JPH03184014A - Optical modulator - Google Patents

Abstract

PURPOSE:To reduce electric induction and mutual interference by the interven tion of an elastic wave nearby the surface of a substrate 1 and to improve the SN ratio of a modulated signal by providing a 1st asymmetrical electrode consisting of a 1st signal electrode and a 1st ground electrode and a 2nd asym metrical electrode consisting of a 2nd signal electrode and a 2nd ground elec trode. CONSTITUTION:The 1st asymmetrical electrode 10 consists of the 1st signal electrode 3a and 1st ground electrode 4b and the 2nd asymmetrical electrode 20 consists of the 2nd signal electrode 3b and 2nd ground electrode 4a and is arranged subordinately to the 1st asymmetrical electrode 10. In this case, the 1st signal electrode 3a and 2nd signal electrode 3b are arranged subordinately to each other, so the distance between the both is large and ground electrodes 4a and 4b can be interposed and arranged between both the signal electrodes 3a and 3b. Consequently, the electric induction and mutual interference by the intervention of the elastic wave nearby the surface of the substrate 1 are made small and the SN ratio of the modulated signal of the fast optical modulator is improved greatly.

Description

[Detailed Description of the Invention] [Table of Contents] Overview Industrial Application Fields Prior Art (Figs. 9 to 11) Problems to be Solved by the Invention Examples of Means and Actions for Solving the Problems (Figs. 1 to 11) (Figure 8) Effects of the invention [Summary] Regarding the optical modulator, there is no fluctuation in the operating point in high-speed drive external light modulation.
In addition, with the aim of realizing an optical modulator that is stable and easy to use over a long period of time, an optical waveguide having first and second branched optical waveguides is provided on a planar substrate having an electro-optic effect, and A first asymmetric electrode consisting of a first signal electrode provided on the branched optical waveguide and a first ground electrode provided in parallel with the first signal electrode on the second branched optical waveguide. a second signal electrode provided on the second branched optical waveguide after the first asymmetric electrode, and a second signal electrode provided on the first branched optical waveguide in parallel with the second signal electrode. An optical modulator is constructed by forming a second asymmetrical electrode consisting of a second ground electrode and a second ground electrode.

In addition, the input electrode lead portions of the first and second signal electrodes are both drawn out to one side of the substrate and connected to an external electric signal circuit, thereby configuring an optical modulator that is easy to use in terms of mounting. .

[Industrial application field]

The present invention relates to an optical modulator that performs optical modulation at high speed and with high stability and is easy to use in terms of mounting, and particularly relates to an electrode configuration thereof.

In the optical transmission system of recent optical communication systems, for example,
In optical communication systems up to about 1.6 GHz,
A method has been used in which a laser diode (LD) is directly modulated, but as the modulation frequency becomes higher, temporal minute fluctuations in the modulated light wavelength, the so-called chirping phenomenon, occur, which limits 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 are faster, more stable, and easier to use.

[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 9 is a diagram showing an example of the configuration of a conventional external light modulator (asymmetric electrode type), where (a) is a top view and (b) is a Y-Y
'This is a cross-sectional view.

In the figure, 1 is a substrate having an electro-optic effect processed into a flat surface;
For example, a LiTaOi substrate. 2 is an optical waveguide, and branch optical waveguides 2a and 2b are formed in the middle. This optical waveguide is usually made by selectively diffusing a metal such as Ti on the surface of a 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. 60 is a buffer layer for reducing absorption of light into the metal electrode layer on the optical waveguide, and is usually made of SiO□. The signal electrode 3'' and the ground electrode 4'' are formed on the optical waveguide via the buffer layer 60 by vapor deposition or plating of a metal such as Au.

