JPH07318874A - Mach-zehunder type optical modulator having improved temperature drift - Google Patents

Abstract

PURPOSE:To provide an optical modulator which is low in voltage drift according to a temp. rise, is high in reliability and is useful for a high-speed optical communication net, etc. CONSTITUTION:This optical modulator has an electro-optical substrate, a progressive wave type electrode system which is arranged on the substrate and consists of a strip electrode 3 and a pair of ground electrodes 4a, 4b and an optical waveguide system which is formed on the substrate surface and has optical waveguide parts 1, 2 extending by passing respectively the undersides of the strip electrode 3 and the ground electrode 4a. The width of the optical waveguide part 1 under the strip electrode 3 is wider than the width of the optical waveguide part 2 under the ground electrode 4a.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Mach-Zehnder type optical modulator useful for high-speed optical communication networks and the like.
More specifically, the present invention relates to a Mach-Zehnder type optical modulator having a significantly reduced temperature drift and higher reliability.

[0002]

2. Description of the Related Art A Mach-Zehnder type optical modulator manufactured by using an electro-optic crystal typified by LiNbO ₃ as a substrate has advantages such as a high band and a low operating voltage, and thus has been attracting attention as a high-speed optical communication system component. There is. However, since an optical modulator used in a high-speed optical communication system requires extremely high reliability and durability, the Mach-Zehnder type optical modulator reduces instability factors such as dc drift and temperature drift. There is a need.

To achieve the purpose of reducing temperature drift, a modulator using x-cut LiNbO ₃ as a substrate is suitable, but this modulator is unsuitable for high-speed applications. A z-cut LiNbO ₃ modulator is most suitable as a modulator intended for high-speed applications, but this has a problem that the operating point voltage greatly changes with temperature change (temperature drift). .

As shown in FIG. 1, conventional LiN
In the bO ₃ high-speed optical modulator, the drift (V) of the DC control voltage applied to the electrodes for adjusting the operating point increases remarkably as the temperature rises.

The cause of the temperature drift of the LiNbO ₃ modulator, which is a problem when the optical modulator is actually operated, is (1) a large pyroelectric effect of the z-cut LiNbO ₃ crystal, and (2) the inside of the device. It is known that the refractive index of the waveguide changes due to thermal stress, and (3) the half-wave voltage of the modulator changes with temperature.

In a general operating temperature environment, the influences of the factors (2) and (3) are small and can be ignored. As a measure against the pyroelectric effect of factor (1), the surface of the optical element is covered with a semiconductor thin film to shield the electric charges generated by the pyroelectric effect, thereby preventing the generation of a local unstable electric field near the electrodes. Are known. By this method, the temperature drift of the z-cut LiNbO ₃ modulator can be significantly reduced. Further, by making the electrode structure bilaterally symmetrical with respect to the symmetry axes of the left and right branch waveguides of the Mach-Zehnder type optical waveguide, it is possible to further reduce the temperature drift (dual electrode structure). However, in general, when it is intended to increase the speed of the modulator, the traveling wave electrode system includes a strip electrode (hot electrode) located on the waveguide on one side,
It is composed of a pair of ground electrodes that surround both sides of it and cover the other waveguide, and the electrode arrangement is asymmetrical. In the modulator having such an asymmetrical electrode, even if the element surface is coated with the semiconductor thin film as described above, a voltage drift of several tens of mV / ° C. is recognized as the temperature rises, as shown in FIG. .

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a highly reliable Mach-Zehnder type optical modulator which has no or little DC control voltage drift due to temperature rise. The present invention is particularly effective for a Mach-Zehnder type optical modulator in which a Mach-Zehnder type optical waveguide is formed on a LiNbO ₃ substrate and an asymmetrical traveling-wave type electrode system is arranged thereon.

[0008]

SUMMARY OF THE INVENTION The Mach-Zehnder interferometer with improved temperature drift of the present invention comprises a substrate formed of a material having an electro-optic effect, and a strip disposed on the substrate. A traveling wave electrode system including an electrode and a pair of ground electrodes arranged on both sides thereof, and below the strip electrode and below one of the pair of ground electrodes formed on the substrate surface portion. An optical waveguide system extending through the strip electrode, wherein the width of the optical waveguide portion located below the strip electrode is wider than the width of the optical waveguide portion located below the ground electrode. Is.

