JP2800339B2 - Method of manufacturing optical modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] Regarding an optical modulator, which aims at driving an external optical modulator at a high speed, an optical waveguide having a bifurcated optical waveguide on a substrate having an electro-optic effect Providing a buffer layer so as to cover each of the branch optical waveguides, and a signal electrode on one of the branch optical waveguides via the buffer layer, and at least the other of the other of the branch optical waveguides Providing each of a ground electrode thereon; covering the signal electrode with a resist film; providing an overhang portion made of a conductor on the ground electrode; and between the overhang portion and the signal electrode. And a step of removing the intervening resist film.

[Industrial applications]

The present invention relates to an optical modulator that operates at high speed and high stability, and particularly to an electrode configuration thereof.

In an optical transmission system of a recent optical communication system, for example, in an optical communication system 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, the modulation light wavelength becomes longer. A minute fluctuation in time, a so-called chirping phenomenon, occurs, which limits the speed up and long-distance communication.

On the other hand, since demands for large-capacity and long-distance communication are increasing in the future, development of an optical modulator that operates at higher speed and with higher stability is required.

[Conventional technology]

As a high-speed light modulation method, there is known an external modulation method for modulating a semiconductor laser beam externally, in particular, a Mach-Zehnder type optical modulator in which a branch optical waveguide is provided on an electro-optic crystal substrate and driven by a traveling wave electrode.

FIG. 4 shows a basic configuration example of an optical modulator (part 1).
(A) is a plan view, and (b) is a cross-sectional view taken along the line YY '.

In the drawing, reference numeral 1 denotes a substrate processed into a plane and having an electro-optical effect, for example, a LiNbO ₃ substrate. Reference numeral 2 denotes an optical waveguide in which branch optical waveguides 2a and 2b are formed in the middle. In an optical waveguide, a metal such as Ti is usually selectively diffused only into the optical waveguide portion on the surface of the substrate so that the refractive index of the portion is slightly larger than that of the surrounding portion. 3 is a signal electrode, for example, a traveling wave signal electrode, and 4 is 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 is usually made of a thin film such as SiO ₂ .
The signal electrode 3 and the ground electrode 4 are formed by depositing or plating metal such as Au on the optical waveguide via the buffer layer 5.

Now, for example, a DC light emitted from a semiconductor laser (not shown) enters from the left optical waveguide 2 and enters the branch optical waveguide 2.
When the signal voltage is applied to the signal electrode 3 from the high-frequency modulation signal source 6, the signal is branched by the electro-optic effect in the branch optical waveguides 2a and 2b provided on the substrate. A phase difference occurs between the two lights. The two lights are combined again, a modulated optical signal output is taken out from one optical waveguide 2 on the right side, and is converted into an electric signal by a photodetector (not shown). The branch optical waveguide
If a drive voltage is applied so that the phase difference between the two lights in 2a and 2b becomes π or 0, for example, the optical signal output becomes
Obtained as ON-OFF pulse signals. Note that _RT is a terminating resistor.

In the optical modulator having this configuration, the ground electrode 4 is larger than the signal electrode 3 as shown in the figure to improve the transmission of high-frequency electric signals, and therefore, the ground electrode 4 is applied to the branch optical waveguides 2a and 2b. Electric field distributions are not equal, and there is a difference of about 3 to 6 times between the two. Therefore, driving by the push-pull operation is difficult, and eventually, the driving voltage for modulation must be increased.

Furthermore, a temperature difference occurs between the branch optical waveguides 2a and 2b due to the asymmetry of the electric field applied to the signal electrode 3 and the ground electrode 4, and the operating point of the optical modulation characteristic shifts due to distortion due to the temperature difference. was there.

On the other hand, FIG. 5 is a diagram (part 2) showing an example of the basic configuration of an optical modulator, which is an example proposed to improve the above-mentioned disadvantages. FIG. 5 (a) is a plan view and FIG. Is a YY 'sectional view.

The same parts as those described in the drawings are denoted by the same reference numerals, and the description of the same parts will be omitted.

In the optical modulator having this configuration, the signal electrode 3 and the ground electrode 4 are formed in parallel with exactly the same width, that is, a so-called symmetric electrode arrangement. There is no disadvantage that the points are shifted, and the driving voltage can be reduced.

However, in this case, the resistance of the ground electrode 4 cannot be sufficiently reduced, and mutual interference based on induction occurs between the signal electrode 3 and the ground electrode 4, so that there is a limit to high-frequency driving.

The problems related to high-speed driving as described above are, after all,
The problem is how to improve the matching between light and a microwave signal, and several proposals have been made.

FIG. 6 is a cross-sectional view (part 1) showing an example of conventional speed matching between a light wave and a microwave signal, in which 3 'is a signal electrode,
4 'is a ground electrode and 5' is a buffer layer.

