JP5447306B2 - Light control element - Google Patents

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.

  FIG. 1 shows an example of a resonant optical modulator that is a light control element using a resonant electrode. An optical waveguide 2 is formed on a substrate 1 having an electro-optic effect, and a light wave propagating through the optical waveguide 2 is further illustrated. A control electrode 3 is provided for optically modulating the light. In FIG. 1, a Mach-Zehnder interferometer having two branch waveguides 21 and 22 in the optical waveguide 2 is used. The control electrode 3 has a resonance electrode (signal electrode) 30 and a control signal is sent to the resonance electrode. The feeding electrode (feeding line) 40 to be introduced is configured, and the control electrode is configured by the signal electrode 30 and the feeding line 40 and a ground electrode 31 surrounding the signal electrode 30 and the feeding line 40. The position where the feeding electrode (feeding line) feeds power to the resonance electrode (signal electrode) is called feeding position (feeding point).

  In the resonance electrode 30 as shown in FIG. 1, when an electric signal having a specific frequency is input from the feeding point 50, a standing wave of the electric signal is generated in the resonance electrode 30. 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 in improving the efficiency, and Non-Patent Document 2 discloses a resonant electrode type optical modulator using lithium niobate as a substrate. The case where good characteristics were obtained by setting the refractive index (nm) of an electric signal to about 2.2 (the refractive index of lithium niobate with respect to light is about 2.2) is introduced.

  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.

  In the resonance type optical modulator, the Q value indicating the resonance efficiency is determined by the attenuation of the control signal, the electrode shape, and the like. In general, the higher the Q value, the higher the efficiency at the resonance frequency. However, in this case, since the frequency dependence of the efficiency in the vicinity of the resonance frequency is large, the operating band (bandwidth at which the efficiency is half of the optimum frequency) is narrowed. For this reason, due to the problem of reproducibility of the manufacturing process, when the shape of the electrode such as the length of the resonant electrode changes, inconvenience in terms of control occurs, such as a large variation in characteristics at the operating frequency. In particular, this problem becomes significant in a resonant optical modulator having a long electrode length that satisfies the speed matching condition.

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-described problems, to expand the operating band while using a resonance electrode, and to suppress deterioration of characteristics due to variations in electrode shape during manufacturing. It is to provide a light control element capable of satisfying the requirements.

  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 resonance electrode and a power supply electrode that feeds a control signal to the resonance electrode, and the resonance electrode includes one resonance electrode. A signal electrode, the length of the signal electrode is set to be longer than the wavelength formed on the signal electrode by the control signal having a predetermined frequency, and the power supply electrode branches one input wiring portion into a plurality of All of the branch wiring sections are connected to different positions on the signal electrode, and a control signal having the same phase or a predetermined phase difference is supplied to the signal electrode by the branch wiring section. It is characterized by.

  According to a second aspect of the present invention, in the light control element according to the first aspect, the optical waveguide constitutes one or a plurality of Mach-Zehnder interferometers, and the resonant electrode constitutes the Mach-Zehnder interferometer. It is characterized by being arranged corresponding to at least one of the two branched waveguides.

  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 feeding point of each branch wiring portion to the signal electrode is different in increase / decrease in impedance from the equivalent excitation point. It is characterized by being arranged with a small amount of displacement in the direction.

  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 power supply electrode has a branch wiring portion obtained by branching one input wiring portion into N pieces, and each branch The impedance of the power supply electrode in the wiring portion and the impedance at the power supply position of each resonance electrode are set to be approximately N times the impedance of the power supply electrode in the input wiring portion.

  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 resonance electrode and a power supply electrode that feeds a control signal to the resonance electrode, and the resonance electrode includes one resonance electrode. A signal electrode, the length of the signal electrode is set to be longer than the wavelength formed on the signal electrode by the control signal having a predetermined frequency, and the power supply electrode branches one input wiring portion into a plurality of All of the branch wiring sections are connected to different positions on the signal electrode, and a control signal having the same phase or a predetermined phase difference is supplied to the signal electrode by the branch wiring section. Apply a control signal to the resonant electrode. Each conductive position (feed point), it is possible to finely adjust the timing for applying the control signal. For example, by providing a portion that is shifted from the optimum feeding point, the resonance condition is slightly disturbed and the efficiency at the optimum frequency is lowered, but the frequency dependence of the efficiency near the resonance frequency is reduced, and as a result, the operating band is reduced. It becomes possible to enlarge.