In the optical modulator of the present invention, the substrate forming material is z-cut lithium niobate, and the optical waveguide is an asymmetric Mach-Zehnder type optical waveguide formed by diffusing titanium in the substrate, It is preferable that the traveling wave electrode system is arranged on the optical waveguide via at least one dielectric layer.

In the optical modulator of the present invention, the width of the optical waveguide portion located below the hot electrode is about 7 μm,
The width of the optical waveguide portion located under the ground electrode is preferably about 6 μm. Such a difference in optical waveguide width is about 1 μm. In addition, a width of about 7 μm is necessary so that the waveguide does not have multiple modes. Therefore,
A suitable combination of the widths of both waveguide portions is about 7 μm and about 6 μm, and is 7 μm / 6 μm, 7.5 μm / 6.5 μm.
There are combinations such as m.

[0011]

In the conventional Mach-Zehnder interferometer type optical modulator, a traveling wave type electrode system having left-right asymmetrical electrodes is arranged with respect to the symmetrical Mach-Zehnder type optical waveguide system. Therefore, due to the effect of the semiconductor layer, the electric field due to the pyroelectric effect evenly dispersed on the modulator surface,
The magnitude of the interaction with the optical waveguides is effectively different from each other in the left and right optical waveguides, which causes temperature drift.

In the present invention, a substrate made of a material having an electro-optical effect, for example, a LiNbO ₃ single crystal, one strip (hot) electrode arranged on this substrate, and both sides thereof are arranged. A Mach-Zehnder having a traveling wave type electrode system including a pair of ground electrodes and an optical waveguide system formed on a surface portion of the substrate and extending under one of the strip electrode and the pair of ground electrodes. Type optical modulator, the width of the optical waveguide portion located below the strip electrode is made wider than the width of the optical waveguide portion located below the ground electrode, thereby causing the temperature drift problem. Is the solution.

A Mach-Zehnder type optical waveguide system in which an optical waveguide portion located below a strip electrode and a ground electrode is asymmetrical is disclosed in US Pat. No. 4,709,9.
Issue 78 (December 1, 1987) and Janet L. Jacke
l and John J. Johnson, IEEE / OSA, J. of Lightwave
Technology, Volume 8, pp. 1348-1351, 1988, which aims to weaken the coupling between waveguides, and the technical problem of reducing temperature drift, and There is no mention of any solution. That is, there is no description about increasing the width of the waveguide located under the strip (hot) electrode.

In the optical modulator of the present invention, the optical waveguide system formed on the surface portion of the substrate (not shown but corresponding to the paper surface) as shown in FIG. 2 has two branches / converges. The traveling wave type electrode system having the optical waveguide portions 1 and 2 is a strip (hot) electrode 3 and a pair of ground electrodes 4.
a, 4b. The optical waveguide portion 1 extends below the strip electrode 3, and the other optical waveguide portion 2 extends below the ground electrode 4a. At this time, the width W ₁ of the optical waveguide portion ₁ is formed wider than the width W ₂ of the optical waveguide portion 2.

As shown in FIG. 3, a buffer layer (made of, for example, SiO ₂ ) 6 is formed on a substrate 5 (for example, a LiNbO ₃ single crystal), and a semiconductor layer (for example, Si) is formed thereon.
7) on which the strip electrode 3 and the pair of ground electrodes 4a and 4b are provided.
The electrodes 3, 4a, 4b have a thickness of, for example, about 10 μm and are made of Au, for example.

When the temperature rises on the surface of the optical modulator (for example, the -z plane of the LiNbO ₃ substrate), a positive charge is generated due to the pyroelectric effect. When the semiconductor thin film layer is arranged under the electrode, this positive charge is almost uniformly dispersed due to its effect. However, since the material forming the modulator is a dielectric, this positive charge disappears immediately. Retained without.
As a result, the amount of positive charges accumulated increases as the temperature of the optical modulator rises.

