant electrodes to bond odd modes together, even if between the resonance electrodes crosstalk (coupling) occurs, the other electrode Since the same amount of electric field energy is received in phase with the amount of light coupled to the light, it works in the same way as when there is no light coupling, and a stable light modulation operation is possible. In addition, this is the same regardless of the magnitude of the coupling between both electrodes. Therefore, it is possible to provide an optical control element that can be reduced in driving voltage and reduced in size. In addition, the supply of the control signal to each resonance type electrode uses a branch wiring portion obtained by branching one input wiring portion into a plurality of parts, and therefore has a predetermined phase difference (including the same phase) and the same magnitude. The control signal can be formed very easily, and the use of expensive equipment such as a differential driver and an external phase shifter is not required, and it is possible to provide a light control element that is reduced in cost.

Further , the optical waveguide constitutes one or a plurality of Mach-Zehnder interferometers, and the two resonant electrodes are arranged corresponding to the two branch waveguides constituting the Mach-Zehnder interferometer. It is possible to provide a light control element with a lower driving voltage, such as a two-electrode optical modulator using a mold electrode.

According to the second aspect of the present invention, the predetermined phase difference is an integer multiple of 2π with respect to the control signal having the predetermined frequency. It can be realized.

According to the invention of claim 3 , the power supply electrode has a branch wiring portion that branches one input wiring portion into two, the impedance of the power supply electrode in each branch wiring portion, and the power supply of each resonance electrode The impedance at the position is set to be approximately twice the impedance of the power feeding electrode in the input wiring portion. Therefore, the control signal supplied to the input wiring portion is caused by impedance mismatching, etc. Reflection can be suppressed, and the modulation efficiency by the control signal can be increased, and a further lower driving voltage can be realized.

According to the invention of claim 4 , the resonant electrode has one signal electrode, and the length of the signal electrode is longer than the wavelength that the control signal having a predetermined frequency forms on the signal electrode. An optical control element that can be driven at a lower voltage can be provided.

According to the invention of claim 5 , since the feeding position is set at a position closest to the center of the resonant electrode, variation in characteristics based on manufacturing errors of the electrode is suppressed, and furthermore, each resonant electrode is optically guided. Since the electric field intensity distribution exerted on the waveguide can be made substantially the same, wavelength chirp can be suppressed.

10 is a graph showing the relationship between electric field strength and distance (interval of optical waveguide) disclosed in Patent Document 1. It is a figure explaining an example (what opened both ends of the signal electrode from the ground electrode) of the light control element which the present applicant proposed in other applications. When the length of the signal electrode in the light control element of FIG. 2 is a half-wavelength λ / 2 (λ: signal wavelength), it is a diagram for explaining the relationship between the feeding position and the impedance and the state of the electric field vector at a specific timing. It is a figure explaining the Example which concerns on the light control element of this invention. It is a figure explaining the other Example which concerns on the light control element of this invention. When the length of the resonance type signal electrode (with both ends of the signal electrode open from the ground electrode) is 3λ / 2 (λ: signal wavelength), the relationship between the feeding position and the impedance, and the electric field vector at a specific timing It is a figure explaining a mode. It is a figure explaining the structure which made resonance type electrode unevenly distributed in a part of MZ interferometer. It is a figure explaining the structure which uses only a part of resonance type electrode as an action part which acts on an optical waveguide. It is a figure which shows a mode that the drive circuit was connected to the light control element of this invention.

Hereinafter, the light control element of the present invention will be described in detail.
As shown in FIG. 4, the light control element of the present invention includes a substrate 1 having an electro-optic effect, a plurality of optical waveguides 2 formed on the substrate, and light that is provided on the substrate and propagates through the optical waveguide. In the light control element having the control electrode 3 for controlling the phase of the control electrode 3, the control electrode 3 transmits at least two resonance electrodes 31 and 32 having the same resonance frequency and a control signal to each of the resonance electrodes. The optical waveguide 2 constitutes a Mach-Zehnder interferometer, and the two resonance-type electrodes 31 and 32 are connected to two branch waveguides constituting the Mach-Zehnder interferometer. The resonance electrodes 31 and 32 are arranged correspondingly and the positions and positions of the resonance electrodes 31 and 32 and the power supply positions of the power supply electrodes to the resonance electrodes are set so that odd mode coupling is possible. 1 input wiring A plurality of branch wiring portions 41 and 42 which branches 40, each resonant electrode, a control signal having an in-phase or a predetermined phase difference by the branch wiring section is powered, of the resonant electrodes, the control signal There position fed is the position other than the end portion of the resonant electrodes 31 and 32, in a state where the control signal is powered, and wherein Rukoto the resonant electrodes 31 and 32 be bonded odd modes together To do.

