JPH07152051A - Optical waveguide element - Google Patents

Abstract

PURPOSE:To reduce variation with respect to the lapse of time in DC drift of a waveguide type optical controlling element. CONSTITUTION:The optical waveguide element 31 is obtd. by forming optical waveguides 33, 34 on a Z-cut lithium niobate crystal substrate 32 by thermal diffusion of titanium. Then an optical buffer layer 35 is deposited thereon and electrodes 37, 38 for controlling are formed on the optical buffer layer 35 near the optical waveguides 33, 34. By controlling the volume resistivity of the optical buffer layer 35 to about 1 to 100 times of the crystal substrate 32, variation in DC drift due to transient response of an electric field can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide element used for, for example, optical wave modulation or optical path switching in optical communication, and in particular, it uses an optical waveguide formed on the upper surface of a crystal substrate to provide a light output end. The present invention relates to an optical waveguide device that is controlled.

[0002]

2. Description of the Related Art Practical application of an optical communication system is progressing, and development of a large-capacity and multifunctional advanced system is under way. Under these circumstances, new functions such as a function of exchanging an optical transmission line and a connection and switching between optical data bus terminals are required, and the need for a so-called optical switching network is increasing.

In an optical communication system which is currently put into practical use, an injection signal of a semiconductor laser or a light emitting diode is directly modulated to obtain an optical signal. However, in such direct modulation, due to the effect of relaxation oscillation, etc.
There is a problem that it becomes difficult to perform high-speed modulation further from around Hz. In addition, there is a problem that it is difficult to apply it to long-distance transmission and coherent optical transmission systems because of wavelength fluctuation. Further, the switch used for optical path switching used in the optical transmission line of such an optical communication system is configured to mechanically move an optical component such as a prism, a mirror, or a fiber, and its response is slow. However, there are also problems such as insufficient reliability, large size, and inadequate arrangement of a large number of switches in a matrix.

To solve these problems, various optical control elements having an optical waveguide formed on a substrate have been proposed. Such a light control element is fast,
Also, many elements can be integrated on a substrate.
Further, since it is not necessary to move the optical parts mechanically, it has excellent characteristics such as high reliability. In particular, the light control element using a ferroelectric band material such as lithium niobate (LiNbO ₃ ) has low absorption of light, low loss, and high efficiency because it has a large electro-optical effect. There is.

However, a waveguide type optical control element using an electro-optic crystal such as a lithium niobate or lithium tantalate (LiTaO ₃ ) substrate as described above is
A phenomenon called operating point shift (DC drift) occurs in which the intensity of light output with respect to the applied voltage varies. This limits the life of the light control element. Therefore, a solution to the DC drift is an indispensable issue when the waveguide type optical control element is put to practical use.

FIG. 6 shows a directional coupling type optical switch as a typical example of a waveguide type optical control element. This directional-coupling type optical switch 11 has optical waveguides 13 and 14 formed by diffusing a metal such as titanium (Ti) on a crystal substrate 12 made of lithium niobate. These optical waveguides 13 and 14 are arranged close to each other in the central portion of the substrate at intervals of about several μm, and form an optical directional coupler 15. A buffer layer (not shown) is arranged on the optical waveguides 13 and 14, and a pair of control electrodes 16 and 17 are arranged via the buffer layer.

The basic operating principle of the directional coupling type optical switch 11 shown in FIG. 6 will be described. As one of the optical waveguides, first, for example, the end face of the optical waveguide 14 will be considered. The light wave 21 incident from the end face of the optical waveguide 14 is
Energy is propagated through the optical waveguide 14 and energy is transferred to the other optical waveguide 13 that is close to the optical directional coupler 15. When the length of the optical directional coupler 15 is matched with the complete coupling length Lc, almost 100% of the energy is transferred to the optical waveguide 13 and becomes the emitted light 22.

On the other hand, when a predetermined voltage is applied between the two control electrodes 16 and 17, the refractive indexes of the optical waveguides 13 and 14 change due to the electro-optical effect, and the refractive indexes of the two become unequal. As a result, a phase mismatch occurs between the light waves propagating through the two, the coupling state changes, and the light is emitted from the incident optical waveguide 14 as light 23.

The DC drift has two control electrodes 16, 1
It is considered that when a DC voltage is applied to 7, the ions move in the buffer layer, the ions are attracted to the negative electrode to form a demagnetizing field, and the externally applied electric field is canceled. In order to suppress this DC drift, it is known to perform high temperature silicon dioxide (SiO ₂ ) annealing in an oxygen atmosphere, or to use a structure in which a buffer layer between the control electrodes 16 and 17 is not used. Further, recently, as a method of more effectively suppressing DC drift, there is also a technique of forming a buffer layer from a material obtained by synthesizing a conductive material and an insulating material as described in JP-A-61-198133. It is disclosed. In addition, JP-A-5-11
As disclosed in Japanese Patent No. 3513, there has been proposed a technique in which a crystal substrate such as lithium niobate is doped with a Group 5 element such as phosphorus (P) or chlorine (cl).