Now, for example, DC light emitted from the semiconductor laser 100 enters from the left optical waveguide 2, and branches into the branch optical waveguide 2a,
When a signal voltage is applied to the signal electrode 3' from 15° to the signal electrode 3', the waveguides are split into two by the electro-optic effect in the branched 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 the photodetector 200 converts it into an electrical signal. If a driving voltage is applied so that the phase difference between the two lights in the branched optical waveguides 2a and 2b becomes π or O, for example, an optical signal output can be obtained as an ON-OFF pulse signal.

Note that R7 is a terminating resistor.

R3 and Bk in the same figure (b) indicate the magnitude and direction of the effective electric field acting on the light in each waveguide. 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, and therefore the branched optical waveguides 2a, 2b The electric field distributions applied at are unequal and therefore the effective electric field magnitude E acting on the light at each.

and E are asymmetric, and usually E is about 3 to 6 times that of E.

Since the modulation efficiency is proportional to (E-+Eb), as mentioned above, if E is very small compared to B, the modulation efficiency will not increase and driving by push-pull operation will be difficult. This means that the drive voltage for modulation must be increased.

Furthermore, due to the asymmetry of the electric field seen 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. There was a drawback.

Figure 10 is a diagram showing an example of the configuration of a conventional external optical modulator (parallel two-signal electrode type), and is an example proposed to improve the above-mentioned drawbacks; (b) is Y-Y
'This is a cross-sectional view. In the figure, 3a and 3b are branched optical waveguides 2a.
, 2b. The second signal electrodes, for example traveling wave signal electrodes, 31a, 31b are the first. The input electrode lead portions 4a, 4b of each of the second signal electrodes form a pair with the first and second signal electrodes (3a, 3b), and the first and second branch optical waveguides 2a are connected to each other by application of a signal voltage. , 2b, a ground electrode arranged to create a phase difference in the light transmitted through it, 15''.

16'' is a high frequency modulation signal source.

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 this example, since the electrodes are arranged completely symmetrically as shown in FIG.
, 2b, and Eb are equally capable of push-pull operation, so the drive voltage can be lowered to about 60% compared to the case of the asymmetric electrode type described above.

Furthermore, since no temperature difference occurs between the branched optical waveguides 2a and 2b due to the symmetrical arrangement of the electrodes, no shift in the operating point of the optical modulation characteristics occurs as in the asymmetric electrode type.

FIG. 11 is a diagram showing an example of the element mounting state of a conventional optical modulator, in which the optical modulator shown in FIG. 10 is mounted and arranged, and FIG. Figure (b) is a side view. In the figure, a brass block 50 serves as a base for the substrate 1, and also serves to improve heat dissipation and grounding effects. 51°, 52゛ are IC5 for external electric signal circuit
3', a circuit board for mounting terminating resistor R7, etc.
For example, it is a hybrid IC made of a ceramic substrate. That is, two ICs 53' are arranged on both sides of the light modulation element and connected to the input electrode lead parts 31a and 31b.

[Problem to be solved by the invention]

However, in the optical modulator with the parallel two-signal electrode structure as described above, since the two signal electrodes 3a and 3b are arranged close to each other and facing each other, electrical induction and elasticity near the surface of the substrate 1 are The mutual interference through the waves is large, and the S/ of the modulated signal is
There was a serious problem of N deterioration, which needed to be solved.

[Means for Solving the Problem] The above problem is solved by an optical waveguide having a substrate having an electro-optic effect processed into a plane, and first and second branched optical waveguides 2a and 2b formed on the substrate 1. 2, and a first signal electrode 3 provided on the first branched optical waveguide 2a.
a and a first ground electrode 4b provided on the second branched optical waveguide 2b in parallel with the first signal electrode 3a;
The second branch optical waveguide 2 is arranged in a downstream stage and is subordinate to the second branch optical waveguide 2.
a second asymmetric electrode consisting of a second signal electrode 3b provided on the second signal electrode 3b and a second ground electrode 4a provided on the first branched optical waveguide 2a in parallel with the second signal electrode 3b; This can be solved by configuring an optical modulator including at least 20.

