JP3556873B2 - Optical modulation method and optical modulator - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical modulation method and an optical modulator, and more particularly, to an optical modulation method and an optical modulator that can be suitably used in a high-speed, large-capacity optical fiber communication system, an optical measuring instrument, and the like.
[0002]
[Prior art]
With the recent progress in high-speed, large-capacity optical fiber communication systems, lithium niobate (LiNbO3: hereafter) has been used instead of the conventional direct modulation of a laser diode for reasons such as broadband characteristics, low chirp characteristics, and small propagation loss. , LN) are being put to practical use.
This optical modulator is roughly classified into three according to the driving method.
[0003]
One of them is called a lumped constant type and has a configuration in which a parallel electrode is arranged at an open end of a modulation electrode of an optical modulator. Then, a signal is directly applied from the output of the signal generator to modulate the light wave. The other type is a traveling wave type, which applies a micro traveling wave to a modulation electrode of an optical modulator and modulates the light wave while maintaining speed matching with the light wave in the optical waveguide.
The last one is called a resonance type, in which a modulation signal is induced in each of the plurality of resonance electrodes by using a plurality of resonance electrodes, and a light wave is modulated by the modulation signal.
[0004]
[Problems to be solved by the invention]
In the lumped-constant type, the modulation region can be expanded by increasing the length of the parallel electrode, whereby the light wave can be more efficiently modulated. However, when the length of the parallel electrodes is increased, the capacitance component of the parallel electrodes increases, and a phenomenon occurs in which the band due to the parallel capacitance is limited. For this reason, there is a problem in that modulation can be performed only up to about several hundred MHz, and cannot be used for high-speed optical modulation.
In addition, the traveling wave type modulates while maintaining the velocity matching between the light wave and the micro traveling wave, and thus is not affected by its own capacitance. For this reason, modulation up to about several tens of GHz can be performed, and a large band can be realized. For this reason, although the mainstream of the current optical modulator is used, there is a problem that the operating voltage is high.
[0005]
Furthermore, the resonance type is in the research stage at present, and has not reached the practical level. Further, since the resonance principle is used, it is limited to narrow band applications. While the operating voltage per unit length can be smaller than that of the traveling wave type, the length of the resonance electrode per unit is as short as several mm or less. Must be used. Therefore, it is extremely difficult to design and arrange the plurality of resonance electrodes so as to achieve speed matching with the light wave.
[0006]
SUMMARY OF THE INVENTION It is an object of the present invention to provide a novel light modulation method and a light modulator capable of efficiently modulating a light wave at a low operating voltage.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, an optical modulation method according to the present invention generates a standing wave type modulation signal over the entire electrode length of a modulation electrode in a so-called waveguide type optical modulator. This modulates the light wave guided in the optical waveguide only by the amplitude component of the same sign in the modulation signal of FIG.
[0008]
Generally, a standing wave is generated by interference between an incident wave and a reflected wave generated by the incident wave being reflected at an open end or a short-circuited end. Therefore, the antinode portion of the standing wave is twice as large as the case of the incident wave alone. That is, the standing wave type modulation signal is twice as large as the incident modulation signal from the external power supply, whose antinode is represented by a micro traveling wave.
In the present invention, such a standing wave type modulation signal is generated over the entire electrode length of the modulation electrode, and is modulated using only the same sign component of the standing wave type modulation signal. I have to.
[0009]
Even when a standing wave type modulation signal having a double amplitude component as described above is used, if the lightwave is modulated using the entire modulation signal (all amplitude components), the amplitude component of a different sign is obtained. Modulations cancel each other out. For this reason, it is almost impossible to modulate the light wave.
On the other hand, in the present invention, the modulation is performed using only the same sign amplitude component of the standing wave type modulation signal. Therefore, the modulation does not cancel each other as described above, and the light wave can be modulated by the double amplitude component of the standing wave type modulation signal. That is, in the optical modulation method of the present invention, modulation is performed using a standing wave type modulation signal having twice the amplitude of an incident modulation signal such as a micro traveling wave of a traveling wave type optical modulator.
[0010]
On the other hand, in the traveling wave type optical modulator, the speed of the light wave and the speed of the micro traveling wave cannot be perfectly matched, so that the actual modulation is performed with an amplitude smaller than the amplitude of the micro traveling wave which is an incident modulation signal. It is.
