JP3592245B2 - Resonant optical modulator - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a configuration of an optical modulator used as a phase modulator, an intensity modulator or a polarization modulator in the field of optical communication, and particularly to a configuration of a resonance type optical modulator having a small power and a high modulation effect.
[0002]
[Prior art]
An optical modulator is a device that converts information of an electric signal into an optical carrier such as intensity, phase, and frequency and outputs the optical carrier. As a modulation method, a method of directly modulating a laser which is a light source, a method of Modulation using an external modulator. Although the above-described direct modulation method can have a simple configuration in terms of a device, it is known that modulation using an external modulator can achieve higher quality modulation. For this reason, in ultra-high-speed communication and long-distance communication, modulation is generally performed using an external modulator.
[0003]
When modulation is performed using an external modulator in this way, a modulator using an electro-optic effect, a modulator using an electric field absorption effect of a semiconductor, and the like are used. Since the present invention relates to the former modulator using the electro-optic effect, this point will be described in more detail below.
[0004]
LiNbO3 is one of the most frequently used substances showing an electro-optic effect in an optical modulator. Modulation can be performed using a change in the refractive index due to the Pockels effect, which is a primary electro-optic effect, which is a characteristic of this substance. At this time, it is well known that there are a type in which an electric field is applied parallel to the surface of the LiNbO3 substrate and a type in which an electric field is applied vertically. These are due to the difference in the crystal direction of the LiNbO3 substrate, and the substrates corresponding to the respective electric fields are called X-cut and Z-cut, respectively.
[0005]
In the LiNbO3, as another characteristic, the refractive index may be easily adjusted by thermally diffusing Ti (titanium) or the like to the substrate. Using this characteristic, a waveguide is often formed inside a LiNbO3 substrate and integrated.
[0006]
It is known that a modulator using such a LiNbO3 substrate includes a traveling-wave optical modulator and a resonant optical modulator. A traveling wave optical modulator is a modulator that guides a light wave and an electric signal in the same direction and modulates light while the light wave and the electric signal are guided. This modulator has a wide bandwidth from a DC signal to a high-frequency signal, but is characterized by a large drive current required for modulation. On the other hand, the resonance type optical modulator modulates using the resonance of the modulation signal. Therefore, the usable band is limited to a narrow band in the microwave region, but has a characteristic of high modulation efficiency. For this reason, it is effective when used in a polarization scrambler, a multiplexer, or the like that is required to operate efficiently in a narrow band.
[0007]
The present invention relates to the configuration of such a resonance type optical modulator, and will be described next. Known resonance optical modulators (Oikawa and three others, "Study of 10 GHZ band resonant LiNbO3 optical modulator", Proceedings of the 2000 IEICE General Conference, page 204, lecture number C- FIGS. 7A and 7B are schematic views of 3-25). The modulator shown in FIG. 7A is a Z-cut type, and includes an optical waveguide, an electrode (modulation electrode) disposed thereon, and a capacitance for impedance matching. The electrode has a size that resonates with the input microwave. The modulator shown in FIG. 7B is composed of a modulation electrode and an open stub for impedance matching. This modulator is characterized in that it is easier to manufacture than the modulator shown in FIG.
[0008]
[Problems to be solved by the invention]
The modulator shown in FIG. 7B has a feature that the modulation electrode and the open stub for impedance matching are formed by processing the same metal layer, so that the modulator is easy to manufacture. However, the common potential wiring layer is provided not only on the input side of the high-frequency signal but also on the side beyond the optical path, and since it is arranged to bypass the modulation electrode, the common potential wiring layer is And the electric field strength effective for modulation applied to the optical path is reduced to almost half compared to the case where the common potential wiring layer is provided only on the input side of the high-frequency signal. Has the disadvantage of becoming
[0009]
The present invention has been proposed in view of the above, and has a configuration that does not impair the electric field effective for modulation applied to the optical path while being easy to manufacture, and has a low power and high resonance effect. It realizes an optical modulator of a type.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, a first aspect of the present invention relates to a resonance type optical modulator, comprising: an optical path having electro-optic effect characteristics; and a modulation electrode for applying an electric field to an optical path formed along the optical path. A common electrode formed opposite to the modulation electrode, a power supply line electromagnetically connected to the modulation electrode at a valley portion of a potential distribution of the modulation electrode by a modulation signal , and a power supply line connected to the power supply line. Including a stub and a common electrode, the power supply line, the stub, and the common electrode are provided on one side of a region divided by the modulation electrode, and the modulation electrode resonates with a modulation signal. It is characterized by having a configuration of size .
