JP4376795B2 - Waveguide type optical modulator - Google Patents

Description

  The present invention relates to a waveguide type optical modulator that modulates intensity (power) of light output from a light source.

  Conventionally, in an optical transmitter used in an optical fiber communication system, a direct modulation method for modulating a current flowing through a laser diode with a data signal has been adopted. However, in the direct modulation method, the influence of dynamic wavelength fluctuation (chirping) of the output optical signal increases as the transmission speed increases due to the chromatic dispersion generated in the optical fiber. It has become.

  In view of such circumstances, an optical transmitter in which an external modulator that hardly causes chirping in principle is applied to CW (continuous wave) light from a light source has been put to practical use. As this type of external modulator, a Mach-Zehnder interferometer is formed by forming an optical waveguide of a predetermined pattern on a waveguide substrate, and an electrode for applying an electric field thereto is provided. It has been known.

  This waveguide type optical modulator connects a first Y-branch optical waveguide that provides an input port, a second Y-branch optical waveguide that provides an output port, and a first and second Y-branch optical waveguide A potential difference is applied between the first and second optical waveguides, the first and second signal electrodes provided on the first and second optical waveguides, and the first and second signal electrodes, respectively. And a ground electrode for applying an electric field to the first and second optical waveguides together with the first and second signal electrodes.

  The light whose power is divided into two by the first Y branch optical waveguide propagates through the first and second optical waveguides, and is in phase (phase difference is 2nπ (n is an integer)) in the second Y branch optical waveguide. When combined, the optical output is turned on, and when combined in the opposite phase (phase difference is (2n + 1) π), the optical output is turned off. By changing the voltage applied to each electrode according to the input signal, the intensity of chirping is low. Modulation is possible.

Referring to FIGS. 1A and 1B, there are shown a plan view and a bb cross-sectional view of a conventional waveguide type optical modulator configured using a Z-cut LiNbO ₃ substrate. This optical modulator guides a Y-branch optical waveguide 4 that provides an input port 2, a Y-branch optical waveguide 8 that provides an output port 6, and optical waveguides 10 and 12 that connect the Y-branch optical waveguides 4 and 8. It is formed on the waveguide substrate 14. The detailed structure and operation principle will be described based on the manufacturing process.

For example, a high refractive index is obtained by heating and diffusing by heating at 1050 ° C. for 7 to 10 hours in a state where Ti is patterned in the same shape as the waveguides 4, 8, 10, and 12 on the waveguide substrate 14 made of LiNbO _3. Waveguides 4, 8, 10, and 12 are formed. When LiNbO ₃ is Z-cut, a strong electric field is required in the Z direction, which is the thickness direction, and thus the signal electrode 16 and the ground electrode 18 are patterned directly above the optical waveguides 10 and 12, respectively. In this case, in order to prevent the light propagating through the optical waveguides 10 and 12 from being absorbed by the electrodes 16 and 18, SiO ₂ is 0 as a transparent buffer layer 20 on each of the waveguides 4, 8, 10 and 12. A film having a thickness of 2 to 1.0 μm is formed, and electrodes 16 and 18 made of Au having a thickness of 3 to 20 μm are formed thereon.

In order to drive this optical modulator, as shown in FIG. 1A, the terminal ends of the signal electrode 10 and the ground electrode 12 are connected by a resistor R to form a traveling wave electrode, and microwaves such as several GHz to 100 GHz are applied from one end. When the electric field 22 is generated between the electrodes 16 and 18 by applying the input signal V, the refractive index of the optical waveguides 16 and 18 changes as + Δn and −Δn, so that the input light of the wavelength λ becomes the input signal. The signal is output from the output port 6 while being on / off-modulated by V.
JP-A-5-210072 JP 7-261135 A Japanese Patent No. 2725341 JP-A-5-142504

  In such an optical modulator having a traveling wave type electrode, the waveguide substrate may act as a microwave resonance box, and the frequency response characteristics may deteriorate due to resonance of microwaves in a specific frequency band. is there. That is, for example, as shown in FIG. 2, a dip occurs in the frequency response characteristic in a graph showing the relationship between the transmission loss from the input to the output of the optical modulator and the frequency.

