JP3548042B2 - Waveguide type optical device - Google Patents

Description

【００２２】
以上のように、本発明に従い、信号電極の入力部と作用部との間の曲部の中心線曲率半径を７５μｍ以上とすることによって、ロスディップ周波数が著しく高周波側へとシフトし、特に４０ＧＨｚ以上の高周波領域へとシフトさせることができた。
【００２３】
【発明の効果】
本発明によれば、導波路型の光デバイスにおいて、４０ＧＨｚ以上の周波数領域に、ロスディップが発生する最小周波数をシフトさせることによって、４０ＧＨｚ以上の広帯域で実質的に使用可能な導波路型光デバイスを提供できる。
【図面の簡単な説明】
【図１】特願平２−５２０２１号公報に基づく、基板断面の対角線の長さｄとロスディップ周波数の計算値との関係を示すグラフである。
【図２】本発明の実施例に係る光デバイス１１を示す平面図、及びそのＩ−Ｉ線断面図である。
【図３】図２の光デバイスにおいて、信号電極２の曲部２ｅと曲部２ｄとの中心線曲率半径Ｒを５０μｍとしたときの、周波数と光伝送特性との測定結果およびロスディップ周波数を示すグラフである。
【図４】図２の光デバイスにおいて、信号電極２の曲部２ｅの中心線曲率半径Ｒを５０μｍとし、曲部２ｄの中心線曲率半径Ｒを７５μｍとしたときの、周波数と光伝送特性との測定結果およびロスディップ周波数を示すグラフである。
【符号の説明】
１ 基板、２ 信号電極、２ａ 入力部、２ｂ 作用部、２ｃ 出力部、２ｄ 入力部２ａと作用部２ｂとの間の曲部、２ｅ 出力部２ｃと作用部２ｂとの間の曲部、３ 光導波路、４、５ 接地電極、７ 信号電極の幅方向の中心線、１１
導波路型光デバイス、Ｒ 中心線曲率半径[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a waveguide type optical device that can be used in a wide band of 40 GHz or more.
[0002]
[Prior art]
At present, communication capacity is increasing dramatically, and in the analog domain, the carrier frequency is increasing from a microwave band (several GHz) to a millimeter wave band (30 GHz or more). Therefore, there is a demand for a high-speed waveguide type optical device that can be used in a wide band of 40 GHz or more.
As a waveguide type optical device, a ground electrode, a signal electrode, and an optical waveguide are formed on a substrate made of a material having an electro-optic effect, a microwave signal voltage is applied to the signal electrode, and a light wave propagating through the optical waveguide is formed. Modulating optical devices are known.
In such an optical device, there is a problem that transmission characteristics are deteriorated at a specific frequency, so-called “loss dip”. The usable frequency band of such an optical device is limited to the lowest frequency at which a loss dip occurs (hereinafter, referred to as a loss dip frequency).
[0003]
A method of shifting the loss dip frequency to a higher frequency side in a waveguide type optical device is disclosed in, for example, Japanese Patent No. 2669097. According to this publication, in a waveguide-type optical device, a dielectric resonator is formed in a cross-sectional direction of a substrate at a connection portion between an external power supply and a traveling-wave signal electrode, and at a resonance frequency, a microwave power Most of them leaked to the substrate side, and there was a problem that the light wave was not modulated. And this loss dip frequency fc is
fc = co / (2Nmd)
(Co is the speed of light in vacuum, Nm is the effective refractive index of the microwave, and d is the length of the diagonal line in the cross section of the rectangular substrate). An object of the present invention is to shift the loss dip frequency to the 10 GHz band. As a result, the product Nmd of the longest length (usually the length of the diagonal line) d of the substrate cross section perpendicular to the optical waveguide and the effective refractive index Nm of the microwave in this direction is larger than 0.8 mm, It is said that by making the diameter smaller than 11 mm, the frequency of the resonance peak was successfully shifted to 10 GHz or more.
[0004]
[Problems to be solved by the invention]
However, there is a limit in a method of shifting the loss dip frequency to a higher frequency side by reducing the length d of the diagonal line in the cross-sectional direction of the substrate. That is, when the optical device is used in a higher frequency band than the frequency band assumed in the above-mentioned patent publication, there is a problem that the dimensions of the substrate become too small to handle.
[0005]
For example, a single crystal of lithium niobate is used as the material of the substrate, and the thickness of the substrate is 0.5 mm. The effective refractive index Nm of the microwave is 5.36 in the Z-axis direction (the thickness direction of the substrate cross section) and 6.59 in the X and Y-axis directions. Assuming these data, a trial calculation of the loss dip frequency from the above equation shows that d is 2.3 mm when the loss dip frequency is 10 GHz and d when the loss dip frequency is 20 GHz as shown in the graph of FIG. In the case of 1.2 mm and 30 GHz, d is 0.8 mm, and when d = 0.6 mm is estimated, the loss dip frequency is 40 GHz, and in this case, the width of the substrate is 0.4 mm.
[0006]
As can be seen from these results, in order to shift the loss dip frequency to 40 GHz or more, it is necessary to further reduce the thickness and width of the substrate, that is, to reduce the width or thickness of the substrate to 0.4 mm or less. In consideration of the strength of the substrate, workability, etc., it was found that this was not practical.
[0007]
An object of the present invention is to provide a waveguide type optical device which can be used substantially in a wide band of 40 GHz or more by shifting a loss dip frequency to a frequency region of 40 GHz or more in a waveguide type optical device. It is.
[0008]
[Means for Solving the Problems]
The present invention relates to a waveguide type optical device for use in a wide band of 40 GHz or more, comprising a substrate including a material having an electro-optic effect, an optical waveguide formed on the substrate, and an optical wave propagating through the optical waveguide. A traveling-wave-type signal electrode provided on the substrate to control the power supply, and at least one ground electrode. The signal electrode includes an input unit, an operation unit, and an output unit. The center line in the width direction of at least one of the curved portion between the curved portion and the acting portion and the curved portion between the output portion and the acting portion has a radius of curvature of 75 μm or more (hereinafter referred to as “center line radius of curvature”). It is characterized by having.
[0009]
The inventor of the present invention has conducted intensive studies on the radiation point of the microwave signal applied to the signal electrode to the substrate, which is the cause of the occurrence of loss dip, and as a result, the input portion or the output portion of the signal electrode and the working portion It was confirmed that a part of the microwave signal was radiated into the substrate in the curved portion formed therebetween. This radiation into the substrate causes a coupling between the microwave signal and a substrate mode that is undesirable for the original light modulation. That is, the microwave signal that has leaked into the substrate from the signal electrode excites the substrate mode, causing a resonance phenomenon and causing a loss dip.