[0011]
In addition, since the modulator is often fed by a coaxial cable from an external signal source, and it is necessary to feed power to a minute area while maintaining impedance matching, the second invention includes, in addition to the first invention, The above-mentioned feeder line is characterized by including a tapered transformer.
[0012]
The stub may be singular or plural. However, in the case where a plurality of stubs are provided, the stubs may be arranged symmetrically to facilitate the design. Therefore, the third invention is described in the first or second invention. In the above-mentioned resonance type optical modulator, the number of the stubs is an even number, and the stubs are provided at symmetrical positions with respect to the feeder line.
[0013]
Further, in the first invention, the position where the stub is provided is desirably at the intersection of the modulation electrode and the wiring. Therefore, in the fourth invention, in addition to the first invention, the modulation electrode and the power supply line are provided. And the stub has an arrangement in contact with the feeder line and the modulation electrode.
[0014]
Further, in any one of the first to fourth inventions, a modulation electrode having an open end can be used. Therefore, the fifth invention is a modification of the first to fourth inventions. And the common electrode formed opposite to the modulation electrode is characterized in that open ends are formed at both ends thereof.
[0015]
Further, in any one of the first to fourth inventions, a modulation electrode having a short-circuited end can be used. Therefore, in the sixth invention, in addition to any one of the first to fourth inventions, a modulation electrode is provided. And the common electrode formed opposite to the modulation electrode is characterized in that short-circuit ends are formed at both ends thereof.
[0016]
Further, since the stub to be used may be an open stub having an open end or a short stub having a short-circuit end, the seventh invention according to any one of the first to sixth inventions, The eighth invention is characterized in that the stub is an open stub, and the stub is a short stub.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
The embodiment of the present invention will be described below in the case of a modulation electrode having an open end as 1) Example 1, the case of a modulation electrode having a short end as 2) Example 2, and the above-described modulator. A case having a longer modulation electrode will be described in detail as a third embodiment with reference to the drawings.
[0018]
1) Example 1
A modulation electrode for applying an electric field to an optical path formed along the optical path, and a common electrode formed opposite the modulation electrode form an asymmetric coplanar waveguide, but a high-frequency signal having an open end is formed. FIG. 1 shows a schematic view of a resonance type optical modulator using a modulation electrode formed by an asymmetric coplanar waveguide. FIG. 1A is a plan view, and FIG. 1B is a sectional view thereof. This resonance type optical modulator shows the configuration of a Mach-Zehnder-interferometer type optical modulator for intensity-modulating light having a wavelength of 1.55 μm with a high-frequency signal having a center frequency of 10 GHz. The substrate used here is a Z-cut LiNbO3 substrate, and its optical path is formed by thermally diffusing Ti (titanium). On this substrate, a 0.55 μm thick silicon oxide layer 9 is formed to suppress attenuation of light propagating through the optical waveguide, and a modulation electrode, a transformer, a stub, or a metal forming a common electrode is formed thereon. A layer (gold, 2 μm thick) is provided.
[0019]
The modulation electrode 1 has a distance of 27 μm between the common electrodes 6 and 7 and a width of 5 μm and a length of 3250 μm, which is 0.19 times the wavelength of the high-frequency signal which is a modulation signal. It can be set to about .22 times (or about 0.67 to 0.70 times). The length of the modulation electrode 1 is desirably slightly shorter than the resonance point for the high-frequency signal by about 20 to 25%, so that the combined impedance with the stub electrode stays in an appropriate region. .