  In a single drive type optical modulator having only one set of signal electrode and ground electrode as shown in FIGS. 1A and 1B, the frequency response is achieved by narrowing the width of the ground electrode and providing a portion having no electrode on the surface of the chip. Dip in the characteristics can be suppressed. However, a dual drive type optical modulator having two sets of signal electrodes, which will be described later, cannot adopt a single drive type countermeasure and cannot suppress a dip in frequency response characteristics. there were.

  An object of the present invention is to provide a waveguide type optical modulator having a structure capable of effectively suppressing a dip in frequency response characteristics in a dual drive type optical modulator having two sets of signal electrodes.

  Other objects of the present invention will become clear from the following description.

According to the present invention, a first Y-branch optical waveguide that provides an input port, a second Y-branch optical waveguide that provides an output port, and a first that connects between the first and second Y-branch optical waveguides. A potential difference is applied between the first and second optical waveguides, the first and second signal electrodes provided on the first and second optical waveguides, and the first and second signal electrodes, respectively. and a ground electrode for applying an electric field to the first and second optical waveguide together with the first and second signal electrodes, the ground electrode is provided between said first and second optical waveguides A central ground electrode; and first and second ground electrodes other than the central electrode and formed outside the first and second signal electrodes, the central ground electrode having an opening. A waveguide type optical modulator is provided.

  According to the present invention, there is an effect that it becomes possible to provide a dual drive waveguide type optical modulator capable of effectively suppressing a dip in frequency response characteristics.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to describing embodiments of the present invention, a description will be given of the prior art that may be useful in understanding the usefulness of the present invention.

  Referring to FIG. 3, a plan view of a conventional dual drive type optical modulator is shown. The Y branch optical waveguides 4 and 8 and the optical waveguides 10 and 12 are formed on the waveguide substrate 14 in the same manner as shown in FIGS. 1A and 1B. On the optical waveguides 10 and 12, signal electrodes 161 and 162 are provided in a traveling wave type, respectively. In addition, ground electrodes 181 and 182 are formed on both sides of the signal electrodes 161 and 162, and a central ground electrode 183 is formed between the signal electrodes 161 and 162.

  Since microwave signals having phases opposite to each other are applied to one end of each of the signal electrodes 161 and 162 and each other end is terminated by a terminating resistor (not shown), efficient binary modulation is possible. However, in the single drive type optical modulator, the dip in the frequency response characteristic can be suppressed by narrowing the width of the ground electrode and providing a portion having no electrode on the surface of the chip. In the dual drive type optical modulator as shown, such an asymmetric structure cannot be adopted for both the optical waveguides 10 and 12, so that the dip in the frequency response characteristic cannot be effectively suppressed.

  FIG. 4 is a plan view showing a first embodiment of a waveguide type optical modulator according to the present invention. Here, the Y-branch optical waveguides 4 and 8 and the optical waveguides 10 and 12 are formed on the waveguide substrate 14, and the signal electrodes 161 and 162 are provided on the optical waveguides 10 and 12, respectively, in a traveling wave type. This is the same as shown in FIG.

  In this embodiment, in order to obtain the configuration of the central ground electrode, which is a feature of the present invention, each electrode is arranged as follows. That is, the ground electrodes 184 and 185 are provided so as to sandwich the signal electrode 161, and the ground electrodes 186 and 187 are provided so as to sandwich the signal electrode 162. There is a gap 24 in which no electrode is formed between the ground electrodes 185 and 187 located between the signal electrodes 161 and 162. Therefore, the ground electrodes 185 and 187 correspond to the center ground electrode, and the gap 24 corresponds to the opening of the center electrode.

  In this way, the central ground electrode located between the signal electrodes 161 and 162 is composed of the two ground electrodes 185 and 186, and a gap 24 (opening) is provided between them, so that it also functions as a microwave resonance box. The characteristic of the waveguide substrate 14 becomes different from that of the prior art shown in FIG. 3, and the dip in the frequency response characteristic can be effectively suppressed.

A method for manufacturing the waveguide type optical modulator will be described. The waveguide substrate 14 is provided by, for example, mirror-polishing the surface of LiNbO ₃ having a size of 40 mm × 2 mm and a thickness of 1 mm. Ti is vacuum-deposited with a thickness of about 100 nm on the waveguide substrate 14, and is processed by a normal photoetching method so that a portion corresponding to the waveguide remains, and is heated at about 1050 ° C. for 10 hours to form Ti with LiNbO _3. The Y branch optical waveguides 4 and 8 and the optical waveguides 10 and 12 are formed by thermal diffusion inside.