Furthermore, in this optical modulator, each input electrode lead portion 31 of the first and second signal electrodes 3a, 3b
If both a and 31b are drawn out to one side of the substrate 1 and connected to an external electric signal circuit, an optical modulator can be constructed which is extremely convenient in terms of mounting, since it is only necessary to place the integrated circuit device on one side. can.

Further, in a parallel two-signal electrode type optical modulator, the input electrode lead portions 31 of each of the first and second signal electrodes 3a and 3b
If both a and 31b are drawn out to one side of the board 1 and connected to an external electrical signal circuit, it is only necessary to place the integrated circuit device on one side, and in this case as well, optical modulation is very convenient for mounting. The vessel can be configured.

[Effect]

According to the configuration of the present invention, the first signal electrode 3a and the second signal electrode 3b are arranged subordinately to each other, so that the distance between them is large, and the ground electrode is provided between both signal electrodes. It is also possible to have a structure in which the optical fibers are interposed and arranged, and therefore, mutual interference through electrical induction and elastic waves near the surface of the substrate 1 is reduced, and the S/N of the modulated signal of the high-speed optical modulator is greatly improved. do.

Furthermore, the input electrode lead portions 31a and 31b of the first and second signal electrodes 3a and 3b are both drawn out to one side of the substrate l and connected to an external electric signal circuit. For example, it is only necessary to place the integrated circuit device on one side, and an optical modulator can be constructed which is extremely convenient in terms of implementation.

〔Example〕

FIG. 1 is a top view showing a first embodiment of the present invention.

The board L has a size of 60 mm x 2 mm and a thickness of 1 mm.
The surface of an iNb0z Z plate was mirror polished and used.

On this substrate, Ti was vacuum-deposited to a thickness of about 90 nm,
After treating the area corresponding to the optical waveguide 2, including the branched optical waveguides 2a and 2b, using a normal photoetching method, Ti is thermally diffused into LiNb0° at about 1050'C to form the entire optical waveguide 2. Formed.

The length of the branched optical waveguide portion was adjusted to 45 mm, and the widths of all optical waveguides were adjusted to 7 to 11 μm.

Next, as a buffer layer, SiO2 is deposited at a concentration of 300 to 1000
It was formed by sputtering to a thickness of nm.

As shown, the signal electrodes 3a and 3b are
In order to arrange traveling wave signal electrodes with an interval of mm, a Ti-Au alloy film was deposited, and then pattern etched into an electrode shape with a width of 10 μm on the branch optical waveguides 2a and 2b. , on which Au with a thickness of 3 μm was deposited by plating. 31a and 31b
is connected to an external electrical signal circuit through an input electrode lead.

The ground electrodes 4b and 4a are formed at the same time as the signal electrode by the same process as the signal electrode. The ground electrode 4b is formed in parallel with the signal electrode 3a, and the ground electrode 4a is formed in parallel with the signal electrode 3b on the branched optical waveguides 2b and 2a, respectively.

That is, the first signal electrode 3a and the first ground electrode 4b form a first asymmetrical electrode lO, and the second signal electrode 3b
and the second ground electrode 4a form a second asymmetrical electrode 20, and the second asymmetrical electrode 20 is arranged subordinately to the first asymmetrical electrode IO.

Now, the light that has entered the optical waveguide 2 as DC light has a phase difference of π in the branched optical waveguides 2a and 2b.

Alternatively, if the driving voltages of the first and second asymmetrical electrodes are matched and applied so that the voltage becomes 0.

For example, the optical signal output is obtained as an ON-OFF pulse signal, and the optical modulator of the present invention operates. 15 and 25
is a high frequency signal source.

The terminating resistor R7 was adjusted to 50Ω in accordance with the characteristic impedance of the traveling wave signal electrodes 3a and 3b.

With the above configuration, in the frequency band up to approximately 5 GHz,
An optical modulator with low driving voltage and low mutual interference and no operating point shift was obtained.

Further, the number of dependent asymmetric electrodes is not limited to two, and an even number of four or more may be provided.