  According to the invention of claim 2, the optical waveguide constitutes one or a plurality of Mach-Zehnder interferometers, and the resonant electrode corresponds to at least one of 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 an optical modulator using a resonant electrode.

  According to the third aspect of the invention, the predetermined phase difference is an integer multiple of 2π with respect to the control signal having the predetermined frequency, so that it is possible to always apply the control signal having the same phase state to each feeding point. . In addition, since it is only necessary to set the branch wiring portions so that the difference in length between them becomes an integer multiple of 2π, the degree of freedom in design can be increased.

  According to the invention according to claim 4, since the feeding point of each branch wiring portion to the signal electrode is arranged with a small amount shifted from the equivalent excitation point in the direction in which the increase or decrease in impedance is different, the resonance condition is slightly Although it is disturbed and the efficiency at the optimum frequency is lowered, the frequency dependence of the efficiency near the resonance frequency is reduced, and as a result, the operating band can be expanded.

  According to the fifth aspect of the present invention, the power supply electrode has a branch wiring portion obtained by branching one input wiring portion into N, the impedance of the power supply electrode in each branch wiring portion, and the power supply of each resonance electrode. Since the impedance at the position is set to be approximately N times the impedance of the power supply electrode in the input wiring portion, it is possible to achieve impedance matching between the input wiring portion and the branch wiring portion. It is possible to suppress the reflection of the control signal at, and efficiently input the control signal to the resonant electrode.

It is a figure which shows an example of the conventional resonance type | mold optical modulator. It is a figure explaining the Example which concerns on the light control element of this invention. It is a figure explaining the relationship between the electric power feeding position and total impedance in a resonance type electrode. It is a figure explaining the relationship between each electric power feeding position of a resonance type electrode, and an electric power feeding electrode. It is a figure explaining the other Example which concerns on the light control element of this invention. 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. 2, 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 includes a resonance type electrode and a power supply electrode for supplying a control signal to the resonance type electrode. The signal electrode 30 has a length that is set longer than the wavelength that the control signal having a predetermined frequency forms on the signal electrode; A branch wiring section 42 and 43 branched into a plurality of sections 41, all of the branch wiring sections are connected to different positions 51 and 52 on the signal electrode, and the signal electrode is connected to the branch wiring section by the branch wiring section; A control signal having the same phase or a predetermined phase difference is fed. To.

  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. In the light control element of the present invention, as shown in FIG. 2, the configuration in which the resonance type electrode is arranged on the optical waveguide can expect the most effective modulation, and therefore a Z-cut type 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.

The resonance type electrode and the feeding electrode constituting the control electrode are constituted by the signal electrode 30, the ground electrode 31, or the feeding lines 41 to 43 and the ground electrode. These control 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.

  Next, as shown in FIG. 3, the resonant electrode used in the light control element of the present invention has a length of one signal electrode 30 constituting the resonant electrode having a predetermined frequency on the signal electrode. The control signal is set longer than the wavelength formed on the signal electrode 30. FIG. 3 shows a signal electrode 30 having a length of 3λ / 2 (λ: signal wavelength). Of course, it goes without saying that the light control element of the present invention is not limited to the length signal electrode shown in FIG.

  Further, a ground electrode 31 is disposed around the signal electrode 30, and both ends of the signal electrode 30 are illustrated as being open from the ground electrode 31. Naturally, either “both ends open type” where both ends of the signal electrode are open from the ground electrode, or “both ends short type” where both ends of the signal electrode are both short-circuited to the ground electrode, and one end is short-circuited, etc. It goes without saying that various combinations such as “one short-circuited and other open type” with open ends are possible.

  The lower side of FIG. 3 shows the state of the electric field vector generated in the signal electrode 30, and the upper side of FIG. 3 shows the relationship between the power feeding position for the signal electrode 30 and the impedance. In the resonance type electrode, the impedance varies depending on the position, and as shown in the upper graph of FIG. 3, the impedance varies from 0Ω to almost infinite depending on the location.