Next, a buffer layer made of SiO ₂ is vacuum-deposited with a thickness of 500 nm, for example, and Au with a thickness of 150 nm is deposited thereon as a metal underlayer. Thereafter, the metal base layer in the remaining portion is removed by a photoetching method while leaving the electrode formation region.

  On the waveguide substrate 14 thus treated, a resist is spin-coated with the same thickness as the electrode to be formed, and a resist pattern is formed in a region other than the electrode formation region by a normal photolithography method. Then, Au plating is performed on the metal base layer on which the resist pattern is not formed, for example, with a thickness that matches the upper surface of the resist pattern, thereby forming the signal electrodes 161 and 162 and the ground electrodes 184 to 187.

  FIG. 5 is a plan view showing a second embodiment of the waveguide type optical modulator according to the present invention. Here, the distance between the optical waveguides 10 and 12 is relatively large. Accordingly, the optical waveguides 10 and 12 are composed of a portion parallel to each other and a curved portion connecting them to the Y-branch optical waveguides 4 and 6.

  In this embodiment, the signal electrodes 163 and 164 formed on the optical waveguides 10 and 12, respectively, are formed not only on the parallel portions of the optical waveguides 10 and 12, but also on the curved portions. The length of the waveguide substrate 14 can be shortened.

  Ground electrodes 188 and 189 are formed on both sides of the signal electrodes 163 and 164, respectively, and a central ground electrode 190 is formed between the signal electrodes 163 and 164. The central ground electrode 190 has an opening 26 characteristic of the present invention.

  Also according to this embodiment, the characteristic of the waveguide substrate 14 that also functions as a microwave resonance box becomes different from that of the prior art shown in FIG. 3, and the dip in the frequency response characteristic can be effectively suppressed. .

  FIG. 6 is a plan view showing a third embodiment of the waveguide type optical modulator according to the present invention. Y branch optical waveguides 4 and 8 and optical waveguides 10 and 12 are formed on the waveguide substrate 14, and signal electrodes 165 and 166, ground electrodes 191 and 192, and a central ground electrode 193 are applied in order to apply an electric field thereto. Is formed. The central ground electrode 193 is provided with an opening 26.

  In this embodiment, both ends 165A and 165B of the signal electrode 165 are exposed on both sides of the waveguide substrate 14 to facilitate the electrical wiring of the microwave circuit associated with the signal electrodes 165 and 166, and the signal Both ends 166A and 166B of the electrode 166 are exposed on the side where the one end 165B of the signal electrode 165 is exposed. In this case, since the shape of the signal electrodes 165 and 166 becomes asymmetric and there is a concern about the delay of the microwave, in order to avoid this, in this embodiment, the delay unit 166C is provided near one end 166B of one signal electrode 166. Is formed. Microwaves for modulation are supplied from the ends 165B and 166B to signal electrodes 165 and 166 formed in a traveling wave type, respectively.

  The ground electrodes 191 to 193 include first and second portions W1 and W2 provided so as to sandwich the optical waveguide 10 and third and fourth portions W3 and W4 provided so as to sandwich the optical waveguide 12. These widths are set to be approximately equal. Thereby, the mode of the microwave propagating through the signal electrodes 165 and 166 is stabilized, and the dip in the frequency response characteristic can be effectively suppressed.

  When implementing the present invention, as in the first and second embodiments, the first and second signal electrodes and the ground electrode are provided substantially symmetrically with respect to the center line in the light propagation direction of the waveguide substrate. Thus, the dip in the frequency response characteristic can be more effectively suppressed.

  In addition, the dip in the frequency response characteristics can be more effectively suppressed by making the portion where the electrode is not attached in the cross section crossing the opening of the ground electrode be 1/3 or more of the width of the waveguide substrate. Can do.

  FIG. 7 is a graph showing the relationship between the transmission loss and the frequency of the waveguide type optical modulator in the embodiment of the present invention. In contrast with the characteristics shown in FIG. 2, it is clear that the dip in the frequency response characteristics is effectively suppressed.

  As described above in detail, according to the present invention, there is an effect that it becomes possible to provide a dual drive type waveguide type optical modulator capable of effectively suppressing the dip in the frequency response characteristic.

