JPH09258152A - Waveguide type optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a waveguide type optical device excellent in temp. stability by eliminating influence of electric field caused by an induced electric charge due to pyroelectricity of a substrate to an optical waveguide part. SOLUTION: This device is provided with the optical waveguide 2 formed on the main surface of the substrate 1 having a pyroelectric effect, a dielectric layer 3 formed on the main surface and controlling electrodes 4, 5 provided in the vicinity of the optical waveguide, and further, at least some parts of the electrodes 4, 5 are buried in the dielectric layer 3, and the device is constituted so that electric flux due to the charge by the pyro-electric effect pass through the dielectric between the electrode side surface and the outside surface of the dielectric layer 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide such as an optical modulator or an optical switch having a structure in which an optical waveguide is formed on a substrate having a pyroelectric effect and an electric field is applied to the optical waveguide by a control electrode. Type optical device.

[0002]

2. Description of the Related Art A Mach-Zehnder type optical modulator using a Z-cut lithium niobate (LiNbO ₃ ) substrate will be described as a conventional example of this type of waveguide type optical device. FIG. 5 is a plan view of a Mach-Zehnder type optical modulator as a conventional example of a waveguide type optical device, and FIG. 6 is a sectional view of a central portion thereof. This optical modulator has a crystal axis Z
Lithium niobate crystal substrate (electro-optical crystal substrate having pyroelectricity) cut so that its axis is in the thickness direction of the substrate
A Mach-Zehnder interference type optical waveguide 2 is formed on the main surface of 1 by thermal diffusion of titanium (Ti), and a dielectric layer 3 and control electrodes 4 and 5 are formed on the optical waveguide 2 so that desired modulation characteristics can be obtained. Arranged and configured. The electrode 4 is an excitation electrode and the electrode 5 is a ground electrode.

A major problem in this type of waveguide type optical device is its operational stability. Particularly, drift such as modulation characteristics with respect to application of a DC voltage or temperature change is caused by a crystal substrate, a dielectric, which is a component of the device. Various proposals have been made to reduce the instability, which depend on the physical properties of the body layer and the structural arrangement thereof.

Of these, the drift with respect to temperature originates from the pyroelectricity of an electro-optic crystal substrate such as a lithium niobate crystal substrate, and its mechanism is considered as follows.

When the temperature of the above-mentioned waveguide type optical device changes, electric charges are generated on the substrate surface (main surface on which the optical waveguide is formed) and the back surface due to the pyroelectric effect of the crystal substrate. Of these, the electric field generated by the charges on the back surface is small when the thickness of the substrate that is normally used (0.5 to 1 mm) is small.
As disclosed in No. 5, this effect can be eliminated by adding a conductive material to the back surface of the substrate and grounding. On the other hand, the electric charge generated on the main surface of the substrate on which the optical waveguide is formed induces the polarization of the dielectric layer normally formed on the substrate,
As a result, the electric charge generated by the pyroelectric effect is replaced by the polarized electric charge induced on the surface of the dielectric layer. Since the electric charges on the surface of the dielectric material are not mobile, the electric field generated by the surface charges is greatly influenced by the electrical arrangement in the vicinity (especially the conductor). Usually, an electrode for controlling the refractive index using the electro-optical effect of the substrate is arranged in the optical modulator, so that an electric field as shown by the lines of electric force in FIG. 6 is generated near the surface. . This electric field fluctuates depending on the condition of temperature change, which causes a drift of the operating point.

Therefore, Japanese Patent Publication No. 22485/1992 discloses that the low resistance film 6 is further formed on the dielectric layer 3 as shown in FIG. 7 to reduce the unevenness of the electric field distribution. (In FIG. 7, the same or corresponding parts as in FIG. 6 are designated by the same reference numerals). For example, a material using silicon or ITO as these low resistance film materials, or JP-A-5-2241.
A device using a metal partial oxide film as shown in No. 64 has been proposed.

[0007]

However, as shown in FIG. 7, the structure in which the low resistance film 6 is uniformly applied has a low resistance even between the excitation electrode 4 and the ground electrode 5 which should originally be insulated. Since the resistance film 6 is straddled, the characteristics at low frequencies are deteriorated, and depending on the configurations of the electrodes 4, 5, the dielectric layer 3, and the optical waveguide 2, the stability with respect to the DC voltage application is also adversely affected. Further, even if the low resistance film is provided, it is difficult to completely eliminate the instability due to temperature due to the asymmetry of the arrangement of the electrode and the optical waveguide and the slight nonuniformity of the remaining charge distribution.