  In FIG. 3, for example, there are three feeding positions (a1 to a3, b1 to b3) in one electric field vector pattern, considering the direction of the electric field vector, in consideration of the direction of the electric field vector. That is, by applying a control signal having the same phase (including a phase difference that is an integer multiple of 2π) to the feeding positions a1 to a3 (or feeding positions b1 to b3), a resonating waveform of the electric field is formed. be able to. Further, the control signal applied to the feeding positions a1 to a3 and the control signal applied to the feeding positions b1 to b3 are fed so that they are in opposite phases (phase difference is π or (2n + 1) π, where n is an integer). By doing so, it is possible to form a resonance waveform.

  FIG. 4 is a diagram showing an example of a state where power is supplied to the signal electrode 30 constituting the resonance electrode, and illustrates three types of power supply methods. When a control signal is input to the symbol A, in-phase control signals are applied to the feeding positions a1 and a2. In addition, when a control signal is input to the code B, an in-phase control signal is applied to the power feeding positions b1 and b3. In the case of the symbol B, since the control signal is input to the power feeding position where the electric field vector is reversed like the power feeding positions b2 and a3, the control signal needs to be in a reverse phase state.

  In order to simultaneously apply control signals to a plurality of power supply positions, as shown in FIG. 4, it is possible to branch the power supply lines and simplify the wiring to each power supply position. In addition, as shown in FIG. 2, when wirings (41 to 43) are also formed on the same substrate 1, each phase can be accurately controlled only by adjusting the wiring pattern. For example, when applying a reverse-phase control signal, such as the wiring pattern of reference C in FIG. 3, the phase difference is easily set by adjusting the length of the feeder line, such as by providing a delay line. be able to.

  Further, when the feeder line is branched in this way, the impedance of the feeder line changes before and after branching. For this reason, in order to perform impedance matching, when the power supply electrode has a branch wiring portion in which one input wiring portion is branched into N, the impedance of the power supply electrode in each branch wiring portion and each resonance The impedance at the feeding position of the mold electrode is set to be approximately N times the impedance of the feeding electrode at the input wiring portion.

  In the present invention, “substantially N times” is indicated. This means that the relationship of N times can reduce the branching loss most, but the impedance value is within a practical range where the effects of the present invention can be expected. Some differences from this N times mean that the present invention is acceptable. It is desirable that the preferable allowable range is within about ± 20% and the reflection within about ± 10% with respect to N times.

  Specifically, for an input having an impedance of 50Ω, the input wiring portion 41 has an impedance of 50Ω, but the branch wiring portions 42 and 43 have an impedance of 100Ω. For this reason, as shown in FIG. 3, even in the signal electrode 30 of the resonance type electrode, a feeding position with an impedance of 100Ω is selected. Resonance-type electrodes always have a feeding position that has the optimum impedance. By appropriately selecting the feeding position, the impedance on the feeding-electrode side and the impedance on the resonance-type electrode side can be easily matched. It becomes. This is a technology that can be realized because the power supply destination is a resonant electrode.

  Although FIG. 2 or FIG. 4 shows an example in which the feeder line is branched into two, naturally, the present invention is not limited to these, and impedance matching can be similarly achieved when branching into three or more. 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 branched path is arranged on the same substrate by adjusting the refractive index of the branch wiring so that the control signal in the frequency band that resonates with the same phase or a predetermined phase difference is fed to the resonance electrode. The frequency band refers to a band within 6 dB. For example, in the case of the same phase, the product of the length of each wiring and the refractive index with respect to the control signal of the wiring in each branch wiring section is set to be equal.

  When a control signal having an in-phase or anti-phase relation is applied to the power supply positions (a1 to a3, b1 to b3) described with reference to FIGS. An ideal resonant electric field is generated. However, such ideal resonance conditions have extremely high resonance efficiency, but the operating band (bandwidth at which the efficiency is half the optimum frequency) is narrow. However, the optical control element is manufactured by controlling the length of the signal electrode constituting the resonance electrode, particularly the length of the plurality of branch wiring portions, the refractive index with respect to the control signal, and the plurality of feeding positions with high precision. As a result, in the light control element of the present invention as shown in FIG. 2, although the resonance condition is slightly disturbed and the efficiency at the optimum frequency is lower than the optimum resonance condition. The frequency dependence of the efficiency near the resonance frequency is reduced, and as a result, the operating band can be expanded.