FIG. 1 is a plan view and a sectional view of a conventional single drive type optical modulator. FIG. 2 is a graph showing the relationship between the transmission loss and the frequency of the optical modulator shown in FIG. FIG. 3 is a plan view of a conventional dual drive type optical modulator. FIG. 4 is a plan view showing a first embodiment of an optical modulator according to the present invention. FIG. 5 is a plan view showing a second embodiment of the optical modulator according to the present invention. FIG. 6 is a plan view showing a third embodiment of the optical modulator according to the present invention. FIG. 7 is a graph showing the relationship between the transmission loss of the optical modulator and the frequency in the embodiment of the present invention.

Claims (7)

A first Y-branch optical waveguide providing an input port;
A second Y-branch optical waveguide providing an output port;
First and second optical waveguides connecting between the first and second Y-branch optical waveguides;
First and second signal electrodes respectively provided on the first and second optical waveguides;
A ground electrode for applying a potential difference between the first and second signal electrodes and applying an electric field to the first and second optical waveguides together with the first and second signal electrodes;
The ground electrode includes a central ground electrode provided between the first and second optical waveguides, and first and second electrodes other than the central ground electrode and formed outside the first and second signal electrodes. Two ground electrodes ,
A waveguide type optical modulator in which the central ground electrode has an opening.   2. The waveguide type optical modulator according to claim 1, wherein the first and second Y branch optical waveguides and the first and second optical waveguides are formed on or near the surface of the waveguide substrate. A first Y-branch optical waveguide providing an input port;
A second Y-branch optical waveguide providing an output port;
First and second optical waveguides connecting between the first and second Y-branch optical waveguides;
First and second signal electrodes respectively provided on the first and second optical waveguides;
A ground electrode for applying a potential difference between the first and second signal electrodes and applying an electric field to the first and second optical waveguides together with the first and second signal electrodes;
The ground electrode includes a central ground electrode provided between the first and second optical waveguides,
The central ground electrode has an opening;
The first and second Y branch optical waveguides and the first and second optical waveguides are formed on or near the surface of a waveguide substrate,
The waveguide type optical modulator, wherein the first and second signal electrodes and the ground electrode are provided substantially symmetrically with respect to a center line in the light propagation direction of the waveguide substrate.   3. The waveguide type optical modulator according to claim 2, wherein a portion where no electrode is attached in a cross section crossing the opening of the ground electrode is 1/3 or more of the width of the waveguide substrate. A first Y-branch optical waveguide providing an input port;
A second Y-branch optical waveguide providing an output port;
First and second optical waveguides connecting between the first and second Y-branch optical waveguides;
First and second signal electrodes respectively provided on the first and second optical waveguides;
A ground electrode for applying a potential difference between the first and second signal electrodes and applying an electric field to the first and second optical waveguides together with the first and second signal electrodes;
The ground electrode includes a central ground electrode provided between the first and second optical waveguides,
The central ground electrode has an opening;
The first and second Y branch optical waveguides and the first and second optical waveguides are formed on or near the surface of a waveguide substrate,
One end of the first signal electrode and one end of the second signal electrode corresponding to the first signal electrode are exposed on both sides of the waveguide substrate,
A waveguide type optical modulator, wherein the other end of the first signal electrode and the other end of the second signal electrode corresponding thereto are exposed on one side of the waveguide substrate.   6. The waveguide type optical modulator according to claim 5, wherein one of the first and second signal electrodes includes a delay unit for compensating a delay of a microwave traveling through the first and second signal electrodes. A first Y-branch optical waveguide providing an input port;
A second Y-branch optical waveguide providing an output port;
First and second optical waveguides connecting between the first and second Y-branch optical waveguides;
First and second signal electrodes respectively provided on the first and second optical waveguides;
A ground electrode for applying a potential difference between the first and second signal electrodes and applying an electric field to the first and second optical waveguides together with the first and second signal electrodes;
The ground electrode includes a central ground electrode provided between the first and second optical waveguides,
The central ground electrode has an opening;
The ground electrode includes first and second portions provided to sandwich the first optical waveguide and third and fourth portions provided to sandwich the second optical waveguide,
A waveguide type optical modulator in which the first to fourth portions have substantially the same width.

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