  As the substrate 1 having an electro-optic effect, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), quartz-based materials, and combinations thereof can be used. In particular, lithium niobate (LN) or lithium tantalate (LT) crystals having a high electro-optic effect are preferably used. The light control element of the present invention has a two-electrode structure as shown in FIG. 4, and a configuration in which a resonant electrode is disposed on an optical waveguide can expect the most effective modulation. Therefore, a Z-cut substrate is preferable. .

  The optical waveguide can be formed by a method of forming a ridge on the substrate, a method of adjusting the refractive index of a part of the substrate, or a method of combining both. In the ridge type waveguide, other portions are removed by mechanical cutting or chemical etching so as to leave a substrate portion to be an optical waveguide. It is also possible to form grooves on both sides of the optical waveguide. In the method of adjusting the refractive index, the refractive index of a part of the substrate surface corresponding to the optical waveguide becomes higher than the refractive index of the substrate itself by using a thermal diffusion method such as Ti or a proton exchange method. Configure as follows.

Control electrodes such as signal electrodes and ground electrodes can be formed by forming a Ti / Au electrode pattern, a gold plating method, or the like. Also, each electrode is optionally arranged via a buffer layer such as SiO ₂ film between a substrate. The buffer layer has an effect of preventing light waves propagating through the optical waveguide from being absorbed or scattered by the control electrode. Moreover, as a structure of a buffer layer, in order to relieve the pyroelectric effect of a thin plate, a Si film or the like can be incorporated as necessary.

The light control element of the present invention employs a configuration in which at least two resonance electrodes are formed on the control electrode, and the modulation efficiency of the control signal (modulation signal) is not affected even if both resonance electrodes cross-talk. is doing. For this purpose, the following two requirements are required.
(1) Both resonance-type electrodes have basically the same shape and are in a condition of being coupled with odd (symmetric) modes.
(2) An in-phase signal is supplied to both resonant electrodes.

  The resonant electrode is mainly composed of one signal electrode and a ground electrode surrounding it. As a combination of the two resonance-type electrodes, as described later, both ends of the signal electrode are both open from the ground electrode, or both ends of the signal electrode are short-circuited to the ground electrode. The combination of “short-circuited at both ends—short-circuited at both ends” is one of the preferred embodiments. It goes without saying that various combinations such as “short-circuited at both ends—one short-circuited and other open” are possible.

  Since the two resonance-type electrodes are basically the same shape electrodes, the resonance frequency is the same, and it is easy to combine them. Normally, when this electrode is arranged so as to be symmetric with respect to the central axis (light propagation direction) of the MZ interferometer, it is supplied to the signal electrode by the position on one signal electrode constituting the resonance electrode. Since the state (the direction of the electric field vector) between the electric field generated by the control signal and the electric field generated by the crosstalk between the electric field formed by the other resonant electrode is different, the control signal interferes in a complicated manner, and the resonant electrode (particularly, The normal propagation of the control signal on one signal electrode) is hindered.

  On the other hand, if both resonant electrodes are placed under the condition that odd-mode coupling occurs even when coupled, and a large signal with the same phase is fed to each other, the same amount of electric field energy as that coupled to the other is fed to the same phase. It works in the same way as when there is no connection. This is the same regardless of the strength of the coupling between both electrodes.

  In the light control element of the present invention, as shown in FIG. 4, the control signal is bifurcated, and the phases are matched to supply power to the resonant electrodes 31 and 32. The paths after branching are arranged on the same substrate by adjusting the refractive index of the branch wiring so that each feeding point is supplied with a control signal in a frequency band that resonates in phase. This eliminates the need for expensive components such as differential electrodes and external phase adjusters. The frequency band refers to a band within 6 dB.