Usually, the speed of light propagating in an optical waveguide is about twice as fast as the speed of a microwave signal propagating in a signal electrode, that is, speed matching is not achieved. Becomes

Therefore, to achieve speed matching, the speed of the microwave signal propagating in the signal electrode may be increased, that is, the dielectric constant may be effectively reduced. In this example, the electrode thickness t _E1
For example, by increasing both the buffer layer thickness t _B1 and t _E0 and t _B0 of the basic configuration example of FIG. 4 to about three times, the modulation frequency bandwidth can be extended about three times. It has become.

FIG. 7 is a cross-sectional view (part 2) showing an example of conventional speed matching between a light wave and a microwave signal, and 3 ″ is a signal electrode, and only its height t _E2 is used as the height t of the ground electrode 4 ′. _In this case, the optical modulator is set to be about three times higher than _E1 , thereby obtaining an optical modulator having a wider modulation frequency bandwidth.

[Problems to be solved by the invention]

However, in the case of the above conventional example (Part 1), the gap d between the signal electrode 3 'and the ground electrode 4' is only about 15 μm, and when the electrode thickness t _E1 becomes 10 μm or more, the photo etching process is performed. The above difficulties arise. If the thickness of the buffer layer is too large, the operating voltage will increase. On the other hand, in the case of the conventional example (No. 2), the signal electrode 3 ″
Can be made thicker, for example, by a plating method. In this case, however, if the height of t _E2 is set to, for example, 20 μm or more, the characteristic impedance of the signal electrode line will produce a manufacturing variation, and There was a problem that the yield was reduced, and the electric field distribution from the signal electrode line to the surrounding space was widened to one side in the horizontal direction. .

[Means for solving the problem]

The above-mentioned problem is caused by the problem that the branched optical waveguide shown in FIGS. 1 and 3 is branched on a substrate 1 having an electro-optical effect.
Providing the optical waveguide 2 having 2a and 2b; providing a buffer layer 5 so as to cover each of the branch optical waveguides 2a and 2b; Providing a signal electrode 3 on one side and a ground electrode 4 on at least the other of the branch optical waveguides 2a and 2b; covering the signal electrode 3 with a resist film 7; Manufacturing an optical modulator configured to include a step of providing an overhang portion 40 made of a conductor on the substrate and a step of removing the resist film 7 interposed between the overhang portion 40 and the signal electrode 3. Solved by the method.

[Action]

According to the configuration of the present invention, since the thickness of the signal electrode 3 is not so large, it can be formed with high precision in the manufacturing process. On the other hand, the signal electrode 3 is configured so that at least its upper portion is widely covered by the overhang portion 40 formed on the ground electrode 4 with the gap-shaped space interposed therebetween, so that the electric field distribution in the peripheral space is improved, and Increasing the number of lines effectively reduces the dielectric constant, greatly improving the speed matching between the transmitted microwave signal and the lightwave, and providing an optical modulator with a wide modulation frequency bandwidth. .

〔Example〕

FIG. 1 is a view showing an embodiment of the present invention. FIG. 1A is a plan view, and FIG. 1B is a sectional view taken along the line YY '.

In the figure, reference numeral 40 denotes an overhang portion of the ground electrode 4 formed so as to cover at least the upper portion of the signal electrode 3 with a gap-shaped space therebetween.

The same parts as those described in the drawings of the conventional example are denoted by the same reference numerals, and the description of the same parts will be omitted.

In the case of the present embodiment, the overhang portion 40 of the ground electrode 4 is formed so as to cover the upper portion of the signal electrode 3 with a gap g of, for example, 15 μm therebetween. As the number of lines increases, the dielectric constant effectively decreases, and the speed matching between the light wave and the microwave signal is greatly improved. For example, the modulation frequency is lower than that of the conventional example (part 1). An optical modulator having a bandwidth of 5 times or more and capable of high-speed operation can be obtained. Further, the signal electrode 3 does not need to be formed so thick that the device yield does not decrease.

FIG. 2 shows another embodiment of the present invention.
1 is a plan view, and FIG. 1B is a sectional view taken along line YY '.

This embodiment is an embodiment in the case of a symmetric electrode arrangement with respect to the asymmetric electrode arrangement of FIG.

Needless to say, the same effects as in the above embodiment can be obtained in this embodiment.

The main steps for specifically manufacturing the above-described embodiment will be described below.

FIG. 3 is a cross-sectional view showing main manufacturing steps of the embodiment of the present invention. FIG. 3A shows a case of an asymmetric electrode type, and FIG. 3B shows a case of a symmetric electrode type.