Therefore, according to the optical modulation method of the present invention, the same modulation can be performed with an operating voltage that is less than half of the conventional optical modulation method in a traveling wave type optical modulator or the like. For this reason, the operating voltage can be greatly reduced as compared with a conventional optical modulator represented by a traveling wave optical modulator. That is, according to the light modulation method of the present invention, light waves can be efficiently modulated at an extremely low operating voltage.
[0011]
Further, in the present invention, the standing wave type modulation signal can be obtained by reflecting an incident modulation signal, which has been incident on a modulation signal from an external power supply, at the end of the modulation electrode. In this case, the length of the modulation electrode of the waveguide type optical modulator used in the present invention is λ / 2 × n with respect to the wavelength λ of the incident modulation signal when the terminal of the modulation electrode is a short-circuited end. (N: natural number), and when the end of the modulation electrode is an open end, (n + 1/2) × λ / 2 (n: natural number). Further, the modulation electrode is bent at a period of λ so that the width of the convex surface or the width of the concave surface is λ / 2.
[0012]
FIG. 1 is a schematic diagram showing the configuration of such a modulation electrode.
The modulation electrode 1 shown in FIG. 1 is formed in a concavo-convex shape so that the length of the bending period represented by A → B → C → D → E becomes equal to the wavelength λ of the incident modulation signal. Then, the width of the convex surface 2 shown by A → B is formed so as to be λ / 2. The terminal Z of the modulation electrode 1 forms a short-circuited end. Therefore, the entire length of the modulation electrode is set to about 7 / 2λ in order to generate a standing wave type modulation signal with respect to the wavelength λ of the incident modulation signal. This is the case where n = 7 in the above equation λ / 2 × n (n: natural number).
[0013]
FIG. 2 is a diagram schematically showing a standing wave type modulation signal generated in the modulation electrode 1 shown in FIG.
In the modulation electrode 1 shown in FIG. 1, when an incident modulation signal is taken in from the point A which is one end, a standing wave type modulation signal as shown in FIG. That is, a standing wave type modulation signal corresponding to a half wavelength is generated on the convex surface 2 having a width of λ / 2. The standing wave type modulation signal has the same amplitude component on each convex surface 2. Therefore, by bringing the convex surface 2 of the modulation electrode 1 close to the optical waveguide, it is possible to modulate the light wave with a standing wave type modulation signal having the same sign.
[0014]
In this case, a standing wave type modulation signal is also generated on the concave surface 3 paired with the convex surface, and the amplitude component of this modulation signal is compared with the amplitude component of the modulation signal on the convex surface 2. Small. Therefore, the convex surface 2 on which a standing wave type modulation signal for half a wavelength is normally generated is brought close to the optical waveguide.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail in accordance with embodiments of the present invention with reference to the drawings.
FIG. 3 is a plan view schematically showing one example of the optical modulator of the present invention. Note that, in order to clarify the features of the present invention, the optical modulator shown in FIG.
The waveguide type optical modulator 20 shown in FIG. 3 includes a substrate 11 made of a material having an electro-optic effect, an optical waveguide 12, and a modulation electrode 14. Further, the optical waveguide 12 has two branch optical waveguides 13-1 and 13-2. Further, the modulation electrode 14 is formed in an uneven shape such that the bending period P → Q → R → S → T is equal to the wavelength λ of the incident modulation signal incident from the external power supply 15. Further, the width W of the concave surface 14A of the modulation electrode 14 is formed so as to be λ / 2.
[0016]
An external power source 15 is attached to one end P of the modulation electrode, and the other end 14E is open. Therefore, in the optical modulator 20 shown in FIG. 3, the incident modulation signal having the wavelength λ is reflected at the end portion 14E, and a standing wave is generated for the incident modulation signal having the wavelength λ over the entirety of the modulation electrode 14. The overall length is set to 7 / 4λ to generate a modulation signal of the type. That is, this is a case where n = 3 in the above equation (n + ／) × λ / 2 (n: natural number).
