JPH09236782A - Optical waveguide device - Google Patents

Abstract

(57) An object of the present invention is to provide an optical waveguide device which can suppress temperature drift as much as possible by an easy process and can be driven by direct current. SOLUTION: A pyroelectric substrate 1 having an optical waveguide 2 formed on one main surface, a buffer layer 3 formed on one main surface of the pyroelectric substrate 1, and an optical waveguide on one region of the buffer layer 3. 2 and the semiconductor layer 5 formed on the other region of the buffer layer 3 and separated from the side end surface of the electrode layer 9 by a predetermined distance.

Description

Detailed Description of the Invention

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical waveguide device such as an optical modulator and an optical switch used in the optical communication field.

[0002]

2. Description of the Related Art With the practical use of next-generation large-capacity optical communication, optical control devices such as high-speed optical modulators and optical switches are required. In particular, lithium niobate (L
A waveguide-type optical control device (hereinafter, referred to as an optical waveguide device) using the electro-optic effect of a ferroelectric substrate made of iNbO ₃ ) or the like is very promising because of its low insertion loss and high-speed operation. I have.

Conventionally, in an optical waveguide device using a ferroelectric substrate of this type, Z-cut has been mainly employed as a substrate orientation in which an electric field acts most effectively on an optical waveguide formed on the surface of the substrate.

However, since the ferroelectric material has pyroelectricity, static electricity is generated on the c-plane, which is the direction of polarization, that is, the (001) plane of the substrate due to a temperature change, and the characteristics change over time. there were.

For the mechanism, see IEICE Technical Report, OQ.
E86-44 p115-121 See Sawaki et al. In detail.
That is, in a portion where a plurality of electrodes for applying an electric field to the optical waveguide cover the substrate surface, charges corresponding to the polarization of the substrate are induced, whereas in a portion where the electrodes do not cover the substrate surface, The charge corresponding to the polarization is not easily supplied to the substrate surface. For this reason, the polarization of the substrate changes due to the temperature change, so that non-uniformity of electric charge occurs in a portion where the electrode does not cover the surface of the substrate, and an electric field due to such non-uniform electric charge is applied to the optical waveguide, which causes temperature drift. It will happen.

Therefore, the following measures have been proposed as measures to solve this problem. FIG. 6 is a cross-sectional view of the optical waveguide device J1 taken in a direction orthogonal to the incident direction of the optical waveguide 22 formed on the surface of the pyroelectric substrate 21. Layer 2
3, a semi-conductive film 24 and an electrode 25 are sequentially laminated. Here, the semi-conductive film 2 having a certain degree of conductivity such as ITO in a portion where the electrode 25 does not cover the surface of the substrate 21 and having a resistivity such that an electric field can be applied between the electrodes.
The surface charge of the substrate 21 caused by the temperature change is made uniform by covering with No. 4 and connecting to the electrode 25 (see, for example, Japanese Patent Publication No. 5-78016).

However, although such a semiconductive film has a resistance value that substantially prevents conduction, since the electrodes to which a voltage is applied are connected by a conductor, the voltage is effective when a DC to low frequency voltage is applied. There was a problem that it was not applied to.

On the other hand, it has been proposed to divide such a semiconductive film between the signal and GND electrodes to enable direct current driving (for example, Japanese Patent Laid-Open No. 3-2.
However, there is a problem that the electric field strength distribution changes and the characteristics change in the region where the electric field acts depending on the frequency of the applied voltage signal. There is also a manufacturing problem that the manufacturing process itself for easily forming the pattern of the semiconductive film has not been established.

Therefore, the present invention provides an optical waveguide device which overcomes the above-mentioned problems, can suppress the temperature drift as much as possible by an easy process, and can be driven by direct current.

[0010]

An optical waveguide device for solving the above problems is a pyroelectric substrate having an optical waveguide formed on one principal surface, and a buffer layer formed on the one principal surface of the pyroelectric substrate. And an electrode layer formed along the optical waveguide on one region of the buffer layer, and a semiconductor layer formed on the other region of the buffer layer at a predetermined distance from the side end face of the electrode layer.

When the width of the semiconductor layer is A and the distance between the semiconductor layer and the side end surface of the electrode layer is B, the following formula is satisfied.

A / 9 <B <3A / 7, that is, 0.1 L <B <0.3 L (however,
For L = A + B) and the distance B, 10% to 30% of L, which is the sum of the width A and the distance B of the semiconductor, is a suitable numerical range.

Further, if the electrode layers are plural layers, the manufacture of the optical waveguide device can be made very easy. For example, a step of forming a buffer layer on one main surface of the pyroelectric substrate on which the optical waveguide is formed, and a step of forming a base layer and a semiconductor layer side by side along the optical waveguide on one region of the buffer layer. And a step of laminating an electrode layer serving as a control electrode on the underlayer, the production can be carried out quickly and easily.