[0010]

The conventional method described above is effective for suppressing or reducing the drift phenomenon due to ion movement. However, such an approach
It has no effect on the fluctuation of the intensity of the output light due to the transient response of the electric field when the C voltage is applied. This is because the transient response of the electric field occurs due to the volume resistivity and the dielectric constant at direct current in the crystal substrate and the buffer layer forming the light control element, and the saturation value and the saturation time thereof are determined.

Therefore, an object of the present invention is to optimize the volume resistivity and the permittivity at direct current in the layer structure of the crystal substrate and the buffer layer of the waveguide type optical control element, so that the DC drift due to the transient response of the electric field is achieved. An object of the present invention is to provide an optical waveguide device capable of reducing the amount of fluctuation itself with respect to the passage of time.

[0012]

According to a first aspect of the present invention, a ferroelectric crystal substrate having an electro-optical effect, an optical waveguide formed on the substrate, and an output end of light incident on the optical waveguide are provided. And a control electrode for controlling the liquid crystal, and the volume resistivity at direct current disposed between the control electrode and the optical waveguide is 1 to 10 times the volume resistivity of the ferroelectric crystal substrate.
The optical waveguide device is provided with an optical buffer layer adjusted to a range of 0 times.

That is, according to the first aspect of the invention, the volume resistivity at direct current of the optical buffer layer arranged between the control electrode and the optical waveguide is set to be 1 to 1 times the volume resistivity of the ferroelectric crystal substrate. The range is set to 100 times, and the amount of fluctuation itself in the DC drift due to the transient response of the electric field over time is reduced.

According to the second aspect of the invention, as a preferred example of the optical waveguide device, the ferroelectric crystal substrate is made of a lithium niobate crystal substrate and the optical buffer layer is made of silicon dioxide.

In the invention described in claim 3, it is clarified that the optical waveguide may constitute a Mach-Zehnder interferometer, and in the invention described in claim 4, the optical waveguide constitutes a directional coupler. Has revealed that it is good.

[0016]

EXAMPLES The present invention will be described in detail below with reference to examples.

FIG. 1 shows a sectional structure of an optical waveguide device in one embodiment of the present invention. In this optical waveguide device 31, titanium (Ti) is thermally diffused on a (Z-cut) lithium niobate crystal substrate (LiNbO ₃ ) 32 cut in the Z-axis direction to form optical waveguides 33 and 34. And
Silicon dioxide (SiO ₂ ) is deposited thereon as an optical buffer layer 35, and control electrodes 37 and 38 are arranged on the optical buffer layer 35 near the optical waveguides 33 and 34. The volume resistivity of the crystal substrate 32 is lower than the order of 10 ¹⁷ Ωcm of the bulk portion, and is 10 ¹⁵ Ω in this embodiment.
It is about cm. This is because titanium is diffused in a water vapor atmosphere in order to prevent lithium outdiffusion from the crystal substrate 32.

In this embodiment, the volume of the optical buffer layer 35 is
The resistivity is controlled by controlling the film forming conditions to 10 ¹⁶Ωcm,
It was set to about 10 times that of the crystal substrate 32. This allows
Minimize the amount of DC drift fluctuation due to the transient response of the electric field
It is possible to prolong the life of the device.

FIG. 2 shows an electrically equivalent circuit of the optical waveguide device shown in FIG. 1 in correspondence with the sectional structure of the device. Each node 41, 42 of the control electrodes 37, 38
Between them, a DC power supply 44 of voltage E is supplied via a switch 43.
Are connected. Control electrodes 37, 38 and crystal substrate 3
In the optical buffer layer 35 between the two corresponding optical waveguides 33 and 34, the capacitance C _fd and the resistance R _fd are respectively provided.
Is equivalently connected, and between these two nodes 43 and 44 in the optical buffer layer 35, the parallel circuit of the capacitance C _ft and the resistance R _ft is equivalently connected. Each optical waveguide 33, 34 in the crystal substrate 32
Can be represented by a parallel circuit of a capacitance C _bd and a resistance R _bd , respectively, and a parallel circuit of a capacitance C _bt and a resistance R _bt is equivalently connected between the corresponding nodes 45 and 46.

Now, at a certain time t, the voltage acting between the nodes 44 and 46 of one optical waveguide 34 is V (44,46,
t). DC drift at this time is D
Assuming (44, 46, t), this can be expressed by the following equation (1).

