JPH03202810A - Optical waveguide device and production thereof - Google Patents

Abstract

PURPOSE:To suppress the leakage of the electric lines of force in a side end part to a small level and to improve the performance of an optical modulator by coating one main surface and one side end face of a substrate with a semiconductor film. CONSTITUTION:The substrate 1 is formed with 1st and 2nd branched optical waveguides 2a, 2b on one main surface thereof and is formed with 1st and 2nd control electrodes 3, 4 thereon. The semiconductive film 6 is formed on the main surface disposed with the electrodes 3, 4 of the substrate 1 and the side end face near the 1st control electrode 3. The leakage of the electric lines of force to the outside space in the side end part of the substrate 1 is suppressed to the low level even if the width of the optical waveguide device is small. There is, therefore, substantially no disturbance in the electric field distribution near the two branched optical waveguides 2a, 2b and the generation of a change in the refractive index occurring in a temp. change is obviated. The generation of a shift in the operating point of the optical modulator and a fluctuation in an extinction ratio is obviated in this way.

Description

[Detailed Description of the Invention] [Summary] Regarding optical waveguide devices and their manufacturing methods, in optical waveguide devices that require high-speed and highly stable operation, such as external optical modulators, it is possible to change the operating point due to changes in ambient temperature. For the purpose of preventing fluctuations, etc., an optical waveguide having first and second branched optical waveguides is provided on one main surface of a substrate having an electro-optic effect and a pyroelectric effect, and an optical waveguide having a first and second branched optical waveguide is provided above the branched optical waveguide. In an optical waveguide device in which a first control electrode and a second control electrode are disposed on at least one of the two principal surfaces of the substrate, at least one principal surface on which the control electrode is disposed; An optical waveguide device is constructed by forming a semiconductive film on at least one of the two side end faces near the first control electrode for input signals. The optical waveguide device includes a step of forming an optical waveguide on a substrate wafer having an electro-optic effect and a pyroelectric effect, a step of forming a buffer layer on the wafer, and a step of forming an optical waveguide in the vicinity of the first branched optical waveguide. forming a groove in the wafer; forming a semiconductive film on the buffer layer and on the inner surface of the groove; The optical waveguide device chip can be manufactured by forming the second control electrode, and cutting the wafer at the groove portion to form an optical waveguide device chip.

[Industrial application field]

The present invention provides a high-speed and highly stable optical waveguide device.

In particular, it relates to improving the temperature characteristics of external optical modulators using electro-optic crystals as substrates.

In the optical transmission system of recent optical communication systems, for example,
In optical communication systems up to about 1.6 GHz, a method of directly modulating a laser diode (LD) has been used, but as the modulation frequency becomes higher, small temporal fluctuations in the modulated light wavelength, the so-called wavelength chirping phenomenon, occur. This is the limit for high-speed and long-distance communication.

On the other hand, as the demand for large-capacity and long-distance communication will become stronger in the future, there is a need to develop optical waveguide devices that are faster and have higher temperature stability, such as external optical modulators.

[Conventional technology]

As a high-speed optical modulation method, an external modulation method in which a semiconductor laser beam is externally modulated is known, and in particular, a Matsuhatsu Enda type optical modulator in which a branched optical waveguide is provided on an electro-optic crystal substrate and is driven by a traveling wave electrode is known.

Figure 8 is a diagram showing an example of the basic configuration of an optical modulator.
is a plan view, and the same figure (b) is a YY' sectional view.

In the figure, 1 is a substrate having an electro-optic effect processed into a flat surface;
For example, a LiNbO3 substrate. 2 is an optical waveguide, and branched optical waveguides 2a and 2b are formed in the middle. This optical waveguide is usually made by selectively diffusing metal such as Ti on the surface of the substrate only to the optical waveguide part, so that the refractive index of that part is slightly larger than that of the surrounding parts) 3 is the first A control electrode, for example a traveling wave signal electrode, 4 is a second
control electrode, for example a ground electrode. Reference numeral 5 denotes a buffer layer for reducing the absorption of light into the metal electrode layer on the optical waveguide, and a thin film such as SiO□ is usually used.

The first control electrode 3 and the second control electrode 4 are formed on the optical waveguide via the buffer layer 5 by vapor deposition or plating of a metal such as Au.