[0020]
The distance between the short stubs 4 and 5 and the common electrodes 6 and 7 is 27 μm, the width is 50 μm, and the length is 875 μm, which is 0.12 times the wavelength of the high frequency signal which is a modulation signal. The short stubs 4 and 5 are connected to substantially the center of the modulation electrode 1 and are provided obliquely with respect to the modulation electrode 1. A power supply line is connected to the connection portion. Although this connection is connected in a direct current manner by a continuous conductor in FIG. 1A, it is not a direct current connection but an electromagnetic connection by a capacitance or an inductance. Even so, the purpose can be achieved. The power supply line includes a wiring or a tapered transformer. In FIG. 1, a 100 μm wiring is provided between the connection portion and the tapered transformer. Even if the short stubs 4 and 5 are moved along the wiring, no particular inconvenience occurs. Here, there is no essential reason for providing the wiring, and the wiring can be omitted. Further, the tapered transformer is for supplying a signal from the coplanar waveguide of the input portion to the short stub without being reflected back, and has a length of 800 μm and a width inclined from 100 μm to 35 μm. Has become. Further, in accordance with the inclination, the distance from the common electrode is also inclined from 325 μm to 107.5 μm. Further, the short stub suppresses an excessive decrease in the input impedance of the modulation electrode with respect to the frequency, and is particularly effective in suppressing an excessive decrease in the impedance at the time of resonance. The short stub needs to be adjusted so that the effect of the input high-frequency signal on the phase change of light (induced phase amount) is maximized.
[0021]
In the waveguide on the modulating electrode of the Mach-Zehnder-interferometer type resonant optical modulator for modulating the intensity of 1.55 micron wavelength light with a high frequency signal having a center frequency of 10 GHz having the above structure. FIG. 2 shows the results of computer simulation of the intensity and phase of the electric field viewed from the light traveling through. The electric field strength in this figure is normalized by the amplitude of the input high-frequency signal, and it can be seen that the electric field strength is high. The phase is increasing from left to right, as viewed from the traveling light. FIG. 3 shows a computer simulation result when optimization is not performed (specifically, the modulation electrode length is set to 0.5 times the wavelength of the high-frequency signal). As can be easily understood from a comparison between FIG. 2 and FIG. 3, it can be seen that the potential on the modulation electrode is increased by the optimization. FIG. 4 shows a computer simulation result when no stub is used (except for the stub, the same conditions as those for obtaining the result of FIG. 2). As can be easily understood from a comparison between FIG. 2 and FIG. 4, it can be seen that the potential on the modulation electrode is increased by using the stub.
[0022]
In FIG. 1, two short stubs are used, but the same effect can be obtained by using one short stub. The structure of the short stub in this case may be 1.42 times as long as the two cases described above.
[0023]
2) Example 2
Next, as a second embodiment, FIG. 5A is a schematic diagram of a resonance type optical modulator using a modulation electrode formed by an asymmetric coplanar waveguide for a high-frequency signal having a short-circuited end, and FIG. It is shown in (b). Similarly to the above, this resonance type optical modulator also shows a configuration of a Mach-Zehnder-interferometer type optical modulator for intensity-modulating light having a wavelength of 1.55 μm with a high-frequency signal having a center frequency of 10 GHz. . Since the substrate and the cross-sectional structure are the same as those in the first embodiment, the description thereof will be omitted.
[0024]
The modulation electrode 8 has a shape in which both ends are connected to the common electrode, the distance between the common electrodes 6 and 7 is 27 μm, the width is 5 μm, and the length is 7010 μm. 0.41 times, but can be set to about 0.41 to 0.44 times (or about 0.91 to 0.94 times). Similarly to the above case, the length of the modulation electrode 8 is slightly from the resonance point for the high-frequency signal by about 15 to 20%, so that the combined impedance with the stub electrode stays in an appropriate region. It is desirable that the lengths are shifted.
[0025]
The short stubs 4 and 5 have a distance of 27 μm from the common electrodes 6 and 7 and a width of 50 μm and a length of 1165 μm, which is 0.16 times the wavelength of the high frequency signal which is a modulation signal. The short stubs 4 and 5 are connected to substantially the center of the modulation electrode 1 and are provided obliquely with respect to the modulation electrode 1. A 100 μm wiring is provided from the connection to the tapered transformer. Also in this case, there is no essential reason for providing the wiring, and it is possible to omit the wiring. The short stub needs to be adjusted so that the effect of the input high-frequency signal on the phase change of light (induced phase amount) is maximized.
