JPH09211503A - Waveguide type optical control element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a waveguide type optical control element which is large in manufacturing tolerance and does not entail the deterioration in various characteristics exclusive of a DC drift. SOLUTION: Optical waveguides 12, 13 are formed on a Z-cut lithium niobate crystal substrate 11 by thermally diffusing thin-film titanium (Ti) into this substrate in such a manner that light propagates in a Y-axis direction. Oxidized film silicon which is optical buffer layers 1, 2 is divided perpendicularly to the crystal substrate 11 and is deposited thereon between control electrodes 15 and 16 and further, the control electrodes 15, 16 are installed near the optical waveguides 12, 13. The optical buffer layers 1, 2 are respectively independently deposited. The relation of the respective resistance values R1 , R2 of the optical buffer layers 1, 2 constituted in such a manner is R1 <R2 . The resistance value R1 of the buffer layers which are divided perpendicularly to the ferroelectric crystal substrate and are held by the optical waveguides 12, 13 and the control electrodes 15, 16 is made smaller than the rest value R2 of the adjacent buffer layers. The DC drift is suppressed by this constitution.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical control element used for optical wave modulation or optical path switching in optical communication or the like.

[0002]

2. Description of the Related Art Conventionally, an optical communication system has been put into practical use, and development of a large-capacity and multifunctional advanced system has been advanced. Under such circumstances, further capacity increase (wide band, high speed operation) and optical transmission line switching function,
New functions such as high-speed connection and switching between optical data bus terminals are required. In order to fulfill these functions, an optical switching network in a so-called waveguide type optical control element is required.

An optical signal in an optical communication system which is currently put into practical use is obtained by directly modulating an injection current of a semiconductor laser or a light emitting diode. In this direct modulation, it is difficult to perform high-speed modulation of about 10 GHz or higher due to effects such as relaxation oscillation. Moreover, the optical path changeover switch of the current optical transmission path mechanically moves a prism, a mirror, a fiber, etc., and is low speed, insufficient reliability, large in shape and unsuitable for matrix formation. There are some drawbacks.

What is being developed to solve these drawbacks is a waveguide type optical control element using an optical waveguide installed on a substrate. This light control element has features such that high speed and multi-element integration are possible, and high reliability is desired. In particular, a Z-cut lithium niobate crystal substrate using a ferroelectric material such as LiNbO ₃ has high efficiency because it has small light absorption and low loss and has a large electro-optical effect. There are features such as.

FIG. 6 shows a perspective view of a directional coupling type optical switch which is an example of a waveguide type optical control element. Optical waveguides 52 and 53 are formed by diffusing a metal such as Ti on a LiNbO ₃ crystal substrate 51 that is cut out and shaped perpendicular to the optical axis. These optical waveguides 52 and 53 are arranged close to each other at an interval of about several μm to form an optical directional coupler 54, and an optical buffer layer (not shown) is provided on the optical waveguides 52 and 53. Control electrodes 55 and 56 are installed.

The basic operating principle of this directional coupling type optical switch is as follows. First, one optical waveguide, for example, the optical waveguide 53.
The incident light 57 incident from the end surface of the optical waveguide 52 propagates through the optical waveguide 53 and is close to the optical directional coupler 54.
When energy is transferred to the optical directional coupler 54 and the length of the optical directional coupler 54 is matched with the complete coupling length Lc, almost 100% of the energy is transferred to the optical waveguide 52 and becomes emitted light 58.

On the other hand, when a voltage is applied between the control electrodes 55 and 56, the optical waveguides 52 and 53 are produced by the electro-optical effect.
The refractive index of the two changes, and the refractive indexes of the two become asymmetric, and the phase mismatch occurs between the light waves propagating through the two, and the coupling state changes. In this state, under an appropriate applied voltage, energy is transferred to the original optical waveguide 53 and becomes emitted light 59, and the optical path is switched. In the case of performing high-speed modulation of about 10 GHz or more, a configuration in which a Mach-Zehnder interference type is formed by an optical waveguide is generally used, and a phase difference is given to a light wave branched into two by the same principle as the switch described above, and the other reference The output light is turned on and off by combining and interfering with the light.
Here, the optical buffer layer is formed for the purpose of preventing the incident light 57 propagating through the optical waveguides 52 and 53 from being absorbed by the control electrodes 55 and 56. Generally, a dielectric film is used as an optical buffer layer in many cases, and it is formed of silicon dioxide (SiO ₂ ) particularly for light with a wavelength of about 1.3 to 1.55 μm.

However, the movement of carriers (impurity ions, charges) generated in the optical buffer layer formed near the optical waveguide between the control electrodes causes the external electric field (DC voltage) acting on the optical waveguide to change with time. The phenomenon that the output light fluctuates occurs. This is the so-called DC drift phenomenon.