Now, for example, DC light emitted from a semiconductor laser (not shown) enters from the optical waveguide 2 on the left side and is divided into two by the branching optical waveguides 2a and 2b. When a signal voltage is applied from the high frequency modulation signal source 8 to the branch optical waveguides 2a and 2 provided on the substrate,
A phase difference occurs between the two branched lights due to the electro-optic effect at b. These two lights are combined again and the modulated optical signal output is taken out from the single optical waveguide 2 on the right side.
It is configured to convert the signal into an electrical signal using a photodetector (not shown). The phase difference between both lights in the branched optical waveguides 2a and 2b is 0. Alternatively, if a driving voltage is applied so as to be π, the optical signal output can be obtained as an ON-OFF pulse signal, for example. Note that RT is a terminating resistor.

FIG. 9 is a cross-sectional view showing the pyroelectric effect due to temperature change.

Usually, the substrate L is made of an oxide single crystal L, which has a large electro-optic effect.
iNb0. Alternatively, a two-cut plate of LiTaO5 is used. All of these single crystals belong to the rhombohedral crystal system and are known to have a relatively large pyroelectric effect. Therefore, as the substrate temperature rises or falls, polarization as shown in the figure occurs, and positive and negative charges are generated on both sides of the substrate. Due to the presence of the two control electrodes 3 and 4, the electric field distribution in the substrate l is disturbed, and as a result, the refractive indexes of the first and second branched optical waveguides (2a', 2b) are different. In terms of optical modulators, this means that the operating point shifts as the temperature changes.
This will lead to unstable operation of the optical modulator.

A method for improving instability caused by such a pyroelectric effect has already been proposed. For example, FIG. 5 is a cross-sectional view showing an example of conventional methods for preventing the influence of the pyroelectric effect. That is,
A semiconductive film 6c, for example, a Si film, having a relatively high resistance that does not affect the operating voltage characteristics is formed on the buffer layer 5.

As a result, as shown in the figure, the electric field distribution inside the substrate 1 becomes uniform, and therefore, no difference in refractive index occurs between the first and second branched optical waveguides (2a, 2b).
Even if there is a temperature change, the operating point of the optical modulator does not shift, and an extremely stable optical modulator can be obtained.

Figure 7 is a diagram showing the operating point shift based on the pyroelectric effect.
The vertical axis shows the operating point shift, and the horizontal axis shows temperature. In the figure, the dotted line (■) indicates the case of the basic configuration example explained in FIGS. 8 and 9, and it can be seen that the operation is extremely unstable with respect to temperature changes. On the other hand, the broken line (■) shows the data of the conventional example in which the semiconductive film 6c described in FIG.

However, recently there has been a strong demand for miniaturization of optical modulator elements, and by reducing the width dimension as much as possible, the number of optical modulator chips that can be made from the same substrate can be increased and the cost of optical modulators can be reduced. In order to achieve this goal, it has come to be canted as close as possible to the first control electrode 3, which is a traveling wave signal electrode.

FIG. 6 is a cross-sectional view showing an optical modulator element with a narrow substrate and electric field distribution. That is, the distance d1 between the edge of the first control electrode 3 and the side edge of the substrate 1 is as follows.

For example, if it is 0.1 mm or less, the do
is, for example, 2 mm or more, but it is an extremely small value of 1/20 or less.

[Problem that the invention sought to solve]

However, as shown in FIG. 6 above, when the distance d1 between the edge of the first control electrode 3 and the side edge of the substrate 1 becomes extremely small, the electric field distribution becomes disordered at the edge as shown in the figure. , As a result, the first and second branch optical waveguides 2a
, 2b will be different, and the difference in refractive index will fluctuate with temperature changes, leading to unstable operation of the optical modulator.

The dash-dotted line (■) in Figure 7 shows an example of the operating point shift, and there is a shift of about 1.5 V in the range from 10oC to 50°C, and it cannot be used practically at this point. There was a problem that needed to be solved.

[Means to solve the problem]

The above problem is a substrate 1 having an electro-optic effect and a pyroelectric effect.
on one main surface of the first and second branch optical waveguides 2a.
, 2b, and a first control electrode 3 and a second control electrode 4 are arranged above the branched optical waveguides 2a and 2b. At least one of the main surfaces on which the control electrodes 3 and 4 are disposed, and 2 of the substrate 1
This problem can be solved by an optical waveguide device in which a semiconductive film 6 is formed on at least one of the two side end faces that is close to the first control electrode 3 for input signals.