[0026]
When comparing the modulation electrode length at the open end with the modulation electrode length at the short-circuit end, the wavelength of the high-frequency signal on the electrode is almost half the wavelength of the modulation electrode at the open end. In the case of a modulation electrode, it corresponds to almost one wavelength. Therefore, they can be selected according to the desired modulation electrode length.
[0027]
3) Example 3
FIG. 8 shows a Mach-Zehnder-interferometer type modulation electrode having a length of 0.69 times the high-frequency signal wavelength for intensity-modulating 1.55 micron wavelength light with a high-frequency signal having a center frequency of 10 GHz. FIG. 8 is a diagram showing a computer simulation result of the intensity and phase of an electric field of the resonant optical modulator viewed from the light traveling in the waveguide on the modulation electrode. In this case, it can be seen that the phase of the electric field becomes 180 degrees or more and is inverted around -0.4λ on the horizontal axis. In this part where the phase is 180 degrees or more, it can be seen that the effect of the input high-frequency signal on the phase change of the light (the amount of induced phase) is opposite to that of the other parts.
[0028]
Due to this phenomenon, the amount of induced phase as a sum may decrease. In order to prevent this decrease, it is desirable that the modulation electrode and the optical path are further separated in a region where the phase exceeds 180 degrees, or the polarization direction of the substrate is reversed. In the case of a separated configuration, the amount of induced phase in this part does not increase, but this inversion of this part turns the aforementioned decrease into an increase. In order to reverse this polarization direction, a high DC voltage of 20 to 25 kV / mm may be applied. For example, the method is described in a paper (H. Murata, K. Kinoshita, G. Miyaji, A. Morimoto and K. K.). Kobayashi, "Quasi-velocity-matched LiTaO3 guided wave optical phase modulator for integrated ultrashort optical pulse generators", ELECTRONIC LETTERS 17 th August 2000, vol.36 No.17, are described in the 1459-1460). This concludes the description of the third embodiment.
[0029]
Any of the modulators described above can be connected in series, as shown in FIG. 6, to enhance the modulation effect. At this time, a high-frequency signal that is a modulation signal is adjusted by a delay unit in accordance with the propagation of light.
[0030]
In the above description, the modulation electrode and the common electrode are configured to directly face each other, but an ICPW (Interdigital Coplanar) in which another conductor equivalent to an isolated state is inserted between the modulation electrode and the common electrode. Waveguide) can still achieve the object of the present invention. The waveguide in which such a power transmitting body is arranged is described in a paper (David A. Thompson and Robert L. Rogers, IEEE MIVROWAVE AND GUIDED WAVE LETTERS, VOL., 8, NO. 7, p. It is described in. The configuration used in the present invention is an asymmetric ICPW in which one of the common electrodes is omitted from the configuration, and this is shown in FIG.
[0031]
Further, in the above embodiment, the intensity modulator has been described. However, since the polarization modulator using the electro-optic effect has the same configuration as the above intensity modulator, it is apparent that the present invention can be applied. It is. In addition, since a phase modulator using the electro-optic effect is a modulator having only an optical path having a modulation electrode, it is obvious that the present invention can be applied.
[0032]
【The invention's effect】
Since the present invention has the above-described configuration, the following effects can be obtained.
[0033]
According to the first and second aspects of the invention, the electric field effective for modulation applied to the optical path is increased while the manufacturing is easy, so that it is possible to realize a resonance type optical modulator having a small power and a high modulation effect. Was.
[0034]
Further, in the third invention, the arrangement is balanced in the left and right directions, and unexpected parasitic vibration is not caused.
[0035]
Further, in the fourth invention, the configuration is simple, and the design is easy.
[0036]
Further, in the fifth and sixth inventions, since the optimum length of the modulation electrode is different, it is possible to select the length in accordance with the length of the modulation electrode to be used.