When the impedance of the input wiring portion 40 fed from the outside of the light control element (chip) is Z ₀ , the impedance of each of the branch wiring portions 41 and 42 that are bifurcated equally without branching loss is as shown in FIG. order to matching, as large as 2Z _0.
In the present invention, “substantially double” is indicated. This means that the double relationship can most reduce the branch loss, but the impedance value is within a practical range where the effects of the present invention can be expected. Even if it is somewhat different from this double, it means that the present invention is acceptable. In addition, it is desirable that a preferable allowable range is within about ± 20% with respect to twice and reflection is within about ± 10%.

  The branched control signal is supplied to the resonance type electrodes 31 and 32. The resonance type electrodes have different impedances depending on the feeding points (feeding positions) by the feeder electrodes (branch wiring portions) 41 and 42. Therefore, it has an impedance from 0Ω to almost infinite. For this reason, there is no reflection loss and proper power feeding can be performed by selecting an appropriate power feeding position regardless of the impedance of the branch wiring portion. This is a technology that can be realized because the power supply destination is a resonant electrode.

  In a resonance type electrode whose electrode length is longer than the signal wavelength, there are a plurality of excitation points that perform the same resonance operation. Therefore, with respect to the power feeding position, it is possible to use the point where the resonance of the same condition is excited as the power feeding point, and there are many variations in the adjustment combination of the phase difference.

  When adopting the shape of an MZ interferometer in the optical waveguide, conventionally, in order to reduce the influence of control signal interference, the distance between the branched optical waveguides is also increased so that the distance between electrodes (between hot electrodes) is 400 μm or more. It was necessary to take this, but this is not necessary in the present invention. It may be 100 μm or less at which crosstalk becomes significant. For this reason, size reduction of a light control element is realizable.

  The resonant electrode has a coplanar type (CPW) structure in which the ground electrode is arranged so as to sandwich or surround the signal electrode, and the speed of the optical signal propagating through the optical waveguide and the speed of the control signal propagating through the electrode are approximately Equal production conditions are used.

  When the speeds are approximately equal (when the speed matching condition is substantially satisfied), the length of the electrode can be made longer than the wavelength of the resonance frequency of the control signal, which is advantageous in reducing the driving voltage. Here, for the purpose of further reducing the driving voltage, a ridge-type optical waveguide in which a control signal is effectively applied to the optical waveguide portion is used. Naturally, any electrode type or optical waveguide may be used as long as the velocity matching is substantially satisfied, regardless of whether the electrode has a non-CPW structure or a non-ridge type waveguide.

  In the light control element shown in FIG. 4, a shape in which both ends of one signal electrode (resonance type electrode) are opened from the ground electrode is used as the resonance type electrode. As shown in FIG. 3, the feeding position is not centered on the resonant electrode, but is asymmetrical on the upper and lower resonant electrodes in consideration of the impedance of the resonant electrode and the intensity waveform of the electric field vector formed on the resonant electrode. It is provided at a position (not symmetrical with respect to the horizontal straight line in the drawing).

  In FIG. 3, the position closest to the center of the resonant electrode among the positions where the impedance is 50Ω on the resonant electrode is used as the feeding point. However, as described above, the feeding electrode branches off the input wiring portion. When two branch wiring portions are formed, since impedance matching is normally performed with an external device at 50Ω, the feeding point of the resonant electrode is mainly at a position where the impedance is 100Ω, and the resonant type It is preferable that a position closest to the center of the electrode is a feeding point. In the present invention, the drive circuit outside the light control element can directly supply the control signal without using the impedance matching circuit.

  In addition, when the position closest to the center of the resonant electrode is used as the feed point, there is a problem in the reproducibility of the manufacturing process compared to the case where power is fed toward the end of the electrode. There is also an advantage that there is little change in the characteristics. Further, since the electric field intensity distribution exerted on the optical waveguide by each resonant electrode can be made substantially the same, wavelength chirp can be suppressed.