The optical waveguide device includes a step of forming an optical waveguide 2 having first and second branched optical waveguides 2a and 2b on a substrate wafer 10 having an electro-optic effect and a pyroelectric effect, and a step of forming a buffer on the wafer IO. a step of forming a layer 5, a step of forming a groove 7 in the wafer 10 close to the first branched optical waveguide 2a, and a step of forming a groove 7 on the buffer layer 5 formed on the wafer IO and inside the groove 7. a step of forming a semiconductive film 6 on the side surface, and the branched optical waveguide 2a;
forming first and second control electrodes 3.4 above the buffer layer 5 and the semiconductive film 6, and cutting the wafer 10 at the groove 7 to form an optical waveguide device. It can be manufactured by a method including at least a step of forming a chip.

[Effect]

According to the configuration of the present invention, even when the width of the optical waveguide device is narrow, since not only the main surface of the substrate 1 but also the side edges thereof are continuously covered with the high-resistance semiconductive film 6, the substrate l Leakage of electric lines of force into the external space at the side ends of the can be kept to an extremely low level.

Therefore, there is almost no disturbance in the electric field distribution in the vicinity of the two branched optical waveguides 2a and 2b, no change in the refractive index due to temperature change occurs, and no shift in the operating point of the optical modulator or fluctuation in the extinction ratio occurs.

〔Example〕

FIG. 1 is a sectional view showing an embodiment of the present invention.

In the figure, 6 (6a, 6c) is a semiconductive film.

Note that the same reference numerals are given to the same parts as those explained in the drawings of the conventional example, and the explanation of the same parts will be omitted.

Board 1 has an L with a size of 60 mm x 2 mm and a thickness of 1 mm.
The surfaces of two tNbOs plates were mirror polished and used.

On this substrate, Ti was vacuum-deposited to a thickness of about 90 nm,
After processing with a normal photoetching method so that Ti remains in the portion corresponding to the optical waveguide 2 including the branched optical waveguides 2a and 2b, the Ti is thermally diffused into LiNb0a at about 800°C to form the entire optical waveguide 2. did.

The length of the branched optical waveguide portion was adjusted to 50 mm, and the widths of all optical waveguides were adjusted to 7 μm. As the buffer layer 5, a SiO□ film with a thickness of 300 nm was used.

The semiconductive film 6 (6a, 6c) is made of Si with a thickness of 1100 nm.
A film is used on one main surface and one side end surface of the substrate 1.

For example, it was formed by vapor deposition, and the film resistance was adjusted to be in the range of 0.1 to 5 MΩ between the first and second control electrodes. The first control electrode 3 is formed by depositing a two-layer film of Au/Ti, then pattern-etching it into an electrode shape with a width of 9 μm on the branch optical waveguide 2a, and then forming a layer with a thickness of 3 μm on top of it.
m of Au was deposited by plating. The distance d1 between the edge of the first control electrode 3 and the side edge of the substrate l is 0.1 mm.
I made it so that

The second control electrode 4 was formed to have a wide width using the same process as the first control electrode 3, that is, it was formed at the same time as the formation of the first control electrode so as to function effectively as a ground electrode.

FIG. 2 is a cross-sectional view showing the effects of the present invention, with the operating point shift plotted on the vertical axis and the temperature plotted on the horizontal axis.

In the figure, the solid line (■) is data showing the operating point shift due to temperature change in the embodiment of the present invention. - On the other hand, the dashed-dotted line in 2 is the same as the dashed-dotted line in ② in FIG.
, 4 are the same as the embodiment described in FIG. It can be seen that compared to the inventive embodiment and the conventional optical modulator, it is possible to operate extremely stably over a wide range of temperature changes.

FIG. 3 is a sectional view showing another embodiment of the present invention. In the case of this embodiment, the semiconductive film 6 (6b) is formed not only on the side surface of the substrate l but also on the other main surface, that is, the back surface, of the substrate l to further enhance the stabilizing effect. This is what I did.

Next, an example of a manufacturing method for specifically forming an element according to the present invention will be shown below.

FIG. 4 is a sectional view showing the main steps of the manufacturing method of the present invention. In the figure, 7 is a groove and IO is a wafer.

Note that the same reference numerals are given to the same parts as those explained in the above drawings, and the explanation of the same parts will be omitted.

Step (1): Wafer 10. For example, a diameter of 100 m
First and second branch optical waveguides 2a, 2 are formed on one main surface of a Z-cut LiNbO5 single crystal plate having a diameter of 1 mm and a thickness of 1 mm.
A large number of optical waveguides 2 having the shape b are formed by thermally diffusing Ti.

Step (2): A buffer layer 5 made of a Si02 film having a thickness of 300 nm is formed on the above-mentioned treated wafer by CVD.

