JPH03204614A - Optical modulator - Google Patents

Abstract

PURPOSE:To drive an external optical modulator at high speed by covering at least the upper part of a signal electrode by an overhang part formed on an earth electrode across a gap-shaped space. CONSTITUTION:The signal electrode 3 is formed not so thickly, so the electrode can accurately be formed in its manufacture process. The signal electrode 3 has at least its upper part covered by the overhang part 40 formed on the earth electrode 4 across the gap-shaped space. The electric field distribution in the peripheral space is therefore improved and the number of lines of electric force increases to decrease the dielectric constant practically; and the speed matching between a microwave signal and a light wave which are propagated is improved greatly and the optical modulator which has a wide modulation frequency band is obtained. Consequently, fast operation characteristics are improved.

Description

Detailed Description of the Invention [Summary] Regarding an optical modulator, for the purpose of driving an external optical modulator at high speed, first and second branched light guides are mounted on a flat substrate having an electro-optic effect. A Matsuhatsu Enda type optical modulator comprising a leading waveguide having a waveguide, and a signal electrode and a ground electrode arranged so as to create a phase difference between the light propagating through the first branched optical waveguide and the second branched optical waveguide. In the optical modulator, at least the upper portion of the signal electrode is covered with an overhang portion formed on the ground electrode with a gap-like space in between.

[Industrial application field]

The present invention is an optical modulator that operates at high speed and with high stability.

In particular, it relates to its electrode configuration.

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 chirping phenomenon, occur. This is the limit to high-speed and long-distance communication.

On the other hand, as the demand for large-capacity and long-distance communications will become stronger in the future, there is a need to develop optical modulators that operate at higher speeds and with higher stability.

[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 4 is a diagram (part 1) showing an example of the basic configuration of an optical modulator.
The figure (a) is a plan view, and the figure (b) is a YY' sectional view.

In the figure, ] indicates a flat substrate with an electro-optic effect;
For example, a LiNb0+ substrate. 2 is an optical waveguide, and branched optical waveguides 2a and 2b are formed in the middle. This guide waveguide is usually made by selectively diffusing a metal such as Ti on the surface of the substrate only to the optical waveguide portion, so that the refractive index of that portion is slightly larger than that of the surrounding portions. 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 absorption of light into the metal electrode layer on the optical waveguide, and a thin film such as SiO□ is usually used. The signal electrode 3 and the ground 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, direct current light emitted from a semiconductor laser (not shown) enters from the left optical waveguide 2, and enters the branch optical waveguide 2a.
, 2b, and when a signal voltage is applied from the high frequency modulation signal source 6 to the signal electrode 3 between them, the two branches are split by the electro-optic effect in the branched optical waveguides 2a and 2b provided on the substrate. A phase difference occurs in the light. The branched optical waveguide is configured to combine these two lights again, take out a modulated optical signal output from one optical waveguide 2 on the right side, and convert it into an electrical signal using a photodetector (not shown). If a driving voltage is applied so that the phase difference between the two lights in the wave paths 2a and 2b becomes π or 0.

For example, the optical signal output is obtained as an ON-OFF pulse signal. Note that R is a terminating resistor.

In the optical modulator with this configuration, the ground electrode 4 is made larger than the signal electrode 3, as shown in the figure, in order to improve the transmission of high-frequency electrical signals. The electric field distributions generated are not equal (there is a difference of about 3 to 6 times between the two. Therefore, it is difficult to drive by push-pull operation, and in the end, the drive voltage for modulation must be increased. become.

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

On the other hand, FIG. 5 is a diagram showing an example of the basic configuration of an optical modulator (part 2).
), this is an example proposed to improve the above drawbacks,
The figure (a) is a plan view, and the figure (b) is a YY' sectional view.

Note that the same reference numerals are given to the same parts as those explained in the drawings, 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 and have exactly the same width. This is a so-called symmetrical electrode arrangement, which does not have the disadvantage that the operating point of the optical modulation characteristic shifts due to distortion as in the above example, and the driving voltage can also be lowered.

However, in this case, the resistance of the ground electrode 4 cannot be made sufficiently low, and mutual interference occurs between the signal electrode 3 and the ground electrode 4 due to induction, which limits high-frequency driving.

The problems related to high-speed drive as described above are as follows.

Ultimately, the problem is how to improve the matching between light and microwave signals, and several proposals have been made in the past.

