JPH06250131A - Optical control element - Google Patents

Abstract

PURPOSE:To provide the optical control element having a desired operating point by providing the rear surface side of a substrate with a stress adjusting film applying a desired stress on the substrate. CONSTITUTION:A pair of optical waveguides 2 and 3 of several to several tens mum width and several to several tens mm length in proximity to each other at several tens mum are installed on a Z-cut LiNbO3 substrate 1 having an electrooptical effect. Input and output waveguides 6 and 7 are connected respectively via Y-branch optical waveguide 4 and 5 at both ends thereof. A pair of electrodes 11, 12 are formed via a buffer layer (SiO2 film) 10 provided in order to prevent light absorption on the optical waveguides 2, 3. The two- phase modulation type optical control elements are constituted of these optical waveguides 2, 3 and the electrodes 11, 12. The rear surface of the LiNbO3 substrate 1 is provided with the stress adjusting film 13 consisting of SiO2 and the adjustment of the value of the stress acting on the substrate is executed by adjusting the thickness and arrangement of the stress adjusting film and the depth, width, etc., of the stress adjusting grooves and, therefore, the operating point of the optical control element can be arbitrarily adjusted in compliance with driving conditions.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical control element for modulating a light wave, switching an optical path, etc., and more particularly to a waveguide type optical control element for controlling using an optical waveguide provided in a substrate.

[0002]

2. Description of the Related Art As an optical communication system is put into practical use, a system having a larger capacity and a higher function is required. It is necessary to add new functions such as generation of higher-speed optical signals, switching of optical transmission lines, and exchange. In current practical systems, optical signals are obtained by directly modulating the injection current of semiconductor lasers and light emitting diodes, but in the future, optical control elements such as high-speed optical modulators and optical switches will be required. ing. As a high-speed optical control element, an optical control element utilizing an electro-optic effect is typical, and there are reports on a directional coupler type optical modulator or switch, a total reflection type optical switch, a Mach-Zehnder type optical modulator or switch. ing.

For example, as a modulator or switching element for ultra-high speed signals of 10 Gb / s or more, there is an element that uses an optical waveguide formed by diffusing Ti in lithium niobate: LiNbO ₃ crystal, which is called microwave. A value of about 20 GHz is obtained as a modulation band by using a traveling waveform electrode that is velocity-matched with a light wave.

FIG. 4 shows an example of a conventional light control element.
FIG. 1 shows a plan view of a Mach-Zehnder type optical intensity modulator formed by diffusing Ti in a LiNbO ₃ crystal and a sectional view taken along the line CC.

As shown in FIGS. 4A and 4B, on a z-cut LiNbO ₃ substrate 101 having an electro-optical effect, a width of several tens of μm and a width of several tens of μm which are close to each other at intervals of several tens μm.
m, a pair of optical waveguides 102 and 1 having a length of several to several tens of mm
03 is installed, and input / output optical waveguides 106 and 107 are connected to both ends thereof via Y-branch optical waveguides 104 and 105, respectively, and the ends of the input / output waveguides 106 and 107 are respectively an incident end 108 and an emission end. Edge 10
It is 9. Then, in this example, the optical waveguide 102
And 103, a pair of electrodes 111 and 112 are formed via a buffer layer (SiO ₂ film) 110 provided to prevent light absorption, and these optical waveguides 102 and 1 are formed.
03 and the electrodes 111 and 112 form two phase modulation type light control elements. Y becomes a 3 dB branch
The branch optical waveguide 104 is an input light Y-branch optical waveguide, the opening angle of which is several mrad, and the interval between the two phase modulator optical waveguides 102 and 103 is several tens of μm. Similarly to the Y-branch optical waveguide 104, the Y-branch optical waveguide 105 serving as the confluent portion is also a Y-branch optical waveguide having an opening angle of mrad. The entrance end 108 and the exit end 109 are respectively connected to the substrate 101.
Are formed on the end faces facing each other, and the width of each optical waveguide pattern is 5 to 10 μm.

Further, in the structure shown in FIG. 4, the device is constructed so that the electro-optic constant is maximized in order to reduce the half-wave voltage Vπ. That is, since the polarization of the light propagating through the optical waveguide is in the z direction and the direction of the applied electric field is also in the z direction, the electrodes 111 and 112 are connected to the optical waveguides 102 and 103, respectively, as shown in FIG. 4B. It is located directly above. Further, in order to prevent light absorption by the electrodes 111 and 112, SiO ₂ is provided between the electrodes 111 and 112 and the optical waveguides 102 and 103.
A buffer layer 110 made of a film is formed.

Next, the operation of the optical modulator shown in FIG. 4 will be described. Incident light 113 incident on the incident end 108 passes through the input optical waveguide 106, is guided to the Y branch optical waveguide 104, and is branched into the optical waveguides 102 and 103. here,
By design, the optical waveguides 102 and 103 are waveguides having exactly the same width and length. Therefore, when drive power is not supplied to the optical modulator, the branched light is combined in the Y-branch optical waveguide 105 which is a combining unit. The light is emitted from the outgoing light 109 as outgoing light 114 through the output optical waveguide 107. At this time, the light is turned on. On the other hand, when drive power is supplied to this optical modulator, an electric field is applied between the electrodes 111 and 112 in this optical modulator. Since the LiNbO ₃ substrate 101 has an electro-optic effect, this electric field causes a change in the refractive index. As a result, the two optical waveguides 102 and 10
The phase of the light propagating through 3 is shifted. This deviation is π
When it becomes (radian), the higher-order mode is excited in the combining portion of the Mach-Zehnder type optical waveguide, and the light is turned off. Therefore, the output light 114 changes sinusoidally with respect to the applied voltage and exhibits the characteristics shown in FIG. In FIG. 5A, the voltage required to turn ON / OFF the light output is set to the half-wave voltage Vπ.