Step (3) First branch optical waveguide 2 of the above-mentioned treated wafer
groove 7.a in the wafer lO, for example, with a spacing d, = 0.15 mm. For example, width 0.3
A groove with a diameter of 0.4 mm and a depth of 0.4 mm is formed using, for example, a cutting machine using a diamond blade.

Step (4) Form a semiconductive film 6 (6a, 6c), for example, a Si film with a thickness of 1100 nm, by vapor deposition on the buffer layer 5 of the second processed wafer and on the inner side surface of the groove 7. do. The membrane resistance may be adjusted to be in the range of 0.1 to 5 MΩ between the first and second control electrodes.

Step (5): Branch optical waveguides 2a, 2 of the above-mentioned treated wafer
Above the first and second control electrodes 3.4, for example, a 0.3 μm thick Au/Ti film is deposited, pattern etched into a predetermined shape, and a 3 μm thick Au/Ti film is deposited on top of that.
Perform Au plating.

Step (6) Second, the above-mentioned treated wafer is heated at the groove 7 portion and the boundary portion of each device perpendicular thereto, for example, by 0.2
It is cut using a mmφ wire saw to form individual optical waveguide device chips.

Thus, the semiconductive film 6 (6a, 6a,
6c) of the present invention, such as an optical modulator, is obtained.

Note that the semiconductive film 6 (6a, 6c) does not cover the entire side end face, but may cover only a portion of the side end face as in this embodiment.
It was confirmed that a great effect can be obtained if the coating is sufficiently deep compared to the depths of a and 2b.

Further, if the semiconductive film 6 has a resistance value within the above range, for example, an optical waveguide device using the semiconductive film 6,
For example, no deterioration of the high frequency characteristics of the optical modulator was observed.

The embodiments described above are just a few examples, and are effective not only for optical modulators but also for other optical waveguide devices using substrates with a pyroelectric effect, such as directional coupler switches and total internal reflection switches. It goes without saying that any preferable materials, configuration dimensions, manufacturing process, etc., or combinations thereof may be used as long as they comply with the spirit of the present invention.

〔Effect of the invention〕

As explained above, according to the present invention, even when the width of the optical waveguide device is narrow, not only the main surface of the substrate l but also the side edges thereof are continuously covered with the high-resistance semiconductive film 6. , leakage of electric lines of force into the external space at the side edges of the substrate l can be suppressed to an extremely low level. Therefore, two branched optical waveguides 2a.

There is almost no disturbance in the electric field distribution near 2b, no refractive index change occurs due to temperature change, and no shift in the operating point of the optical modulator occurs, improving the performance and quality of optical waveguide devices, such as optical modulators. The contribution made to this is extremely large.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing an embodiment of the present invention, FIG. 2 is a view showing the effects of the present invention, FIG. 3 is a cross-sectional view showing another embodiment of the present invention, and FIG. 4 is a manufacturing method of the present invention. A cross-sectional view showing the main steps of the method, Figure 5 is a cross-sectional view showing an example of preventing the effects of the conventional pyroelectric effect, Figure 7 is a view showing the operating point shift based on the pyroelectric effect, Figure 8 9 is a diagram showing an example of the basic configuration of an optical modulator, and FIG. 9 is a sectional view showing the pyroelectric effect due to temperature change. In the figure, 1 is a substrate, 2 is an optical waveguide, 2a and 2b are first and second branched optical waveguides, 3 is a first control electrode, 4 is a second control electrode, 5 is a buffer layer, 6 (6a , 6b, 6c) are semiconductive films, 7 is a groove,
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Claims (2)

[Claims] (1) First and second branch optical waveguides (2a,
2b), and a first control electrode (3) and a second control electrode (4) are arranged above the branched optical waveguide (2a, 2b). At least one main surface of the two main surfaces of the substrate (1) on which the control electrodes (3, 4) are disposed, and at least one of the two side end surfaces of the substrate (1) An optical waveguide device characterized in that a semiconductive film (6) is formed on one side end surface close to a first control electrode (3) for signals. (2) First and second branch optical waveguides (2a, 2b
), forming a buffer layer (5) on the wafer (10); ) forming a groove (7) on the buffer layer (5) formed on the wafer (10) and the groove (7) on the buffer layer (5) formed on the wafer (10);
7) forming a semi-conductive film (6) on the inner surface of the branch optical waveguide (2a, 2b); and a step of forming second control electrodes (3, 4), and a step of cutting the wafer (10) at the groove (7) to form an optical waveguide device chip. The method for manufacturing an optical waveguide device according to claim (1).