[0017]
When an incident modulation signal having a wavelength λ is incident on the modulation electrode 14 from the external power supply 15, the concave surfaces 14A and 14C and the convex surface of the modulation electrode 14 are changed according to the bending period of the modulation electrode 14 shown in FIG. At 14B and 14D, a standing wave type modulation signal as shown in FIG. 5 is generated. That is, a standing wave type modulation signal having a wavelength of λ / 2 is generated on the concave surfaces 14A and 14C, and if the width of the convex surface 14B is W / 3, for example, the convex surface 14B has a concave surface. A modulation signal having a wavelength of about λ / 6 having a different amplitude and polarity from the generated modulation signal is generated.
The magnitude of the wavelength of the modulation signal generated on the concave surface and the convex surface is determined by the width P of the concave surface and the width of the convex surface with respect to the bending period P → Q → R → S → T of the modulation electrode 14. It is determined according to R → S. That is, the concave surface has a width of about half of the bending period, and the convex surface has a width of about 1/6 of the bending period. A modulated signal of a standing wave type having a wavelength of various sizes is generated.
[0018]
The end portion 14E of the modulation electrode 14 is an open end, and reflects an incident modulation signal at this portion to generate a standing wave type modulation signal over the entire modulation electrode 14. Therefore, the antinode of the standing wave type modulation signal is located at the end 14E. And the convex part surface 14D in which the end part E is located has about half width of the convex part surface 14B. Therefore, the standing wave type modulation signal generated on the convex surface 14D has the same polarity amplitude as the standing wave type modulation signal generated on the convex surface 14B, and the magnitude of the wavelength is approximately It is λ / 12 which is half.
[0019]
The light wave introduced from the entrance 12A of the optical waveguide 12 in the optical modulator 20 branches at the branch point 12B, and then propagates through the branch optical waveguides 13-1 and 13-2. The light wave guided through the branch optical waveguide 13-1 is modulated by the standing wave type modulation signal generated on the concave surfaces 14A and 14C of the modulation electrode 14 shown in FIG. On the other hand, the light wave guided through the branch optical waveguide 13-2 is modulated by a standing wave type modulation signal generated on the convex surfaces 14B and 14D of the modulation electrode 14. Then, the modulated light waves are coupled at the coupling point 12C, and are extracted from the output port 12D of the optical waveguide 12.
[0020]
As is clear from FIG. 5 and the above description, the amplitude polarities of the standing wave type modulation signals generated on the concave surface and the convex surface are different from each other. For this reason, the light waves guided through the branch optical waveguides 13-1 and 13-2 are respectively subjected to modulations in opposite directions.
As described above, since the standing wave type modulation signal itself has a large amplitude, the light wave itself can be modulated larger than the traveling wave type modulation signal. Further, in the optical modulator 20 shown in FIG. 3, the respective light waves guided through the branch optical waveguide are modulated in opposite directions by two kinds of standing wave type modulation signals having different amplitude polarities. Therefore, the light wave extracted from the optical waveguide 12 undergoes a considerably large modulation.
[0021]
When the optical modulator having the configuration shown in FIG. 3 is used, it is possible to perform 2 to 4 times the modulation with respect to the traveling wave type optical modulator having the same length modulation electrodes. Therefore, when the same level of modulation is performed on such a traveling-wave optical modulator, the operating voltage can be reduced to about 1/2 to 1/4.
[0022]
The optical modulator 20 shown in FIG. 3 has an open end at the end 14E corresponding to the end of the modulation electrode 14, but may have a short-circuited end similarly to the case of the modulation electrode shown in FIG.
By setting the terminal end for reflecting the incident modulation signal to be an open end or a short-circuited end, the above-described expressions of λ / 2 × n (n: natural number) and (n + ／) × λ / 2 (n: natural number) As is clear from FIG. 5, a standing wave type modulation signal having a half wavelength pitch can be generated on the modulation electrode. For this reason, the magnitude of the amplitude of the antinode of the standing wave type can be made approximately twice as large as that of the incident modulation signal. Therefore, the light wave can be modulated with a larger modulation signal, and the operating voltage of the light modulator can be further reduced.
[0023]
However, it is preferable that the terminal end of the modulation electrode be a short-circuited end as shown in FIG. Assuming that the end of the modulation electrode is an open end, the antinode of the standing wave type modulation signal is located at the end. Then, for example, in the case of the optical modulator shown in FIG. 3, the standing wave type modulation signal generated on the convex surface 14D where the end 14E corresponding to the end of the modulation electrode is located does not have the end. Only a signal having a size corresponding to a half wavelength of the modulation signal generated on the convex surface 14B is generated.