[0021]

[Equation 1]

FIG. 3 shows an electric field transient response simulation result by the equivalent circuit shown in FIG. Each curve in the figure is a DC drift with the volume resistivity ratio R _fd / R _bd with the film thickness of the optical buffer layer 35 being constant with respect to the crystal substrate 32 in the equivalent circuit shown in FIG. 2 as a parameter. It is an example of the behavior of D (44, 46, t). When the volume resistivity ratio R _fd / R _bd is 10 times, it is shown by a white circle in the figure, but DC due to the electric field transient response is shown.
It can be seen that the drift is suppressed to about 20%.
When the volume resistivity ratio R _fd / R _bd is 100 times, it is indicated by a black circle in the figure. If the ratio is larger than this, the DC drift amount (%) at the time of saturation approaches 100%, which is not preferable. Further, the volume resistivity ratio R _fd / R _bd is, for example, 0.
When it is 1 time, it is shown by a black diamond in the figure, but the DC drift amount (%) is drastically changed from “0” at the initial stage of the elapsed time (for example, at the time of 1 hour in the figure). When the film thickness is increased, the change is further increased, which is not preferable. When the volume resistivity ratio R _fd / R _bd is 1 times, it is shown by a black square in the figure, but this is the limit of the preferable characteristics when the magnification is low.

Therefore, in the direct current of the optical buffer layer,
The volume resistivity of the ferroelectric crystal substrate corresponds to that of the ferroelectric crystal substrate.
It is better to adjust the range from 1 to 100 times.
It is about 2 to 20 times, though not shown in this figure.
Is more preferable. That is, the crystal substrate 32
Volume resistivity is 10¹⁵10 if Ωcm ¹⁶
Silicon dioxide having a volume resistivity on the order of Ωcm
(SiO₂) Using the film as the optical buffer layer 35
Effectively suppresses DC drift due to electric field transient response for a long period of time.
Can be obtained.

Modification

FIG. 4 shows a sectional structure of the optical waveguide element of the optical waveguide element according to the first modification of the present invention. In this figure, the same parts as those in FIG. 1 are designated by the same reference numerals, and the description thereof will be appropriately omitted. In the case of the optical waveguide device 51 of the first modification, the optical buffer layer 35
A is removed from the portion between the control electrodes 37 and 38. As a result, formation of a de-electric field due to ion movement in the optical buffer layer 35A can be prevented, and DC drift due to a transient response of the electric field can be suppressed.

FIG. 5 shows a cross-sectional structure of the optical waveguide element of the optical waveguide element according to the second modification of the present invention. In this figure, the same parts as those in FIG. 1 are designated by the same reference numerals, and the description thereof will be appropriately omitted. In the case of the optical waveguide device 61 of the second modification, the optical buffer layer 35
B is made to exist only in the portion sandwiched between the control electrodes 37 and 38 and the optical waveguides 33 and 34. Then, an antistatic film 62, which is known to have the effect of stabilizing the characteristics against an increase in internal temperature, is formed from above the crystal substrate 32. Also in this case, it is possible to prevent the formation of a counter electric field due to ion movement in the optical buffer layer 35B, and suppress the DC drift due to the transient response of the electric field.

In the embodiments and the modifications described above, the optical waveguide of the optical waveguide element is realized as a directional coupler. It can also be realized as an interferometer. Also in this case, as described above, the volume resistivity of the optical buffer layer at direct current is preferably adjusted to be in the range of 1 to 100 times the volume resistivity of the ferroelectric crystal substrate.

[0028]

As described above, according to the present invention, the volume resistivity at direct current of the optical buffer layer arranged between the control electrode and the optical waveguide is compared with the volume resistivity of the ferroelectric crystal substrate. I set the range from 1 to 100 times,
It is possible to reduce the fluctuation amount of the DC drift due to the transient response of the electric field with the passage of time and extend the life of the optical waveguide device.

[Brief description of drawings]

FIG. 1 is a sectional view of an optical waveguide device according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an electrically equivalent circuit of the optical waveguide device shown in FIG. 1 corresponding to a cross-sectional structure of the device.

FIG. 3 is a characteristic diagram showing a simulation result of electric field transient response by the equivalent circuit shown in FIG.

FIG. 4 is a sectional view of an optical waveguide device according to a first modified example of the present invention.

FIG. 5 is a sectional view of an optical waveguide device according to a second modification of the present invention.

FIG. 6 is a perspective view showing a general configuration of a directional coupling type optical switch which is a kind of waveguide type optical control element.

[Explanation of symbols]

 31, 51, 61 Optical Waveguide Element 32 Lithium Niobate Crystal Substrate 33, 34 Optical Waveguide 35, 35A, 35B Optical Buffer Layer 37, 38 Control Electrode 41-46 Node 62 Antistatic Film

Claims (4)

[Claims] 1. A ferroelectric crystal substrate having an electro-optical effect, an optical waveguide formed on the substrate, a control electrode for controlling an output end of light incident on the optical waveguide, and the control electrode. And an optical buffer layer disposed between the optical waveguide and having a direct current volume resistivity adjusted to a range of 1 to 100 times the volume resistivity of the ferroelectric crystal substrate. Characteristic optical waveguide device. 2. The optical waveguide device according to claim 1, wherein the ferroelectric crystal substrate is a lithium niobate crystal substrate, and the optical buffer layer is silicon dioxide. 3. The optical waveguide element according to claim 1, wherein the optical waveguide constitutes a Mach-Zehnder interferometer. 4. The optical waveguide element according to claim 1, wherein the optical waveguide constitutes a directional coupler.