Step (1): Ti is vacuum-deposited to a thickness of about 90 nm on a wafer 10 having a mirror-polished surface of a 1 mm-thick LiNbO ₃ Z plate,
After processing by a usual photoetching method so that Ti remains in a portion corresponding to the optical waveguide 2 including the branched optical waveguides 2a and 2b, Ti is thermally diffused into LiNbO ₃ at about 800 ° C.
To form

The length of the branch optical waveguide was adjusted to 25 mm, the width of the optical waveguide was adjusted to 7 to 11 μm, and the optical modulator including a large number of optical waveguides 2 was arranged and formed on the wafer 10 so as to be cut. .

Step (2): Next, SiO ₂ is formed as a buffer layer to a thickness of 1 μm by sputtering.

Step (3): After depositing an Au / Ti2 layer film on the processed substrate, photoetching is performed into a required electrode pattern shape having an electrode width of 10 μm and a gap between the signal electrode and the ground electrode of 15 μm. Then, Au having a thickness of 3 μm is formed by plating.

Step (4): A resist pattern 7 is formed on the processed substrate by a normal exposure / etching process so that a portion of the ground electrode 4 remains exposed.

Step (5): Electroplating of Au, for example, is performed on the exposed surface of the ground electrode 4 of the processed substrate. If the thickness of the Au plating layer is, for example, 30 to 40 μm, the tip of the plating layer grows on the resist pattern 7 to form an overhanging portion 40 as shown in the figure.

Step (6): Remove resist pattern 7 with a remover, for example,
The overhang portion 40 is formed by dissolving and removing with a registry mover.

Step (7): If the wafer 10 is diced by a cutting machine and separated into a number of modulator elements of a predetermined size, the elements of the optical modulator of the present invention can be obtained.

With the above configuration, an optical modulator that can operate from the conventional frequency band of 5 to 15 GHz to 30 GHz or more is obtained.

In the above embodiment, the overhang portion 40 of the ground electrode 4 is used.
Covers the upper part of the signal electrode 3, however, the effect can be further enhanced to the opposite side, that is, to cover the entire signal electrode 3.

In addition, it goes without saying that the overhang portion 40 of the ground electrode 4 may be formed by sputtering or CVD instead of plating.

The embodiment described above shows only a few examples, and the material, configuration, dimensions,
Needless to say, it is possible to use a preferable one such as a manufacturing process or a combination thereof.

〔The invention's effect〕

As described above, the optical modulator according to the present invention includes the signal electrode 3
Since the thickness is not so large, it can be formed with high accuracy on the production process. On the other hand, the signal electrode 3 is configured such that at least its upper part is widely covered by the overhang portion 40 formed on the ground electrode 4 with the gap-shaped space interposed therebetween, so that the electric field distribution in the peripheral space is improved, and the electric force is improved. Increasing the number of lines effectively reduces the dielectric constant, greatly improving the speed matching between the transmitted microwave signal and the lightwave, and improving the performance of the optical modulator, especially high-speed operation characteristics. It is extremely large that contributes to the improvement of the quality.

[Brief description of the drawings]

FIG. 1 is a view showing an embodiment of the present invention, FIG. 2 is a view showing another embodiment of the present invention, FIG. 3 is a sectional view showing main manufacturing steps of the embodiment of the present invention, FIG. FIG. 1 shows an example of a basic configuration of an optical modulator (No. 1), FIG. 5 is a diagram showing an example of a basic configuration of an optical modulator (No. 2), and FIG. FIG. 7 is a sectional view (part 2) showing an example of conventional speed matching between a lightwave and a microwave signal. In the figure, 1 is a substrate, 2 is an optical waveguide, 2a and 2b are first and second branch optical waveguides, 3 is a signal electrode, 4 is a ground electrode, 5 is a buffer layer, and 40 is an overhang portion.

Continuation of the front page (56) References JP-A-1-204020 (JP, A) JP-A-64-48021 (JP, A) JP-A-64-91111 (JP, A) JP-A-2-51124 (JP) , A) JP-A-56-17321 (JP, A) JP-A-1-210928 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G02F 1/00-1/035 G02F 1/29-1/313

Claims (2)

(57) [Claims] A step of providing an optical waveguide having two branched optical waveguides on a substrate having an electro-optical effect; a step of providing a buffer layer so as to cover each of the branched optical waveguides; Providing a signal electrode on one of the branch optical waveguides and a ground electrode on at least the other of the branch optical waveguides via a layer; covering the signal electrode with a resist film; A method of manufacturing an optical modulator, comprising: providing an overhang portion made of a conductor on a ground electrode; and removing the resist film interposed between the overhang portion and the signal electrode. 2. The method of manufacturing an optical modulator according to claim 1, wherein said overhang portion is formed by electroplating on said ground electrode.