[0024]
On the other hand, if the end 14E is a short-circuited end, for example, a node portion of the standing wave type modulation signal is located in this portion. Therefore, the modulation signal generated on the convex surface 14D and the modulation signal generated on the convex surface 14B have the same wavelength. For this reason, compared with the case where the terminal end of the modulation electrode is an open end, the component amount of the same sign amplitude component of the modulation signal increases. Therefore, the light wave guided through the optical waveguide can be modulated to a greater extent, and as a result, the operating voltage of the optical modulator can be further reduced.
[0025]
In the optical modulator 20 shown in FIG. 3, as described above, each light wave guided through the branch optical waveguides 13-1 and 13-2 is generated on the concave surface and the convex surface, and has the same sign having different polarities from each other. Simultaneous modulation is performed with a standing wave type modulation signal having an amplitude component.
However, one of the branch optical waveguides 13-1 and 13-2 is guided only by the standing wave type modulation signal having the same amplitude component generated on one of the concave surface and the convex surface. It is also possible to modulate only the light waves that are emitted.
The amplitude components of the same sign formed on the concave surface and the convex surface can be obtained by increasing one of the amplitude components as shown in FIG. 2 and modulating one of the branch optical waveguides more greatly. it can. Also, the amplitude of the amplitude component formed on the concave surface and the convex surface can be made the same, and the light waves guided through both branch optical waveguides can be equally modulated.
[0026]
A standing wave type modulation signal has an extremely large amplitude component contributing to modulation as compared with a traveling wave type modulation signal such as a micro traveling wave in a traveling wave type optical modulator. Therefore, without modulating the light waves guided through the two branch optical waveguides in opposite directions with the modulation signals having the same sign but different polarities, only the light waves guided through one of the branch optical waveguides can be used. Even if modulation is performed with a modulation signal of the same sign amplitude component of either polarity, sufficiently large modulation can be applied. Therefore, also in this case, the operating voltage can be reduced as compared with the traveling wave type optical modulator.
[0027]
For example, in the case of the optical modulator 20 shown in FIG. 3, the concave surfaces 14A and 14C of the modulation electrode 14 are arranged close to the branch optical waveguide 13-1, and the convex surfaces 14B and 14D are connected to the branch optical waveguide 13-. By arranging them separately from the second, only a light wave guided through the branch optical waveguide 13-1 is modulated by a modulation signal having the same sign amplitude component of the magnitude of λ / 2.
Also, by arranging the concave surfaces 14A and 14C of the modulation electrode 14 so as to be separated from the branch optical waveguide 13-1, and arranging the convex surfaces 14B and 14D close to the branch optical waveguide 13-2, about A modulation signal having the same sign amplitude component of λ / 6 and about λ / 12 modulates only the light wave guided through the branch optical waveguide 13-2.
[0028]
Although the Mach-Zehnder type optical waveguide has been described above, the optical modulation method and the optical modulator of the present invention are not limited to the Mach-Zehnder type optical waveguide. For example, the present invention can be used for a phase modulator including a single optical waveguide without a branch optical waveguide.
[0029]
【Example】
Hereinafter, the present invention will be specifically described based on examples.
(Example)
In the present embodiment, an optical modulator 30 as shown in FIG. 6 was used, and its modulation characteristics and operating voltage were examined.
A Z-cut plate of lithium niobate is used as the substrate 21. A photoresist is formed on the main surface 21A of the substrate 21 to a thickness of 0.5 μm using a spin coater, and then exposed and developed to perform development. An optical waveguide pattern having a width of 7 μm was formed. Next, a layer made of titanium is formed to a thickness of 800 ° on this optical waveguide pattern by a vapor deposition method, and heat treatment is performed in an electric furnace at 1000 ° C. for 20 hours to diffuse the titanium into the substrate 21 to have a width of 9 μm. Was formed.
[0030]
Thereafter, an SiO ₂ layer was formed to a thickness of 0.55 μm on the substrate 21 through a predetermined mask by an evaporation method. Next, the modulation electrode 24 was formed with a thickness of 20 μm on the branch optical waveguide 23-1 of the optical waveguide 22 by using both the vapor deposition method and the plating method. An external power supply 25 is connected to one end 24A of the modulation electrode, and the other end 24E corresponding to the end of the modulation electrode is grounded. The width L of the convex surface 24B of the modulation electrode 24 was set to 58.3 mm.