Various measures have been taken to suppress such light output fluctuations. Examples of these are listed below. 1) Remove the optical buffer layer between the control electrodes, as described, for example, in Ohm's "Optical Integrated Circuits", section 7.4. 2) As described in JP-A-61-198133, the conductive material of the optical buffer layer is formed by synthesizing an insulating material. 3) As described in JP-A-4-280219, a difference is provided between the widths of the two waveguides that act on the electric field, and the DC bias itself is unnecessary.

[0010]

However, in the DC drift suppressing measures of the above-mentioned conventional example, for example, by removing the optical buffer layer between the electrodes, the stress distribution over the entire surface of the substrate is greatly different, and the stress distribution against the temperature change is large. The characteristics are extremely deteriorated. Further, in a case where a synthetic film of a conductive material and an insulating material and a waveguide width are provided with a difference, the manufacturing tolerances are strict and it is difficult to manufacture with a high yield.

An object of the present invention is to provide a waveguide type optical control element which has a large manufacturing tolerance and is free from deterioration of various characteristics other than DC drift.

[0012]

To achieve the above object, a waveguide type optical control element of the present invention comprises an optical waveguide, an optical buffer layer and a control electrode on a ferroelectric crystal substrate having an electro-optical effect. It is a waveguide type optical control element formed and formed, wherein the optical buffer layer is composed of at least two types having different resistance values and is formed between the control electrodes perpendicular to the ferroelectric crystal substrate. I am trying.

Further, the resistance value R ₁ of the optical buffer layer sandwiched between the optical waveguide and the control electrode and the resistance values R ₂ and R _{6 of} the optical buffer layers adjacent thereto are R ₁ <R ₂ ,
It is preferable to have the relationship of R ₆ .

It is preferable that the dielectric crystal substrate is a lithium niobate crystal substrate, the optical waveguide is a directional coupler, and a Mach-Zehnder interferometer is used.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a waveguide type optical control element according to the present invention will be described in detail with reference to the accompanying drawings. 1 to 5, there is shown an embodiment of the waveguide type optical control element of the present invention. FIG. 1 is a diagram showing a first embodiment, FIGS. 2 to 4 are diagrams showing a second embodiment, and FIG.
FIG. 3 is a perspective view of the first and second embodiments.

FIG. 1 shows a sectional structure of a waveguide type optical control element according to a first embodiment of the present invention. Z-c
to ut lithium niobate crystal substrate LiNbO ₃ 11, such that the light propagates in the Y-axis direction, 50 [mu] m approximately thin titanium (T
i) is thermally diffused at about 1000 ° C. to obtain optical waveguides 12, 13
To form Oxide film silicon, which is the optical buffer layers 1 to 6, is deposited on the control electrodes by dividing the control electrodes perpendicularly to the crystal substrate, and further, in the vicinity of the optical waveguide.
Is installed. The optical buffer layers 1-6 are deposited independently.

The deposition method is as follows: a film forming apparatus (sputtering, vapor deposition, CVD, etc.) target material (SiO, SiO ₂ ,
The desired resistance value can be realized relatively freely depending on the conditions (Al ₂ , O ₃ , PSG, etc.), film forming conditions (atmosphere, pressure, power, heating temperature, etc.). For example, by sputtering a SiO ₂ target at a low incident power of about 500 W or less under 0.3 Pa or in a low vacuum atmosphere of about 0.5 Pa or more at a power of 2 KW, 10 ¹² Ωcm.
The resistance value of the degree can be selected. Or 0.3P
a, a resistance value of 10 ¹⁶ Ωcm can be selected by 2 KW.

The optical buffer layers and the resistance values of the waveguide type optical control element of the embodiment shown in FIG. 1 have the following relationship. That is, the optical buffer layer 2 / the resistance value R ₂ ,
Optical buffer layer 3 / resistance value R ₃ , optical buffer layer 4
/ Resistance value R ₄ , optical buffer layer 5 / resistance value R ₅ , and optical buffer layer 6 / resistance value R ₆ . Furthermore, R ₁ ≠ R
₂ , R ₂ ≠ R ₃ , R ₃ ≠ R ₄ , R ₄ ≠ R ₅ , R ₅ ≠ R
₆ , R ₆ ≠ R ₁ .

FIG. 2 shows a second film manufactured by two film formations.
FIG. 3 is a cross-sectional view of a waveguide type light control element that is the embodiment of FIG.
That is, R ₃ = R ₅ = R ₁ , R ₄ = R ₆ = R in FIG.
It is a form of ₂ . Further, FIG. 3 is a diagram showing an example of a manufacturing process of the element of FIG.