  As shown in FIG. 4, the two resonance electrodes have the same shape and are located on a plane equidistant from the action part of the MZ interferometer (the part where the electric field formed by each resonance electrode acts on the optical waveguide). An arbitrary point to be rotated is set at a position that is 180 ° rotationally symmetric (point symmetry, see fixed point O in FIG. 4). A control signal having the same phase and the same magnitude is fed to the feeding point (feeding position) of one signal electrode (31, 32) of each resonance type electrode using the feeding electrodes 41,.

  Further, in a resonant electrode whose electrode length is longer than the signal wavelength, there are a plurality of excitation points that perform the same resonant operation. In the example of FIG. 4, the point near the center of the electrode is the feeding position. However, as shown in FIG. 5, the position where the same excitation action as this position is obtained is any of the feeding points (any of a1 to a3 and any of b1 to b3). Can be selected). Therefore, in the case of a long resonant electrode, there are various combinations for selecting each feeding point.

  FIG. 6 is a diagram showing a resonance type electrode having one signal electrode when the length of the signal electrode (with both ends of the signal electrode opened from the ground electrode) is 3λ / 2 (λ: signal wavelength). It is a figure explaining the relationship with an impedance, and the mode of the electric field vector in specific timing. The impedance of the resonance-type electrode varies depending on the position, and as shown in the upper graph of FIG.

  In FIG. 6, there are three feeding positions for realizing an impedance of 100Ω, which are three places in one electric field vector pattern in consideration of the direction of the electric field vector. Then, the feeding positions of the resonance electrodes are set so that the electric field vectors formed by the two resonance electrodes are opposite to each other as shown in FIG.

  In this way, impedance matching can be realized by selecting an appropriate power feeding position regardless of the magnitude of the impedance of the branch wiring, and appropriate power feeding can be performed without reflection loss. In addition, the product of the length of each wiring and the refractive index with respect to the control signal of wiring in each branch wiring part is set so that it may become equal, and it is comprised so that it may input into a resonance type electrode in an in-phase state.

  Here, the structure of the power supply electrode is CPW (configuration in which the ground electrode is arranged so as to sandwich the signal electrode), but CPS (configuration in which the ground electrode is arranged on one side of the signal electrode), G-CPW (of the substrate) It may be a configuration in which CPW is formed on the front surface and a ground electrode is disposed on the rear surface, a strip line, or a combination thereof. Further, in order to suppress the loss of the control signal, the impedance is set to be constant when the electrode structure is changed in the middle.

  Similarly, any configuration of CPW, CPS, and G-CPW may be adopted for the resonant electrode. However, in order to facilitate the connection between the resonance type electrode and the power feeding electrode, it is preferable that both adopt the same electrode configuration. The branch wiring portion of the power supply electrode is possible as long as it is electrically continuous even when the power supply wire such as a coupler or a hybrid is discontinuous.

  In the electrode arrangement condition and the control signal power supply condition adopted by the light control element of the present invention, even if coupling occurs between both electrodes, the mode becomes an odd symmetry mode, so the control signal on each resonant electrode is disturbed. Never happen.

  Accordingly, the amount of phase change of light in each branch optical waveguide has the same reverse sign, and an ON / OFF pulse optical signal without wavelength chirp is generated at the optical output unit of the MZ interferometer. This ON / OFF pulse optical signal without wavelength chirp is the most desirable characteristic as an optical clock.

  When the optical control element of the present invention is used as an optical pulsar (optical clock generator) instead of a resonant modulator, the phase difference to the feeding point of each resonant electrode is not zero (in phase) but 2π. Needless to say, it can be an integral multiple of. In addition, as described above, the combination of the selection of the feeding point with substantially equivalent effects and the phase difference satisfying an integer multiple of 2π increases the degree of freedom of the shape and arrangement of the control electrode (resonance type electrode and feeding electrode), Product design is also easy.

  Note that the length of the wiring necessary for shifting the phase by 2π is the same as the interval between the points having the same excitation effect on the resonant electrode shown in FIG. 6 when the refractive index of the feeding electrode is the same as that of the resonant electrode. become. Therefore, if the power supply electrode has substantially the same configuration as the resonance electrode, it is convenient for the arrangement of the power supply electrode.