[0010]
Therefore, by increasing the center line radius of curvature of at least one of a curved portion between the input portion and the acting portion of the signal electrode and a curved portion between the output portion and the acting portion, the curved portion is moved from the curved portion into the substrate. The amount of radiation of microwave signals radiated to the antenna is suppressed, thereby suppressing the leakage of energy that causes the resonance phenomenon, and succeeding in shifting the loss dip frequency to higher frequencies.
[0011]
Further, the present inventor has found that the leakage energy of the microwave signal in the curved portion between the input portion and the working portion of the signal electrode is higher than that in the curved portion between the output portion and the working portion. It is larger than the leakage energy of the wave signal. As a result, when the radius of curvature of the center line of the curved portion between the input portion and the acting portion is set to 75 μm or more, the loss dip frequency is shifted to a high frequency side of 40 GHz or more. And it was even more effective.
[0012]
Particularly preferably, both the curved portion between the input portion and the acting portion and the curved portion between the output portion and the acting portion have a centerline radius of curvature of 75 μm or more. In the present invention, it is particularly preferable that the center line radius of curvature of at least one of the curved portion between the input portion and the acting portion and the curved portion between the output portion and the acting portion is 100 μm or more.
[0013]
In the case where the radius of curvature of one center line of the curved portion between the input portion and the acting portion and the curved portion between the output portion and the acting portion is 75 μm or more, and the radius of the other center line is less than 75 μm. It is particularly preferable that the radius of curvature of the other center line be 50 μm or more.
[0014]
In the present invention, the ground electrode is provided on at least one side of the signal electrode. Particularly preferably, a so-called symmetrical ground electrode arrangement (at least one pair of ground electrodes is provided on both sides of the traveling wave signal electrode). ). By arranging the ground electrode symmetrically with the signal electrode interposed therebetween, the microwave output from the signal electrode can be easily applied to the ground electrodes disposed on the left and right of the signal electrode. The radiation to the side can be further suppressed.
[0015]
As the material of the substrate, an electro-optic crystal such as lithium niobate, lithium tantalate, or a lithium niobate-lithium tantalate solid solution is particularly preferable. As a method for forming the optical waveguide, an internal diffusion method such as a titanium diffusion method or a proton exchange method can be used. Further, the waveguide type optical device of the present invention can be suitably used as an optical modulator, an optical phase modulator, a polarization scrambler, an optical switching element, an optical logic element for an optical computer, and the like.
[0016]
FIG. 2A is a plan view showing an example of the waveguide type optical device of the present invention, and FIG. 2B is a cross-sectional view taken along line II of FIG. 2A. The optical device 11 is a so-called Mach-Zehnder type modulator. On the main surface of the substrate 1, for example, a titanium diffused optical waveguide 3 is formed. The optical waveguide 3 includes an input section 3a, branch sections 3b and 3c, and an output section 3d. A pair of ground electrodes 4 and 5 and a signal electrode 2 sandwiched between the pair of ground electrodes are provided on the main surface of the substrate 1. An insulating region is provided between each of the ground electrodes 4 and 5 and the signal electrode 2.
[0017]
In the optical device of this example, a ground electrode and a signal electrode having a so-called coplanar wave guide (CPW) form are used in order to further improve the operation speed. The signal electrode 2 includes an input section 2a, an action section 2b, an output section 2c, a curved section 2d on the input side, and a curved section 2e on the output side. Reference numeral 7 denotes a center line in the width direction of the signal electrode. According to the present invention, the center line radius of curvature R of at least one of the curved portions 2d and 2e is set to 75 μm or more.
[0018]
【Example】
The optical device 11 as shown in FIG. 2 was manufactured. On a Z-cut lithium niobate wafer, titanium was patterned by a photolithography method, and titanium was diffused by a thermal diffusion method to form an optical waveguide 3. The conditions at this time were that the thickness of titanium was 800 Å, the diffusion temperature was 1000 ° C., and the diffusion time was 20 hours. An insulating buffer layer of SiO 2 was formed on the main surface of the substrate 1 (thickness: 0.5 to 2 μm). Next, electrodes 2, 4, and 5 made of metal plating having a thickness of 15 to 30 μm were formed on these. Next, the wafer was cut to produce an optical device 11. However, the length of the substrate 1 was 60 mm, the thickness was 0.5 mm, and the width was 0.8 mm.
[0019]
In each optical device, the center line radius of curvature R of the curved portion 2d between the input portion 2a and the acting portion 2b and the curved portion 2e between the output portion 2c and the acting portion 2b are changed as shown in Table 1. did.
[0020]
Each optical device and the measuring instrument were connected by a high-frequency cable. A sweep signal whose frequency gradually changed was output from the measuring instrument, the transmission characteristics at each frequency of 0 to 75 GHz were measured, and the loss dip frequency was confirmed. Table 1 shows the results. FIG. 3 shows a case where the center line curvature radius R of the curved portion 2d between the input portion 2a and the acting portion 2b and the curved portion 2e between the output portion 2c and the acting portion 2b is 50 μm. 4 shows the measurement results of the frequency and the optical transmission characteristics (Comparative Example 1). FIG. 4 shows a case where the center line radius of curvature R of the curved portion 2e is 50 μm and the center line radius of curvature R of the curved portion 2d is 75 μm. 5 shows the measurement results of the frequency and the optical transmission characteristics of the first embodiment (Example 1).
[0021]
[Table 1]