FIG. 2 is a top view showing a second embodiment of the invention.

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.

In this embodiment, the optical waveguide is bent in the middle of the branched optical waveguide and shifted in parallel, so that mutual electrical interference between the electrodes can be further reduced.

FIG. 3 is a top view showing a third embodiment of the present invention.

In this example, both the manual electrode lead parts 31a and 31b of the first and second signal electrodes 3a and 3b are pulled out to one side, for example, the bottom side, on the substrate 1,
When configured to connect to an external electrical signal circuit, it is only necessary to place the driving integrated circuit device on one side.
An optical modulator that is extremely convenient in terms of implementation can be constructed. N'
As shown in the figure, since the electrode configurations on the branched optical waveguides 3a and 3b are not completely symmetrical, a slight shift in the operating point may occur.

FIG. 4 is a top view showing a fourth embodiment of the present invention.

In this embodiment as well, the input electrode lead parts 31a and 31b of the first and second signal electrodes 3a and 3b are both drawn out to the lower side of the substrate 1, and the branched optical waveguides 3a and 3b are
The upper electrode structure is symmetrical, and the lead-out portions of both ground electrodes are interposed between both signal electrodes to guard them, so mutual interference is extremely small and there is no shift in the operating point. Moreover, it has a configuration convenient for implementation.

FIG. 5 is a diagram showing the mounting state of the device according to the embodiment of the present invention, and is an example in which the light modulation device shown in FIG. 4 is mounted and arranged. 〉 is a side view. In the figure, 50 is a brass block that is the base of the board 1 and is also useful for effective heat dissipation and grounding. 51 and 52 are external signal circuits. This is a hybrid IC consisting of a circuit board, for example, a ceramic board, on which the IC 53 for use with the IC 53, the terminating resistor R7, and others are mounted.In other words, in this embodiment, the IC 53 only needs to be placed on one side, which is extremely convenient for mounting. .

FIG. 6 is a top view showing a fifth embodiment of the present invention. This example basically has a parallel two-electrode configuration, but the
The input electrode lead portions 31a and 31b of the second signal electrodes 3a and 3b are both drawn out to the lower side of the substrate 1, aiming at a mounting effect. Tarishi,
Mutual interference between both signal electrodes and a slight shift in the operating point are expected, so it is necessary to use them appropriately depending on the application.

FIG. 7 is a top view showing a sixth embodiment of the present invention. This example also basically has a parallel two-electrode type configuration, but both input electrode lead portions 31a and 31b are drawn out to the lower side of the substrate 1, aiming at a mounting effect. Although the symmetry of the electrodes is ensured and there is no shift in the operating point, mutual interference between both signal electrodes remains, so it is necessary to use them selectively depending on the application, as in the fifth embodiment.

FIG. 8 is a top view showing a seventh embodiment of the present invention. This example is also basically a parallel two-electrode type structure, but both input electrode lead parts 31a and 31b are drawn out to the bottom of the board 1, aiming at a mounting effect. In addition, the symmetry of the electrodes is ensured and there is no operating point shift, and since the common ground electrode 4 with a wide width is arranged between both signal electrodes, mutual interference is greatly suppressed. , with low driving voltage.

In addition, it is very suitable for short and compact optical modulators.

The embodiments described above are just a few examples, and it goes without saying that preferred materials, configuration dimensions, manufacturing processes, etc., or combinations thereof may be used as appropriate, as long as they comply with the spirit of the present invention. stomach.

〔Effect of the invention〕

As explained above, the optical modulator of the present invention has low driving voltage, low mutual interference between signal electrodes, and suppresses operating point shift in high-speed driving for high-frequency modulation. Since the lead portions can be placed on the same side for ease of mounting, this greatly contributes to improving the performance and quality of optical modulators for high-frequency, long-distance optical communications.