In the above-mentioned optical waveguide device, a gap is provided along the side edge of the electrode layer where the electrode layer does not cover the substrate surface in the region where the electric field is applied to the optical waveguide, so that high frequency transmission is not hindered. It is preferable to form a so-called semiconductor layer having electrical conductivity, and to connect the semiconductor layer to the signal electrode with high resistance in a region sufficiently distant from a region in which an electric field is applied.

Further, according to the above-mentioned method for manufacturing an optical waveguide device, the patterns of the semiconductor layer and the underlying layer are simultaneously formed by the lift-off method, and then the electrode layer is laminated only on the underlying layer, which is very simple and quick. It is possible to realize various manufacturing.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION An optical modulator, which is one of optical waveguide devices, will be described as an embodiment according to the present invention with reference to the drawings. First, the schematic configuration of the optical modulator will be described.
As shown in FIGS. 1 (a) to 1 (c), the optical modulator S1 is a focal point made of, for example, lithium niobate, lithium tantalate, or lithium tetraborate having an optical waveguide 2 formed on one main surface. An electrically conductive substrate (hereinafter referred to as “substrate”) 1, a buffer layer 5 formed on one main surface of the substrate 1 as a dielectric made of SiO ₂ , Al ₂ O _3, etc., and a region of the buffer layer 5 A ground electrode 1 formed by laminating an electrode layer described below on a base layer described below formed along the optical waveguide 2.
0a, the signal electrode 10b, the ground (hereinafter, GND) electrode 10c, and Si and S formed on the other region of the buffer layer 5.
The semiconductor layer 5 is made of iO _x (silicon-rich silicon oxide) or the like.

Further, as shown in FIG. 1B, the semiconductor layer 5 is connected to the signal electrodes 10b at both ends of the semiconductor layer 5, whereby electric charges are supplied to the semiconductor layer 5, and the semiconductor layer 5 is further improved. Temperature drift can be suppressed.

The semiconductor layer 5 is formed along the optical waveguide 2,
For example, it is formed in a strip shape at a distance B from the respective side end faces of the GND electrode 10a, the signal electrode 10b, and the GND electrode 10c. Here, the distance B is 0.1 L <B
<0.3 L (where L = width A of semiconductor layer + distance B) is within the numerical range.

With this structure, the charges generated on the surface of the substrate 1 due to the pyroelectricity can be made uniform to the extent that there is practically no problem, and the temperature drift can be suppressed as much as possible. Further, the signal electrode layer and the GND electrode layer have an electrically sufficient insulating property, and low frequency and direct current driving are also possible. Furthermore, since the resistance between the semiconductor layer 5 and each electrode layer is high, the electric field intensity distribution acting on the optical waveguide 2 does not change depending on the frequency. Furthermore, the semiconductor layer 5
The resistivity of can be controlled over a wide range to the extent that high-frequency propagation is not hindered, and quality control becomes extremely easy in manufacturing.

Next, a specific and typical manufacturing method of the optical modulator S1 will be described. As shown in FIG. 2A, a double-sided mirror-polished optical-grade lithium niobate single crystal (Z-cut;
A Ti thin film pattern was formed on the (001) plane substrate 1 by the lift-off method, and then the optical waveguide 2 was formed by heat diffusion from the Ti thin film pattern to the substrate 1 at about 1050 ° C.

Then, a buffer layer 3 of a SiO ₂ thin film was deposited on the substrate 1 to a thickness of about 1 μm by sputtering, and a heat treatment was carried out at about 600 ° C. for the purpose of improving the resistivity. After that, the gap of the semiconductor layer and CPW to be described later
Photolithography for lift-off of a (coplanar wave guide) type electrode was performed at once using the same photomask. In the figure, 4 is a photoresist.

Next, as shown in FIGS. 2B and 2C,
As the semiconductor layer 5, silicon (Si) having a thickness of 0.1 μm,
Chromium (Cr) as the first underlayer 6 has a thickness of 0.02 μm.
m and gold (Au) as the second base electrode 7 having a thickness of 0.3 are sequentially deposited and then lifted off to form the semiconductor layer 5
The gap pattern and the base electrode pattern of the CPW type electrode were simultaneously formed.

Here, in the lift-off photolithography, in order to improve the disconnection of the pattern edge portion,
Normally, it is necessary to make the cross-sectional shape of the photoresist pattern into an inverse taper shape at the pattern edge portion. However, since the semiconductor layer 5 and the underlayer are formed at once as described above, the substrate 1 is exposed at the time of photolithography. The back surface of the photoresist can be exposed without being blocked by a light-shielding material such as a thin film as in the conventional case, and the cross-sectional shape of the photoresist pattern can be formed into a good inverse taper shape. It was possible to form a good thin film pattern containing no burr and the like at the pattern edge portion.

Then, as shown in FIGS. 2D to 2G, the electrode portion is covered with a photoresist 8 by photolithography except for the electrode portion, and a gold electrode layer 9 is laminated on the underlayer and plated. An unnecessary portion of the ground layer was etched to expose the semiconductor layer 5 between the electrode layers 9 and 9, and finally a Mach-Zehnder interferometer type optical (intensity) modulator was manufactured.