FIG. 6 is a cross-sectional view (part 1) showing an example of conventional speed matching of light waves and microwave signals. In the figure, 3° is a signal electrode, 4° is a ground electrode, and 5° is a buffer layer.

Normally, the speed of light propagating in the leading wavepath is about twice the speed of the microwave signal propagating in the signal electrode (i.e., the speeds are not matched, and this is one of the reasons why high-speed drive is not possible). It becomes a limit.

Therefore, in order to achieve speed matching, the speed of the microphone S-wave signal propagating through the signal electrode is increased. In other words, it is sufficient to effectively lower the dielectric constant, and in this example, the electrode film thickness tEl
and the film thickness tBI of the buffer layer, for example, t in the basic configuration example of FIG. and tao's 3
By increasing the modulation frequency bandwidth by about 3 times,
It is possible to expand it by about twice as much.

In addition, FIG. 7 is a cross-sectional view (part 2) showing an example of conventional speed matching of light waves and microwave signals, and 3" is a cross-sectional view showing an example of speed matching between light waves and microwave signals. 3" is a height tEI of the ground electrode 4" in which only the height tE□ of the signal electrode is This is a case where the value is increased to about 3 times higher than that of the original value.

As a result, an optical modulator with a wider modulation frequency band width is obtained.

[Problem that the invention sought to solve]

However, in the case of the above conventional example (part 1), the signal electrode 3
The cap d between '' and the ground electrode 4' is only about 15 μm (but if the electrode thickness tEI becomes more than 10 μm, difficulties arise in the photo-etching process. If it is made larger, the operating voltage will become higher.On the other hand, in the case of the conventional example (Part 2), it is possible to thicken only the signal electrode 3', for example, by plating, but in this case as well, t. For example, set the height to 20
If it is larger than μm, manufacturing variations will occur in the characteristic impedance of the signal electrode line, leading to a decrease in device yield, and the electric field distribution from the signal electrode line to the surrounding space will spread widely in the lateral direction on one side. There were also problems such as limitations in speed matching, which needed to be resolved.

[Means to solve the problem]

The above problem is solved by forming the first and second branched optical waveguides 2a and 2b on a substrate l having an electro-optic effect that is processed into a flat surface.
an optical waveguide 2 having the first branch optical waveguide 2 is provided;
In the Matsuhatsu Enda type optical modulator in which a signal electrode 3 and a ground electrode 4 are arranged so as to cause a phase difference between the light propagating through the second branch optical waveguide 2b and the signal electrode 3, at least the upper part of the signal electrode 3 is This can be solved by configuring the optical modulator so that it is covered by the overhang part 40 formed on the ground electrode 4 with a cap-shaped space in between. It can be formed by a plating method.

[Effect]

According to the configuration of the present invention, the thickness of the signal electrode 3 is too thick (
Since the signal electrode 3 is not formed in the ground electrode 4, the signal electrode 3 can be formed with high precision in terms of manufacturing process. As a result, the electric field distribution in the surrounding space improves, and the number of electric lines of force increases, which effectively reduces the permittivity and greatly improves the velocity matching between the propagated microwave signal and the light wave. An optical modulator with a wide modulation frequency bandwidth can be obtained. [Embodiment] Fig. 1 shows an embodiment of the present invention, where (A) is a plan view and (B) is a Y- It is a Y' sectional view.

In the figure, 40 is an overhang portion of the ground electrode 4 formed to cover at least the upper part of the signal electrode 3 with a cap-shaped space in between.

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.

In the case of this embodiment, since the overhang part 40 of the ground electrode 4 is formed to cover the upper part of the signal electrode 3 with, for example, a 15 μm cap g in between, the electric field distribution in the surrounding space is improved, and the electric field is As the number of wires increases, the dielectric constant effectively decreases, and the speed matching between light waves and microwave signals is greatly improved. For example, in the conventional example (Part 1)
An optical modulator capable of high-speed operation with a modulation frequency bandwidth five times or more can be obtained compared to the case of . Furthermore, since the signal electrode 3 does not have to be formed very thick, it does not cause a decrease in device yield.

FIG. 2 is a diagram showing another embodiment of the present invention, in which (a) is a plan view and (b) is a sectional view taken along Y-Y'.

This example is an example in which the electrode arrangement is symmetrical compared to the asymmetric electrode arrangement shown in FIG. .