In view of the above points, the present invention optimizes the arrangement of the electrode, the dielectric layer, and the pyroelectric substrate on which the optical waveguide is formed, so that the electric field generated by the induced charge due to the pyroelectricity of the substrate is optimized. It is an object of the present invention to provide a waveguide-type optical device having excellent temperature stability by substantially eliminating the influence on the optical waveguide part.

Other objects and novel features of the present invention will be clarified in embodiments described later.

[0010]

In order to achieve the above object, a waveguide type optical device of the present invention comprises an optical waveguide formed on the main surface of a substrate having a pyroelectric effect, and an optical waveguide formed on the main surface. In a structure having a dielectric layer formed thereon and a control electrode provided in the vicinity of the optical waveguide, at least a part of the electrode is embedded in the dielectric layer, and is caused by the charge due to the pyroelectric effect. The lines of electric force are made to pass through the dielectric between the side surface of the electrode and the outer surface of the dielectric layer.

Further, the dielectric layer may be composed of a plurality of types of dielectrics, and the bottom dielectric layer may be interposed between the main surface and the electrode.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a waveguide type optical device according to the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing a Mach-Zehnder type optical modulator using a Z-cut lithium niobate (LiNbO ₃ ) substrate as an embodiment of a waveguide type optical device according to the present invention. The plan view is the same as FIG. 5 described in the conventional example. In FIG. 1, thermal diffusion of titanium (Ti) is carried out on the main surface of a lithium niobate crystal substrate (electro-optical crystal substrate having pyroelectricity) 1 which is cut so that the Z axis of the crystal axis is in the thickness direction of the substrate. To form a Mach-Zehnder interference type optical waveguide 2, and a dielectric layer 3 thereon. The lower portions of the excitation electrode 4 and the ground electrode 5 as control electrodes are embedded in the dielectric layer 3. In other words, the outer surface of the dielectric layer 3 between the electrodes 4 and 5 is formed higher than the bottom surface of the electrodes 4 and 5. As a result, even if polarization charges are generated on the outer surface of the dielectric layer 3 due to the pyroelectric effect of the substrate 1, the lines of electric force (indicated by solid lines in FIG. 1) due to these polarization charges are different from each other. It passes through the dielectric body between the side surfaces of the electrodes 4 and 5 and the outer surface of the dielectric layer 3, the electric field at those portions is strong, and the electric field around the optical waveguide 2 due to the polarization charge is almost eliminated.

Therefore, the change of the electric field caused by the pyroelectric effect of the substrate 1 does not affect the light propagating in the optical waveguide 2, and the low resistance film of the conventional structure shown in FIG. It is possible to reduce the temperature drift and improve the temperature stability without addition.

[0015]

EXAMPLE FIG. 2 shows Example 1 according to the present invention, FIG. 3 shows temperature drift data of the optical modulator in the case of Example 1, and FIG. 4 shows Example 2 according to the present invention. Both Embodiments 1 and 2 exemplify a Mach-Zehnder type optical modulator.

Example 1 First, as shown in FIG. 2, titanium (Ti) having a film thickness of 800 Å and a width of 6 μm was formed on the main surface of a Z-cut lithium niobate (LiNbO ₃ ) substrate 1.
Is deposited by vacuum evaporation and lift-off, and is thermally diffused in a dry oxygen (Dry-O ₂ ) atmosphere at 1050 ° C. for 7.5 hours to form the optical waveguide 2. Next, after depositing silica (SiO ₂ ) as a lower layer dielectric 3a by 1 μm, a base film of chromium (Cr) 50 nm and gold (Au) 50 nm is formed thereon, a guide resist is formed, and then a thickness is formed by electrolytic plating. A gold (Au) electrode having a thickness of 8 μm is formed to serve as the excitation electrode 4 and the ground electrode 5. Finally, an epoxy resin as the upper dielectric 3b was applied between the electrodes 4 and 5 by 7 μm to obtain a desired shape. As a result, the lower portions of the electrodes 4 and 5 are located at the lower layer dielectric 3a and the upper layer dielectric 3a.
It is in a state of being embedded in the dielectric layer 3 including b. The lower-layer dielectric 3a serves as a buffer layer interposed between the main surface of the substrate 1 and the electrodes 4 and 5, has a property of not absorbing light, and prevents loss of light propagating in the optical waveguide 2. The purpose is to prevent the absorption of light by the metal of the electrodes 4 and 5.