[Brief Description of the Drawings] Fig. 1 is a top view showing a first embodiment of the present invention, Fig. 2 is a top view showing a second embodiment of the invention, and Fig. 3 is a top view showing a third embodiment of the invention.
FIG. 4 is a top view showing the fourth embodiment of the present invention, FIG. 5 is a diagram showing the element mounting state of the embodiment of the present invention, and FIG. 6 is the fifth embodiment of the present invention. 7 is a top view showing a sixth embodiment of the present invention, FIG. 8 is a top view showing a seventh embodiment of the present invention, and FIG. 9 is a configuration example of a conventional external optical modulator. (asymmetric electrode type), Figure 10 is a diagram showing an example of the configuration of a conventional external optical modulator (parallel two-signal electrode type), and Figure 11 is a diagram showing the element mounting state of a conventional optical modulator. be. In the figure, 1 is a substrate, 2 is an optical waveguide, 2a and 2b are first and second branch optical waveguides, 3a
and 3b are first and second signal electrodes, 4 (4a, 4
b) is a ground electrode; 10 is a first asymmetric electrode; 20 is a second asymmetric electrode; 31a and 31b are input electrode lead parts.

Claims (5)

[Claims] (1) Substrate with electro-optic effect processed into a flat surface (1)
and an optical waveguide (2) having first and second branched optical waveguides (2a, 2b) formed on the substrate (1), and an optical waveguide (2) provided on the first branched optical waveguide (2a). A first signal electrode (3a) and a first ground electrode (4b) provided on the second branched optical waveguide (2b) in parallel with the first signal electrode (3a). Asymmetric electrode (10)
a second signal electrode (3b) disposed downstream of the first asymmetrical electrode (10) and provided on the second branch optical waveguide (2b); Branch optical waveguide (2
a) an optical modulator comprising at least a second asymmetrical electrode (20) consisting of a second ground electrode (4a) provided above the second signal electrode (3b) and in parallel; . (2) In the optical modulator according to claim (1), the first and second branch optical waveguides (2a, 2b) are bent between the at least two sets of asymmetric electrodes (10, 20), An optical modulator characterized in that the at least two sets of asymmetric electrodes (10, 20) are not arranged on the same straight line. (3) In the optical modulator according to claim (1), both input electrode lead portions (31a, 31b) of the first and second signal electrodes (3a, 3b) are connected to the substrate (1). An optical modulator characterized in that it is configured to be drawn out on one side of the top and connected to an external electrical signal circuit. (4) Substrate with electro-optic effect processed into a flat surface (1)
and an optical waveguide (2) having first and second branched optical waveguides (2a, 2b) formed on the substrate (1), and the first and second branched optical waveguides (2a, 2b). First and second signal electrodes (3
a, 3b), which form a pair with the first and second signal electrodes (3a, 3b), and are transmitted through the first and second branched optical waveguides (2a, 2b) by application of a signal voltage. at least one ground electrode (4) arranged to create a phase difference in the light;
), wherein the input electrode lead portions (31a, 31b) of the first and second signal electrodes (3a, 3b) are both connected to one side on the substrate (1). 1. An optical modulator characterized in that the optical modulator is configured to be pulled out to the side thereof and connected to an external electrical signal circuit. (5) Substrate with electro-optic effect processed into a flat surface (1)
and an optical waveguide (2) having first and second branch optical waveguides (2a, 2b) formed on the substrate (1);
first and second signal electrodes (3a, 3b) arranged on the first and second branched optical waveguides (2a, 2b), and the first and second signal electrodes (3a, 3b). A ground electrode is provided so as to be electrically isolated, and this ground electrode is connected to the first and second signal electrodes (3a, 3b).
an optical modulator that is paired with at least one of the optical modulators and arranged so as to create a phase difference in the light transmitted through the first and second branch optical waveguides (2a, 2b) by applying a signal voltage. The input electrode lead portions (31a, 31b) of the first and second signal electrodes (3a, 3b) are both pulled out to one side of the substrate (1) and connected to an external electrical signal circuit. An optical modulator characterized in that it is configured to be connected.