In FIG. 3A, the optical waveguides 12, 1
Z-cut lithium niobate crystal substrate Li on which 3 is formed
An optical buffer layer 1 having a resistance value R ₁ is formed on NbO ₃ 11. After that, the optical buffer layer 1 having a resistance value R ₁ is formed, and a resist pattern 9 is formed on the portion where the resistance value is desired to be changed by using the photolithography technique (2). The unnecessary portion is removed by etching (3), and a film of the optical buffer layer 2 having a resistance value R ₂ is formed thereon (4).

Using the above-mentioned manufacturing conditions, the resistance value R ₂ is 0.3 Pa, the resistance value is 10 ¹⁶ Ωcm at ₂ kW, and the resistance value R ₁ is 0.3.
Assuming that the resistance value is about 10 ¹² Ωcm at Pa and 0.5 KW, the external electric field is applied to the optical waveguides 12 and 13 and the control electrode 15,
It can be concentrated on the optical buffer layer 1 having the resistance value R ₁ sandwiched between the 16 layers. FIG. 4 shows the state of the electric field when the power source 20 is applied between the control electrodes 15 and 16.
In this way, the external electric field can be more effectively applied to the optical waveguides 12 and 13. As a result, the operating voltage is expected to drop.

The optical buffer layer 2 formed at least between the control electrodes 15 and 16 is divided perpendicularly to the crystal substrate 11, and the resistance values of adjacent portions are different (R ₁
≠ R ₂ ), the film interface serves as a barrier to restrict the movement of movable carriers scattered in each layer. Therefore,
The carrier movement that hinders the external electric field from the control electrodes 15 and 16 to the optical waveguides 12 and 13 is reduced, so that it is possible to suppress the temporal change of the output light intensity, to ensure the stable operation of the element, and to secure the reliability in the life and the like.

Further, in particular, by making the resistance value R ₁ of the optical buffer layer 1 sandwiched between the optical waveguides 12 and 13 and the control electrodes 15 and 16 smaller than the resistance value R ₂ of the adjacent film,
The spread of the external electric field is suppressed. Furthermore, the optical waveguide 1
It is possible to apply an electric field to 2 and 13 more effectively,
It is also possible to reduce the operating voltage.

The above embodiment is an example of a preferred embodiment of the present invention, but the present invention is not limited thereto, and various modifications can be made without departing from the gist of the present invention.

[0025]

As is apparent from the above description, in the waveguide type optical control element of the present invention, the optical waveguide, the optical buffer layer and the control electrode are formed on the ferroelectric crystal substrate having the electro-optical effect. However, this optical buffer layer is composed of at least two types having different resistance values and is perpendicular to the ferroelectric crystal substrate between the control electrodes. Therefore, the DC drift caused by the movable carriers scattered in the optical buffer layer can be suppressed, and the stable operation of the element and the reliability in the life etc. can be ensured. Further, the operating voltage can be reduced by selectively setting the resistance value of the optical buffer layer.

[Brief description of drawings]

FIG. 1 is a sectional view showing a configuration example of a first embodiment of a waveguide type light control element of the present invention.

FIG. 2 is a second embodiment and includes two optical buffer layers.
It is sectional drawing which shows the structural example of the waveguide type | mold optical control element manufactured by the film-forming of one time.

FIG. 3 is a manufacturing process diagram of the optical buffer layer shown in FIG.

4A and 4B are views for explaining the operation of the present invention, using the waveguide control element of FIG. 2 as an example.

FIG. 5 is a perspective view showing a configuration of a waveguide type optical control element of the first and second embodiments.

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

[Explanation of symbols]

 1, 2, 3, 4, 5, 6 Optical buffer layer 9 Photoresist 11, 51 Lithium niobate crystal substrate 12, 13, 52, 53 Optical waveguide 15, 16, 55, 56 Control electrode 54 Directional coupler 57 Incident light 58, 59 Emitted light

Claims (5)

[Claims] 1. A waveguide type optical control element comprising an optical waveguide, an optical buffer layer and a control electrode formed on a ferroelectric crystal substrate having an electro-optical effect, wherein the optical buffer layer is a resistor. At least 2 with different values
2. A waveguide type optical control element, characterized in that it is formed between the control electrodes perpendicularly to the ferroelectric crystal substrate. Wherein said optical waveguide and the resistance value R ₂ of the optical buffer layer adjacent to the buffer layer of the resistance value R ₁ and the resistance value R ₁ of the optical buffer layer sandwiched between the said control electrode, R
_{6. The} waveguide type optical control element according to claim 1, wherein ₆ and R have a relationship of R ₁ <R ₂ and R ₆ . 3. The waveguide type optical control element according to claim 1, wherein the dielectric crystal substrate is a lithium niobate crystal substrate. 4. The waveguide type optical control element according to claim 1, wherein the optical waveguide constitutes a directional coupler. 5. The waveguide type optical control element according to claim 1, wherein the optical waveguide constitutes a Mach-Zehnder interferometer.