The main steps for specifically manufacturing the above examples are shown below.

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

Step (1) Ti is vacuum-deposited to a thickness of about 90 nm on the wafer IO, the surface of which is a mirror-polished L+NbO Z plate with a thickness of 1 mm, which corresponds to the optical waveguide 2 including the branch optical waveguides 2a and 2b. After processing with the usual photoetching method so that Ti remains in the exposed area, the Ti is removed by L at about 8008C.
Thermal diffusion is performed in iNbO5 to form an optical waveguide 2.

The length of the branched optical waveguide part was adjusted to 25 mm, the width of all leading waveguides was adjusted to 7 to 11 μm, and the optical modulator part including a large number of optical waveguides 2 was arranged on the Eva 1O so that it could be cut. Formed.

Step (2): Next, 1μ of SiO□ is used as a buffer layer.
It is formed by sputtering to a thickness of m.

Step (3): After depositing the Au/Ti two-layer film on the above-mentioned treated substrate, photoetching it into the required electrode pattern shape with an electrode width of 10 μm and a gap between the signal electrode and the ground electrode of 15 μm, and then Au with a thickness of 3 μm is deposited by plating.

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

Step (5) The exposed surface of the ground electrode 4 portion of the processed substrate is electroplated with, for example, Au.

If the thickness of the Au plating layer is, for example, 30 to 40 μm, the tip of the plating layer will grow up to the top of the resist pattern 7, forming an overhanging portion that will become the overbank portion 40 as shown.

Step (6), remove the resist pattern 7 with a remover, for example,
The overbank portion 40 is formed by dissolving and removing it with a resist remover.

Step (7): The wafer IO is die-sinked using a cut-ink machine and separated into a large number of modulator elements of a predetermined size, thereby obtaining the optical modulator elements of the present invention.

With the above configuration, an optical modulator that can operate in a frequency band that was conventionally 5 to 15 GHz or more than 30 GHz was obtained.

Note that in the above embodiment, the overbank portion 40 of the ground electrode 4
The overhang of the ground electrode 4 can be made to cover the upper part of the signal electrode 3 or even to the opposite side, that is, to cover the entire signal electrode 3 to further enhance the effect. The portion 40 may be formed by plating (of course, sputtering or CVD may also be used).

The embodiments described above are just a few examples, and as long as they comply with the spirit of the present invention, it is possible to use suitable materials, configuration dimensions, manufacturing processes, or combinations thereof. Needless to say, [Effects of the Invention] As explained above, in the optical modulator of the present invention, since the thickness of the signal electrode 3 is not made too large, it can be formed with high precision in terms of manufacturing process. On the other hand, since at least the upper part of the signal electrode 3 is configured to be widely covered by the overhang part 40 formed on the ground electrode 4 with a cap-shaped space in between, the electric field distribution in the surrounding space is improved (the electric field is The increase in the number of field lines effectively reduces the dielectric constant, which greatly improves the velocity matching between the propagated microwave signal and the light wave, improving the performance of optical modulators, especially for high-speed operation. It greatly contributes to improving characteristics.

[Brief explanation of drawings]

Fig. 1 is a diagram showing an embodiment of the present invention, Fig. 2 is a diagram showing another embodiment of the invention, Fig. 3 is a sectional view showing the main manufacturing process of the embodiment of the present invention, FIG. 5 is a diagram showing an example of the basic configuration of an optical modulator (part 1); FIG. Figure 1 is the substrate, is the optical waveguide, 2a and 2b are the first and second branch optical waveguides, is the signal electrode, is the ground electrode, is the buffer layer, and 40 is the overhang hank. . * is likened to the feet of the Ming Dynasty. Figure 2.

Claims (2)

[Claims] (1) Substrate with electro-optic effect processed into a flat surface (1)
An optical waveguide (2) having first and second branched optical waveguides (2a, 2b) is provided above, and the first branched optical waveguide (2a) and the second branched optical waveguide (2b) propagate. In a Mach-Zehnder optical modulator that includes a signal electrode (3) and a ground electrode (4) arranged so as to produce a phase difference in light, the signal electrode (3) has at least its upper part sandwiched by a gap-like space. . An optical modulator, characterized in that it is covered by an overhang (40) formed on the ground electrode (4). (2) The optical modulator according to claim (1), wherein the overhang portion (40) of the ground electrode (4) is formed by a plating method.