The data of the temperature drift of the optical modulator having the configuration of the first embodiment is shown by the line (a) in FIG. However, the horizontal axis of FIG. 3 is temperature (° C.), and the vertical axis is operating point variation (V) of the optical modulator.
Is represented. Further, for comparison, data obtained by separately producing the structure of the conventional example of FIG. 6 is also shown by a line (b). It can be seen that the configuration of Example 1 can reduce the temperature drift without providing the low resistance film between the electrodes.

In the first embodiment, since the dielectric layer 3 can be composed of a plurality of types of dielectrics, the height of embedding the electrodes 4 and 5 in the dielectric layer 3 can be made sufficiently large, and the lowermost dielectric layer Can be made of a material suitable for the buffer layer that does not absorb light.

(Embodiment 2) In Embodiment 2 of FIG. 4, the optical waveguide 2 is formed on the main surface of the substrate 1 by the same method as that of Embodiment 1. Then, the silica (Si
After depositing O ₂ ) for 1 μm, a groove 11 and a step 12 having a depth of 0.6 μm for burying the excitation electrode 4 and the ground electrode 5 were formed by photolithography and ion milling. After that, the electrodes 4 and 5 were formed in the groove 11 and the step 12 on the dielectric layer 3 in the same manner as in Example 1. The thin dielectric portion remaining on the bottoms of the groove 11 and the step 12 serves as a buffer layer interposed between the main surface of the substrate and the electrodes 4 and 5.

Also in the structure of the second embodiment, the temperature drift is almost the same as that of the first embodiment.

Although not shown, by adding a conductive material to the back side of the substrate 1 and grounding it, the influence of charges on the back side of the substrate can be eliminated.

The substrate 1, the optical waveguide 2, the dielectric layer 3,
The materials of the electrodes 4 and 5 may be other than the materials exemplified in the first and second embodiments, and in principle, the upper surfaces of the electrodes 4 and 5 may be embedded in the dielectric layer 3. It is possible.

Although the embodiment of the present invention has been described above, it is obvious to those skilled in the art that the present invention is not limited to this and various modifications and changes can be made within the scope of the claims. Ah

[0024]

As described above, according to the present invention,
Since it is possible to prevent the change in the electric field caused by the pyroelectric effect of the substrate on which the optical waveguide is formed from affecting the light in the optical waveguide, a waveguide type optical device with excellent temperature stability is realized. be able to.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an embodiment of a waveguide type optical device according to the present invention and a cross section of an optical modulator as a waveguide type optical device and an electric field due to a pyroelectric charge.

FIG. 2 is a sectional view of a first embodiment according to the present invention.

FIG. 3 is a graph showing measurement results of temperature characteristics of the optical modulator according to the first embodiment.

FIG. 4 is a sectional view of a second embodiment according to the present invention.

FIG. 5 is a plan view showing an optical modulator as a conventional example of a waveguide type optical device.

6 is a VI-VI sectional view of FIG. 5 showing a cross section of an optical modulator as a conventional example and an electric field due to a pyroelectric charge.

FIG. 7 is a cross-sectional view showing a cross section of an optical modulator as another conventional example to which a low resistance film is added and an electric field due to a pyroelectric charge.

[Explanation of symbols]

 1 Substrate 2 Optical Waveguide 3 Dielectric Layer 3a Lower Layer Dielectric 3b Upper Layer Dielectric 4 Excitation Electrode 5 Grounding Electrode 6 Low Resistance Film 11 Groove 12 Step

Claims (2)

[Claims] 1. An optical waveguide formed on a main surface of a substrate having a pyroelectric effect, a dielectric layer formed on the main surface, and a control electrode provided near the optical waveguide. In the waveguide type optical device, at least a part of the electrode is embedded in the dielectric layer, and lines of electric force caused by the charges due to the pyroelectric effect are generated between the electrode side surface and the outer surface of the dielectric layer. A waveguide type optical device characterized by being configured to pass through a dielectric body. 2. The waveguide type optical device according to claim 1, wherein the dielectric layer is composed of a plurality of types of dielectrics, and the lowermost dielectric is interposed between the main surface and the electrode.