[0037]
Further, in the seventh and eighth inventions, the shape of a stub to be used can be selected according to the presence or absence of a DC bias or a low frequency bias.
[Brief description of the drawings]
FIGS. 1A and 1B are schematic diagrams of a resonance type optical modulator using a modulation electrode having an open end, where FIG. 1A is a plan view and FIG. 1B is a cross-sectional view.
FIG. 2 shows a Mach-Zehnder-interferometer type resonant optical modulator for intensity-modulating 1.55 micron wavelength light with a high-frequency signal having a center frequency of 10 GHz in a waveguide on a modulation electrode. It is a figure showing the computer simulation result about the intensity and the phase of the electric field seen from the light which advances.
FIG. 3 shows a case where light having a wavelength of 1.55 μm is intensity-converted by a high-frequency signal having a center frequency of 10 GHz when optimization is not performed (specifically, a modulation electrode length is set to 0.5 times the wavelength of a high-frequency signal). FIG. 9 is a diagram illustrating a computer simulation result on the intensity and phase of an electric field of a Mach-Zehnder-interferometer type resonant optical modulator for modulating light, as viewed from light traveling in a waveguide on a modulation electrode.
FIG. 4 shows a modulation electrode of a Mach-Zehnder-interferometer type resonant optical modulator for intensity-modulating 1.55 micron wavelength light with a high-frequency signal having a center frequency of 10 GHz without using a stub. It is a figure which shows the computer simulation result about the intensity | strength and phase of the electric field seen from the light which propagates in the above-mentioned waveguide.
5A and 5B are schematic diagrams of a resonance type optical modulator using a modulation electrode having a short-circuited end, where FIG. 5A is a plan view and FIG. 5B is a cross-sectional view.
FIG. 6 is a block diagram showing a configuration in which a plurality of modulators are connected in series to perform modulation.
FIG. 7 is a schematic diagram showing a known resonance type optical modulator.
FIG. 8 is a Mach-Zehnder-interferometer type modulation electrode having a length of 0.69 times the high-frequency signal wavelength for intensity-modulating light having a wavelength of 1.55 μm with a high-frequency signal having a center frequency of 10 GHz. FIG. 10 is a diagram showing a computer simulation result of the intensity and phase of an electric field of the resonant optical modulator viewed from light traveling in the waveguide on the modulation electrode.
FIG. 9 is a schematic diagram showing a modulator using an asymmetric ICPW (Interdigital Coplanar Waveguide) that can be used as a modulation electrode.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 modulation electrode 2 wiring 3 tapered transformer 4, 5 stub 6, 7 common electrode 8 modulation electrode 9 silicon oxide layers 10, 11, 12 waveguide

Claims (8)

An optical path having electro-optic effect characteristics, a modulation electrode for applying an electric field to an optical path formed along the optical path, a common electrode formed opposite to the modulation electrode, and the modulation electrode formed by a modulation signal. A power supply line electromagnetically connected to the modulation electrode at a valley portion of the potential distribution, a stub connected to the power supply line, and a common electrode, and one of the regions divided by the modulation electrode. A resonance-type optical modulator, wherein the power supply line, the stub, and the common electrode are provided on the side , and the modulation electrode has a size that resonates with a modulation signal . The resonance type optical modulator according to claim 1, wherein the power supply line includes a tapered transformer. 3. The resonance type optical modulator according to claim 1, wherein the number of the stubs is an even number, and the stubs are provided at symmetric positions with respect to the feeder line. The resonance type optical modulator according to claim 1, wherein the modulation electrode and the power supply line intersect at a right angle, and the stub has an arrangement in contact with the power supply line and the modulation electrode. 5. The resonance type optical modulator according to claim 1, wherein the modulation electrode and the common electrode formed opposite to the modulation electrode form open ends at both ends. 5. The resonance type optical modulator according to claim 1, wherein the modulation electrode and a common electrode formed opposite to the modulation electrode form short-circuited ends at both ends. 7. The resonance type optical modulator according to claim 1, wherein the stub is an open stub. 7. The resonance type optical modulator according to claim 1, wherein the stub is a short stub.

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