[0022]
As described above, according to the present invention, by setting the center line radius of curvature of the curved portion between the input portion and the operating portion of the signal electrode to be 75 μm or more, the loss dip frequency is remarkably shifted to the high frequency side, and particularly, 40 GHz. It was possible to shift to the above high frequency region.
[0023]
【The invention's effect】
According to the present invention, in a waveguide type optical device, a waveguide type optical device which can be substantially used in a wide band of 40 GHz or more by shifting a minimum frequency at which a loss dip occurs to a frequency region of 40 GHz or more. Can be provided.
[Brief description of the drawings]
FIG. 1 is a graph showing a relationship between a length d of a diagonal line of a substrate cross section and a calculated value of a loss dip frequency based on Japanese Patent Application No. 2-52021.
FIG. 2 is a plan view showing an optical device 11 according to an embodiment of the present invention, and a cross-sectional view taken along the line II.
FIG. 3 shows a measurement result of a frequency and an optical transmission characteristic and a loss dip frequency when a center line radius of curvature R between a curved portion 2e and a curved portion 2d of the signal electrode 2 is 50 μm in the optical device of FIG. It is a graph shown.
FIG. 4 shows the frequency and optical transmission characteristics of the optical device of FIG. 2 when the radius R of the center line of the curved portion 2e of the signal electrode 2 is 50 μm and the radius R of the central line of the curved portion 2d is 75 μm. 5 is a graph showing the measurement results and the loss dip frequency.
[Explanation of symbols]
Reference Signs List 1 substrate, 2 signal electrode, 2a input section, 2b action section, 2c output section, 2d curve section between input section 2a and action section 2b, 2e curve section between output section 2c and action section 2b, 3 Optical waveguides, 4, 5 ground electrodes, 7 center lines in the width direction of signal electrodes, 11
Waveguide type optical device, R centerline radius of curvature

Claims (2)

A waveguide type optical device for use in a wide band of 40 GHz or more,
A substrate including a material having an electro-optic effect, an optical waveguide formed on the substrate, and a traveling-wave signal electrode provided on the substrate for controlling a light wave propagating through the optical waveguide; And the signal electrode has an input section, an action section, and an output section, and a curve section between the input section and the action section and a curve section between the output section and the action section. Wherein at least one of the center lines in the width direction has a radius of curvature of 75 μm or more. 2. The waveguide type optical device according to claim 1, wherein a center line in a width direction of the curved portion between the input portion and the acting portion of the signal electrode has a radius of curvature of 75 [mu] m or more.

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