The underlayer used for plating connects the signal electrode and the GND electrode on the dicing line, and the electrodes were separated during dicing.

The semiconductor layer 5 was not annealed and used with a resistivity of the order of 10 ⁴ Ω · cm. The semiconductor layer 5 was connected to the GND electrode with a line having a width of 100 μm and a length of 20 μm, and a resistance of the order of 10 ⁷ Ω was provided between the semiconductor layer 5 and the GND electrode. The gap (B × 100 / L) [%] between the electrode layer and the semiconductor layer 5 and the temperature drift ΔD [%] (ΔD is ΔD = Δ as shown in FIG.
It is defined by V × 100 / Vπ. Here, ΔV is a drift voltage, and Vπ is a half-wave voltage. 4) is as shown in FIG. 4, and it was found that the temperature drift was within approximately ± 5% when the gap was 10 to 30%.
Further, it was found that the temperature drift becomes almost zero when the gap is 12 to 25%.

The reason for this is that the electric field due to pyroelectricity that causes the temperature drift is caused by the potential difference generated between the signal and GND electrodes, and that it remains in the vicinity of the center of the gap of the semiconductor layer 5 without being neutralized. It is considered that the two are generated by electric charges, and that the electric field intensity distributions produced by each act on the optical waveguide so that the drift directions are opposite, and the balance changes depending on the width of the gap.

Therefore, the imbalance of the electric charges caused by the rapid temperature change is as shown in FIGS. 5 (a) and 5 (b) when the semiconductor layer 5 is loaded on the entire surface (the cross section is, for example, FIG. 6). However, it takes some time to move in the high-resistance semiconductor layer 5, and the potential difference due to pyroelectricity between the signal and GND electrodes is not immediately eliminated. However, according to the present invention, the potential difference between the signal and GND electrodes is high. It is considered that the potential difference due to the pyroelectricity can be canceled out rapidly by the electric field generated by the electric charges remaining without being neutralized in the vicinity of the center of the gap, and the temperature drift characteristic is remarkably improved.

Further, compared with the conventional optical modulator as shown in FIG. 6, the optical modulator exhibits extremely good temperature characteristics, and the applied voltage-optical output characteristics of the optical modulator measured in a state close to DC application are also very good. It was confirmed that excellent static characteristics were obtained and that DC drive was possible. Furthermore, the half-wave voltage V of the optical modulator
π showed stable operation with no change due to frequency.

In addition to the present embodiment, the temperature drift can be suppressed to a minimum by adjusting the width of the gap, and the temperature drift is included in the scope of the present invention without departing from the spirit of the invention. Further, although the optical modulator has been described as the optical waveguide device in the present embodiment, the present invention is not limited to this, and various optical waveguides in which optical waveguides and electrodes are formed on various pyroelectric substrates such as optical switches. It can be applied to a device, and can be appropriately modified and implemented without departing from the scope of the present invention.

[0031]

As is apparent from the above description, according to the optical waveguide device of the present invention, the temperature drift can be reduced to almost zero even with a sudden temperature change with the extremely simple structure and the easy manufacturing process. It is possible to provide an optical waveguide device that can be suppressed and can perform stable operation with no characteristic fluctuation in the entire band from direct current to the frequency at which the device operates, and that has excellent reliability.

[Brief description of drawings]

1A is a plan view showing an example of an optical waveguide device according to the present invention, FIG. 1B is an enlarged view of a portion B in FIG. 1A, and FIG. 1C is a cc ′ line of FIG. Sectional view.

2A to 2G are cross-sectional views showing the steps of manufacturing an optical waveguide device according to the present invention.

FIG. 3 is a diagram illustrating temperature drift of an optical modulator.

FIG. 4 is a graph illustrating a relationship between an electrode layer-semiconductor interlayer gap and temperature drift.

FIG. 5A is a graph showing how the temperature changes; FIG.
Is a graph illustrating the relationship between temperature change and drift characteristics.

FIG. 6 is a sectional view showing an example of a conventional optical waveguide device.

[Explanation of symbols]

 1 ... Substrate 2 ... Optical waveguide 3 ... Buffer layer 4,8 ... Photoresist 5 ... Semiconductor layer 6 ... First underlayer 7 ... Second underlayer 9 ... Electrode layers 10a, 10c ... GND electrode 10b ... Signal electrode S1 ... Optical modulator

Claims (3)

[Claims] 1. A pyroelectric substrate having an optical waveguide formed on one main surface, a buffer layer formed on one main surface of the pyroelectric substrate, and the optical waveguide provided on one region of the buffer layer. An optical waveguide device comprising an electrode layer formed along with the semiconductor layer, and a semiconductor layer formed on the other region of the buffer layer and separated from the side end surface of the electrode layer by a predetermined distance. 2. The optical waveguide device according to claim 1, wherein when the width of the semiconductor layer is A and the distance between the semiconductor layer and the side end surface of the electrode layer is B, the following formula is satisfied. An optical waveguide device characterized by: A / 9 <B <3A / 7 3. The optical waveguide device according to claim 1, wherein the electrode layer is composed of a plurality of layers.