A microwave having a frequency of 820 MHz (wavelength: 37 mm) was incident on the modulation electrode 24 from an external power supply, and a light wave was incident on the optical waveguide 22. Then, only the light wave guided through the branch optical waveguide 23-1 is modulated.
[0031]
The modulated signal thus obtained is +5 dB compared to the traveling wave, and it can be seen that the response is improved. The operating voltage was 2.8V.
By connecting an oscilloscope, it was confirmed that a standing wave having a half size of the wavelength of the incident modulation signal was generated on the convex surface 24B of the modulation electrode.
[0032]
(Comparative example)
In this comparative example, a traveling wave type optical modulator 40 as shown in FIG. 7 was manufactured, and its modulation characteristics and operating voltage were examined. The optical modulator shown in FIG. 6 differs from the optical modulator only in the configuration of the modulation electrode, and the other portions have the same configuration. Therefore, similar parts are represented using the same numerals.
[0033]
Further, a substrate to be used, an optical waveguide, and a modulation electrode were formed in the same manner as in the example. However, the modulation electrode is composed of the signal electrode 34 and the ground electrodes 36-1 and 36-2 to produce a traveling-wave-type electrode, and only the light wave guided through the branch optical waveguide 23-1 is used as in the embodiment. Was modulated. An external power supply 25 is connected to one end of the signal electrode 34, and the other end is connected to ground electrodes 36-1 and 36-2 via a terminating resistor 37.
In the optical modulator 40 of this comparative example, the length of the region where the light wave and the modulation signal substantially interact, which corresponds to the width L of the convex surface of the modulation electrode 24 of the optical modulator 30, is set to L is the same.
[0034]
A light wave is introduced into the optical waveguide 22 of such an optical modulator 40, and a microwave having the same frequency (wavelength) as that of the embodiment is incident on the signal electrode 34, and its modulation characteristics and operating voltage are examined. Was. As a result, the operating voltage was 5V.
[0035]
As is clear from the above examples and comparative examples, in the embodiment according to the optical modulation method and the optical modulator of the present invention, the operation for obtaining the same level of modulation as the conventional traveling wave type optical modulator. It can be seen that the voltage can be extremely small, about 0.56 times.
[0036]
As described above, the present invention has been described in detail based on the embodiments of the present invention with reference to specific examples. However, the present invention is not limited to the above contents, and any modifications and changes are possible without departing from the scope of the present invention. Is possible.
[0037]
【The invention's effect】
As described above, according to the light modulation method of the present invention, a modulation signal of the standing wave type is generated on the modulation electrode, and the light wave is modulated only by the amplitude component of the same sign of the modulation signal. I have. As a result, the light wave can be largely modulated, and the operating voltage can be reduced as compared with a conventional traveling wave type light modulator or the like.
Further, according to the optical modulator of the present invention, the entire length of the modulation electrode is appropriately adjusted depending on whether the end of the modulation electrode is an open end or a short-circuited end. Therefore, by merely reflecting the incident modulation signal at the end of the modulation electrode, a standing wave type modulation signal can be easily generated without complicating the electrode structure.
[Brief description of the drawings]
FIG. 1 schematically shows the structure of a modulation electrode in a light modulation method and a light modulator according to the present invention.
FIG. 2 is a diagram showing a standing wave type modulation signal obtained by the modulation electrode shown in FIG.
FIG. 3 is a plan view showing an example of the optical modulator of the present invention.
FIG. 4 is a diagram showing a modulation electrode of the optical modulator shown in FIG.
5 is a diagram showing a standing wave type modulation signal obtained by the modulation electrode shown in FIG. 4;
FIG. 6 is a plan view showing another example of the optical modulator of the present invention.
FIG. 7 is a plan view showing a conventional traveling wave optical modulator.