When such positive charges are accumulated, as shown in FIG.
Strip electrode and ground electrode as shown
In addition to the electric field applied between the
Occur. Therefore, the asymmetric Mach-Zehnder waveguide
Change of light wave passing through branch waveguides 1 and 2 in optical system
φ₁, Φ₂Is expressed by the following equations (1) and (2)
It φ₁= (Β₁+ K₀Δn₁) L (1) φ₂= (Β₂+ K₀Δn₂) L (2) where β₁And β₂Are strip electrodes 3 respectively
And an optical waveguide portion located below the ground electrode 4a
1 and 2 represent the light propagation constants, and L is the length of the electrode (see Fig. 2).
)), And Δn₁And Δn₂Is the following formula
It is defined by (3) and (4). Δn₁= Δn_h+ Δn_p(T) (3) Δn₂= Δn_g-Δn_p(T) (4) Δn_hAnd Δn_gAre strip electrodes 3 and
And the optical waveguide portions 1 and 2 located below the ground electrode 4a
Of the refractive index change due to the applied electric field
_p(T) is generated by the pyroelectric effect depending on the temperature T
It is the amount of change in the refractive index due to the electric field. Electric field due to pyroelectric effect
The strip (hot) electrode and ground
They are identical to each other underneath, but on the ground electrode side,
The direction of this electric field weakens the electric field due to the applied voltage for control.
Direction (see FIG. 3), Δn in equation (4)_pSign of
Becomes negative.

The electric field E of the light wave output from the Mach-Zehnder waveguide system is the electrode length L and the electric field E of the input light is E _0.
The following formula (5):

[Equation 1] Is represented by

Therefore, the power P of the output light is expressed by the following equation (6):

[Equation 2] Is represented by R (T) and s in equation (6)
(T) is the following formulas (7) and (8), respectively.

[Equation 3] Is defined by

The refractive index changes Δn _h and Δn _g of the optical waveguide portions 1 and 2 below the strip electrode 3 and the ground electrode 4a due to the electro-optical effect also depend on the temperature, but the magnitude thereof is the pyroelectric effect. It is extremely small compared to the change in refractive index due to. Therefore, the phase of the output light expressed by the equation (8) is hardly affected by the temperature. On the other hand, from the equation (7), it can be seen that the intensity of the output light changes in a cosine cycle with temperature change. Furthermore, by adjusting the propagation constants β ₁ and β _{2 of} the waveguide and the refractive indices Δn ₁ and Δn ₂ from the equation (7), the temperature T, that is, the refractive index Δn _p (T)
It can be seen that it is possible to increase or decrease the change of the r (T) value with respect to the change of.
In other words, by bringing the initial position near the minimum or maximum value of the cosine curve that is a function of the temperature T,
The temperature drift can be reduced. In the present invention, the width W ₁ of the optical waveguide portion ₁ is the width W ₁ of the optical waveguide portion 2.
_Wider than ₂ , preferably W ₁ ≈ 7 μm, W ₂ ≈ 6 μ
m. Increasing the waveguide width has the effect of strengthening the confinement of guided light (see Table 1). Therefore, it is also possible to apply a method of producing the same effect, for example, a method of changing the diffusion concentration of Ti into LiNbO ₃ (making the strip electrode side high concentration).

[0021]

The present invention will be further described by the following examples. Examples and Comparative Examples A z-cut LiNbO ₃ substrate was coated with a Ti thermal diffusion method,
Two types of asymmetric Mach-Zehnder type waveguides were formed.
Type A is a structure according to the present invention, the waveguide width under the hot electrode is 7 μm, and the waveguide width under the ground electrode is 6 μm.
Met. In the case of Type B, conversely, the waveguide width under the hot electrode is 6 μm and the waveguide width under the ground electrode is 7 μm.
Met. As will be shown later, in the structure of type B, the temperature drift did not become small in both waveguides as in the conventional symmetrical waveguide type with a width of 7 μm. An SiO ₂ buffer layer is provided between the waveguide surface and the asymmetrical traveling wave type electrode,
A semiconductor layer was provided. Parameters such as the electrode size and the mode field diameter (W _x , W _z ) of light propagating in the waveguide are shown in FIG. 3 and Table 1.