  In the light control element of the present invention, such characteristics are positively utilized, and the feeding points (51, 52) of the branch wiring portions 42, 43 to the signal electrode 30 are equivalent excitation points (a1 to a3, a3). From b1 to b3), it is also possible to arrange them by shifting a minute amount in directions in which the increase or decrease in impedance is different from each other. Naturally, the amount of shifting is not a problem as long as it is within a range where a resonant electric field required for driving the light control element such as light modulation is generated. For example, when the impedance of an ideal feeding point is 75Ω, the actual feeding point is slightly shifted from the equivalent position, and one is fed to a position where the impedance is 73Ω and the other is 77Ω.

  The method of shifting from the resonance condition includes not only adjusting the power supply position as described above, but also adjusting the phase of the control signal applied to each power supply position so as to shift from the in-phase or reverse-phase state. As a method for adjusting such a phase, it is possible to easily set and adjust the length of the branch wiring portions 42 and 43 in FIG. 2 and the refractive index with respect to the control signal.

  As shown in FIGS. 2 and 3, the resonant electrode has a coplanar (CPW) structure in which the ground electrode 31 is disposed so as to sandwich or surround the signal electrode 30, and an optical waveguide (branch waveguide 22). Manufacturing conditions are used in which the speed of the optical signal propagating through the electrode and the speed of the control signal propagating through the electrode are approximately equal.

  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. 2, as the resonance type electrode, the “both ends open type” shape in which both ends of one signal electrode (resonance type electrode) are opened from the ground electrode is used. In addition, a “both ends short-circuited type” and “one short-circuited other open type” may be used. Further, the signal electrode is not limited to a straight line, but may be in a curved state according to the shape of the optical waveguide to which an electric field is applied.

  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 optical waveguide 2 of FIG. 2, a Mach-Zehnder interferometer (MZ interferometer) having two branch waveguides 21 and 22 is illustrated. As described above, the signal electrode of the resonance type electrode is arranged so that the position of the resonance type electrode with respect to the optical waveguide constituting the MZ interferometer is within the length range of one branch waveguide 22 of the MZ interferometer. For example, a resonance type electrode is arranged in each of the two branch waveguides 21 and 22, or only a part of the resonance type electrode 30 overlaps the branch waveguide 22 as shown in FIG. 5. It is also possible to arrange so that the resonant electrode is composed of an action part (range S) acting on the optical waveguide and a non-action part. In the case as shown in FIG. 5, the shape and arrangement of the resonance electrode of the non-acting part increases the degree of design freedom compared to the case where the entire resonance electrode described above becomes the action part.

  In order to drive the light control element of the present invention, as shown in FIG. 6, it can be constituted by a single 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 portion 41 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 an optical pulser (optical clock generator), the following effects can be expected. Of course, it is needless to say that the phase difference to the feeding point to the resonance type electrode can be not only zero (in-phase) but an integer multiple of 2π.
・ 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 expanding the operating band while suppressing the deterioration of characteristics due to variations in the electrode shape during manufacturing while using the resonant electrode. It becomes possible to provide.

DESCRIPTION OF SYMBOLS 1 Substrate having electro-optic effect 2 Optical waveguides 21 and 22 Branched waveguide 3 Control electrode 30 Signal electrode (resonance type electrode)
31 Ground electrodes 41 to 43 Feed line (feed electrode)

Claims (5)

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 a resonance electrode and a power supply electrode that supplies a control signal to the resonance electrode.
The resonant electrode has one signal electrode, and the length of the signal electrode is set longer than the wavelength that the control signal having a predetermined frequency forms on the signal electrode,
The power supply electrode has a branch wiring part that branches one input wiring part into a plurality of parts,
All of the branch wiring portions are connected to different positions on the signal electrode,
A light control element, wherein a control signal having the same phase or a predetermined phase difference is fed to the signal electrode by the branch wiring portion.   2. The light control element according to claim 1, wherein the optical waveguide constitutes a single or a plurality of Mach-Zehnder interferometers, and the resonant electrode includes at least two branch waveguides constituting the Mach-Zehnder interferometer. A light control element that is arranged corresponding to one.   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 feeding point of each branch wiring portion to the signal electrode is shifted from the equivalent excitation point by a minute amount in a direction in which the increase or decrease in impedance is different from each other. A light control element characterized by comprising:   5. The light control element according to claim 1, wherein the power feeding electrode has a branch wiring portion that branches one input wiring portion into N pieces, and impedance of the power feeding electrode in each branch wiring portion. And the impedance at the power feeding position of each resonant electrode is set to be approximately N times the impedance of the power feeding electrode at the input wiring portion.

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