[Explanation of symbols]
1, 14, 24 Modulation electrode 2, 14B, 14D Modulation electrode convex surface 3, 14A, 14C Modulation electrode concave surface 11, 21 Substrate 12, 22 Optical waveguide 13-1, 13-2, 23- 1, 23-2 Branch optical waveguide 14E End (end) of modulation electrode
15, 25 External power supply 20, 30, 40 Optical modulator 34 Signal electrode 36-1, 36-2 Ground electrode 37 Termination resistance λ Wavelength of incident modulation signal W Width of concave surface of modulation electrode L Projection of modulation electrode Face width

Claims (8)

A waveguide type optical modulation comprising a substrate made of a material having an electro-optic effect, an optical waveguide for guiding a light wave, and a modulation electrode for applying a modulation signal to the light wave to modulate the light wave. Light modulation method of the vessel,
With respect to the wavelength λ of the modulation signal, the modulation electrode is bent at a period of λ to form an uneven shape, and the convex surface or the concave surface of the modulation electrode is arranged close to the optical waveguide. At the same time, the end of the modulation electrode is an open end or a short-circuited end, and a predetermined incident wave to the modulation electrode and the incident wave are generated by reflection at the open end or the short-circuited end of the modulation electrode. A reflected wave and a standing wave type modulation signal generated by interfering with each other are generated over the entire electrode length of the modulation electrode, and the amplitude component of the same sign in the standing wave type modulation signal is used as the modulation component. An optical modulation method, wherein an optical wave is modulated. The control is performed by adjusting the entire length of the modulation electrode to λ / 2 × n or (n + ／) × λ / 2 (n: natural number) with respect to the wavelength λ of the modulation signal. The light modulation method according to claim 1, wherein The optical waveguide is a Mach-Zehnder type optical waveguide, and a light wave guided through one of the two branched optical waveguides of the Mach-Zehnder type optical waveguide is converted into an amplitude component having the same sign in the standing wave type modulation signal. The optical modulation method according to claim 1, wherein the light is modulated by: The modulation electrode, the convex surface is arranged close to one branch optical waveguide of the Mach-Zehnder optical waveguide, and the concave surface of the modulation electrode is the other of the Mach-Zehnder optical waveguide. Are disposed in close proximity to the branch optical waveguides, and each of the light waves guided through the two branch optical waveguides has a different polarity generated in the convex surface and the concave surface of the modulation electrode. 4. The optical modulation method according to claim 3, wherein modulation is performed with a standing wave type modulation signal. A waveguide type light comprising a substrate made of a material having an electro-optic effect, an optical waveguide for guiding a light wave, and a modulation electrode for applying a modulation signal to the light wave to modulate the light wave. A modulator,
The end of the modulation electrode is a short-circuited end, and the entire length of the modulation electrode is set to λ / 2 × n with respect to an incident modulation signal wavelength λ applied to the modulation electrode from an external power supply. n: a natural number), the modulation electrode is bent at a period of λ to form irregularities, the width of the convex surface or the concave surface is set to λ / 2, and one of the convex surface or the concave surface is used. Is arranged in close proximity to the optical waveguide, the light wave guided in the optical waveguide, is modulated by a standing wave type modulation signal of the same sign generated in the convex surface or the concave surface. Light modulator. A waveguide type light comprising a substrate made of a material having an electro-optic effect, an optical waveguide for guiding a light wave, and a modulation electrode for applying a modulation signal to the light wave to modulate the light wave. A modulator,
The end of the modulation electrode is set to an open end, and the entire length of the modulation electrode is set to (n + ／) × with respect to an incident modulation signal wavelength λ applied to the modulation electrode from an external power supply. λ / 2 (n: natural number), the modulation electrode is bent at a period of λ to form irregularities, and the width of the convex surface or concave surface is λ / 2, and the convex surface or the convex surface One of the concave surfaces is arranged close to the optical waveguide, and a light wave guided in the optical waveguide is modulated by a standing wave type modulation signal having the same sign generated on the convex surface or the concave surface. An optical modulator, characterized in that: The optical waveguide is a Mach-Zehnder optical waveguide, and the convex or concave surface having a width of λ / 2 of the modulation electrode formed in the concavo-convex pattern is formed with the Mach-Zehnder optical waveguide. The optical modulator according to claim 5, wherein the optical modulator is arranged close to one of the two branch optical waveguides. In the modulation electrode formed in the concavo-convex pattern, the other concave surface or convex surface paired with the convex surface or the concave surface having the width of λ / 2 is formed by the Mach-Zehnder type optical waveguide. The optical modulator according to claim 7, wherein the optical modulator is arranged close to the other of the two branch optical waveguides.

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