[0022]

[Table 1]

The measurement results of the temperature drift of the modulator samples A and B are shown in FIGS. The temperature drift was measured using a modulator sample with an optical wavelength of 1.55 μm and an AC applied voltage of 1
The operation was performed under the conditions of kHz · V _pp = 20 V, and the change in the peak position (unit V) of the output light intensity due to the temperature change was tracked. From FIGS. 4 and 5, it can be seen that the temperature drift in the modulator A according to the present invention is significantly smaller than that in the conventional type (FIG. 1) and the comparative modulator B.

The reason why the temperature drift of the modulator A according to the present invention is small can be explained as follows. The difference in the propagation constant (β ₁ −β ₂ ) in the first term of Expression (7) is positive in the type A waveguide and negative in the type B and conventional symmetric waveguides. The difference in propagation constant between the titanium diffusion waveguide having a width of 6 μm and the titanium diffusion waveguide having a width of 7 μm is on the order of 10 ⁻⁴ μm ⁻¹ (see FIG. 6), and | β ₁ −β ₂ | L / 2 to 1. 0
Becomes On the other hand, the term k ₀ (L /
2) (Δn _h −Δn _g ) is always positive in both types A and B.

From the parameters shown in Table 1, the magnitude of the applied electric field reduction coefficient (overlap integrals) Γ _h −Γ _g is
5.71 × 10 ⁴ m ^-1 (A), 5.48 × 10 ⁴ m
^-1 (B) can be calculated numerically, and the difference between types A and B does not significantly affect the result of equation (7). Therefore, due to the difference between the positive and negative of these two terms, especially (β ₁ −β ₂ ),
In the modulator A, the initial position (the position at the temperature of 0 ° C. according to FIG. 4) is adjusted to the region from the maximum of the cosine curve (equation (7)) where temperature is a variable to the downward direction. On the other hand, the modulator B is adjusted to a position having a positive steep slope of the cosine curve. That is, the temperature drift in the modulator A is small and the tilt direction is negative, and the temperature tilt in the modulator B is positive and a large temperature drift is shown.

[0026]

According to the present invention, it is possible to provide a Mach-Zehnder interferometer type optical modulator having a remarkably small temperature drift.

[Brief description of drawings]

FIG. 1 is a graph showing a temperature-drift relationship of a conventional optical modulator.

FIG. 2 is an explanatory plan view showing an arrangement of an optical waveguide system and an electrode system of an example of the optical modulator of the present invention.

FIG. 3 is a front cross-sectional explanatory view showing the configuration of an example of the optical modulator of the present invention.

FIG. 4 is a graph showing a temperature-drift relationship of an example of the optical modulator of the present invention.

FIG. 5 is a graph showing a temperature-drift relationship of a comparative optical modulator.

FIG. 6 is a graph showing the relationship between the width of an optical waveguide and the standard propagation coefficient of a light wave.

[Explanation of symbols]

 1, 2 ... Optical waveguide part 3 ... Strip electrodes 4a, 4b ... Ground electrode 5 ... Substrate 6 ... Buffer layer 7 ... Semiconductor layer 8 ... Charging by pyroelectric effect

Claims (3)

[Claims] 1. A traveling wave type electrode system including a substrate formed of a material having an electro-optical effect, a strip electrode disposed on the substrate, and a pair of ground electrodes disposed on both sides of the strip electrode. And an optical waveguide system formed on the surface of the substrate and extending under the strip electrode and under one of the pair of ground electrodes, the optical waveguide being located under the strip electrode. A Mach-Zehnder interferometer type optical modulator having improved temperature drift, characterized in that the width of the portion is wider than the width of the optical waveguide portion located below the ground electrode. 2. The substrate forming material is z-cut lithium niobate, the optical waveguide is an asymmetric Mach-Zehnder optical waveguide formed by diffusing titanium in the substrate, and the traveling wave electrode system is used. The optical modulator according to claim 1, wherein is disposed on the optical waveguide via at least one dielectric layer. 3. The light according to claim 1, wherein the width of the optical waveguide portion located below the strip electrode is about 7 μm, and the width of the optical waveguide portion located below the ground electrode is about 6 μm. Modulator.