  In the above description, the linear signal electrode (resonance type electrode) has been mainly described. However, the present invention is not limited to this, and when the branched waveguide is curved or bent, it resonates according to the waveguide. The electrode may be bent or bent. In addition, if the operational efficiency applied to the branched optical waveguides is the same and the requirements to be arranged at positions where they are odd-mode coupled to each other are satisfied, each signal electrode (resonance type electrode) is not limited to a linear type but a ring type. There is no problem.

  Further, as described above, the position of the resonance electrode with respect to the optical waveguide constituting the MZ interferometer is not limited to the arrangement of the two resonance electrodes so as to coincide with the center of the MZ interferometer. 7, the resonance type electrodes 31 and 32 are configured to be unevenly distributed in a part of the branching waveguide of the MZ interferometer, or only part of the resonance type electrode is branched as shown in FIG. It is also possible to arrange so that it overlaps with the waveguide, and the resonance type electrode is constituted by an action part (range S) acting on the optical waveguide and a non-action part. In the case of FIG. 8, the shape and arrangement of the resonance electrode of the non-acting portion increases the degree of design freedom compared to the case where the entire resonance electrode described above becomes the action portion.

  In order to drive the light control element of the present invention, as shown in FIG. 9, it can be constituted by one drive circuit. As an example of a drive circuit, a signal having a predetermined frequency from a signal source is input to a driver, amplified to a predetermined signal voltage, and then input to the input wiring section 40 of the light control element through a bandpass filter that removes noise. . Unlike the present invention, an optical control element that feeds two series of control signals with the same phase and the same magnitude requires a differential driver, an external phase shifter, and the like. However, the optical control element of the present invention has a single drive circuit. It becomes possible to drive only by this, and the whole apparatus can be reduced in cost.

Further, when the light control element of the present invention is applied to a pulser, the following effects can be expected.
・ Realization of high-speed, ultra-low voltage, small pulsar ・ Innovative reduction of power consumption ・ Reduction of user cost by using low-cost drive system ・ Size reduction including peripheral circuits, improvement of integration ・ Device by size reduction Cost reduction by increasing the number

  As described above, according to the present invention, there is provided a light control element capable of reducing the driving voltage and reducing the size, and in particular, the light capable of reducing the cost by using low-cost driving system components. A control element can be provided.

DESCRIPTION OF SYMBOLS 1 Substrate having electro-optic effect 2 Optical waveguide 21, 22 Branched waveguide 3 Control electrode 31, 32 Signal electrode (resonance type electrode)
33 Ground electrode 41, 42 Feed electrode

Claims (6)

In a light control element having a substrate having an electro-optic effect, a plurality of optical waveguides formed on the substrate, and a control electrode provided on the substrate for controlling the phase of light propagating through the optical waveguide,
The control electrode includes at least two resonance electrodes having the same resonance frequency, and a feeding electrode that supplies a control signal to each of the resonance electrodes,
The shape and formation position of each resonance electrode, and the power supply position by the power supply electrode to each resonance electrode are set so that odd mode coupling is possible with each other,
The power supply electrode has a branch wiring part that branches one input wiring part into a plurality of parts,
An optical control element, wherein a control signal having the same phase or a predetermined phase difference is fed to each resonance electrode by the branch wiring portion.   2. The light control element according to claim 1, wherein the optical waveguide constitutes one or a plurality of Mach-Zehnder interferometers, and the two resonant electrodes constitute two branch waveguides constituting the Mach-Zehnder interferometer. The light control element is arranged corresponding to the above.   3. The light control element according to claim 1, wherein the predetermined phase difference is an integer multiple of 2π with respect to a control signal having a predetermined frequency.   4. The light control element according to claim 1, wherein the power supply electrode has a branch wiring portion obtained by branching one input wiring portion into two, and the impedance of the power supply electrode in each branch wiring portion. 5. And the impedance at the power feeding position of each resonant electrode is set to be approximately twice the impedance of the power feeding electrode at the input wiring portion.   5. The light control element according to claim 1, wherein the resonant electrode has one signal electrode, and the length of the signal electrode is such that a control signal having a predetermined frequency is on the signal electrode. A light control element characterized in that the light control element is longer than the wavelength to be formed.   7. The light control element according to claim 1, wherein the feeding position is set at a position closest to the center of the resonance electrode.

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