[0008]

However, in the conventional branching interferometer type optical modulator, in the optical waveguides 102 and 103 of the two branched phase modulator sections, the thickness and width of Ti are several percent when the optical waveguides are manufactured. Difference in the refractive index caused by the change in the mask
A slight difference occurs in the effective optical path length of the phase modulator portion due to an error of about m, nonuniformity / distortion of the buffer layer thickness, electrode structure, and the like. For this reason, a stationary phase difference occurs between the two branched light waves, and the operating point of the modulator shifts. Since it is impossible to eliminate such a change in optical path length during manufacturing, it is virtually impossible to manufacture a modulator having an operating point as designed.
Therefore, in order to drive the conventional modulator, there is a problem that the DC bias voltage must be superimposed on the modulation signal according to the modulation method and the element. Further, since the value of the required DC bias voltage differs depending on the element, there is a problem that the same drive circuit cannot be used.

As described above, the conventional light control element has not obtained sufficient characteristics with respect to reproducibility of operating characteristics.

Therefore, an object of the present invention is to provide an optical control element having a desired operating point, excluding the above-mentioned drawbacks of the conventional optical control device.

[0011]

In order to achieve the above object, a light control element according to the present invention is formed on a substrate having at least one optical waveguide and having an electro-optical effect, and formed on the substrate. An optical control element comprising a buffer layer formed on the substrate and an electrode arranged on the buffer layer, the substrate comprising a stress adjusting film for applying a desired stress to the substrate on the back surface side of the substrate. To do.

Further, the light control element according to the present invention includes a substrate having at least one optical waveguide and having an electro-optical effect, a buffer layer formed on the substrate, and arranged on the buffer layer. In the light control element including the formed electrode, a stress adjusting groove for applying a stress of a desired value to the substrate is formed in the substrate.

[0013]

In the light control element of the present invention, the value of the stress acting on the substrate can be adjusted by adjusting the thickness and arrangement of the stress adjustment film or the depth and width of the stress adjustment groove. The operating point of can be set arbitrarily according to the driving conditions.

[0014]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 shows a Mach-Zehnder type optical modulator for explaining an embodiment of the optical control element according to the present invention.
FIG. 1A is a plan view and FIG. 1B is a sectional view taken along the line AA.

As shown in FIG. 1, on a z-cut LiNbO ₃ substrate 1 having an electro-optical effect, a pair of several tens of μm wide and several tens of mm long, which are close to each other at intervals of several tens μm. Optical waveguides 2 and 3 are installed, and input / output optical waveguides 6 and 7 are connected to both ends thereof via Y branch optical waveguides 4 and 5, respectively.
The end portions of are the entrance end 8 and the exit end 9, respectively. In this example, the buffer layer (SiO ₂ film) 10 provided on the optical waveguides 2 and 3 to prevent light absorption.
A pair of electrodes 11 and 12 is formed via the optical waveguides 2 and 3 and the electrodes 11 and 12
Two phase modulation type light control elements are configured. The Y-branch optical waveguide 4 serving as a 3 dB branch portion is an input light Y-branch optical waveguide, the opening angle thereof is several mrad, and the interval between the two phase modulation portion optical waveguides 2 and 3 is several tens μm. Similarly to the Y-branch optical waveguide 4, the Y-branch optical waveguide 5 serving as the confluent portion is also a Y-branch optical waveguide having an opening angle of mrad. The entrance end 8 and the exit end 9 are formed on opposite end faces of the substrate 1, and the width of each optical waveguide pattern is 5 to 10 μm.
m.

The above structure is similar to that shown in FIG. 4, but is different from the conventional example of FIG. 4 in that the stress adjusting film 13 made of SiO ₂ is provided on the back surface of the LiNbO ₃ substrate 1.

In general, plasma chemical vapor deposition (PCV)
When the stress adjusting film (SiO ₂ film) 13 is formed by the D method) or the like, an internal stress of about 10 ⁹ dyne / cm ^{2 is} usually generated in the thin film. Since the LiNbO ₃ substrate 1 is stressed by this internal stress, when the cross section of the actual device is enlarged, as shown in FIG.
The O ₃ substrate 1 is warped. As an example, the thickness of the substrate is 0.
In an optical modulator with a width of 5 mm, a width of 10 mm, and a length of 40 mm, the central portion is 20 μm smaller than the peripheral portion due to the influence of the buffer layer.
It is convex about m.

In FIG. 2, when the influence of the buffer layer 10 is ignored, the stress σ _LN in the LiNbO ₃ substrate 1 caused by the warp is given by the following equation when the stress in the stress adjusting film 13 is σ _S.

[0020]

[Equation 1]

Where a is a proportional constant and d _LN is LiNbO ₃
The thickness of the substrate 1 and d _S are the thickness of the stress adjusting film 13.

When the LiNbO ₃ substrate 1 is warped, it is strained, so that polarization P is induced in the substrate by the piezoelectric effect. Here, in the example shown in FIG. 1, since the z-cut LiNbO ₃ substrate 1 is used and the light is controlled by using the electric field in the z direction, the z direction of the polarization induced by the strain is the z direction. Component Pz affects the applied voltage-optical output characteristics. When the piezoelectric constant of the LiNbO ₃ substrate 1 is dz and the shape of the device is considered, Pz is given by the following equation.

[0023]

[Number 2] Pz = dzσ _LN where sigma _LN is tensile stress acting on a plane perpendicular to the z-axis. This induced polarization Pz is equivalent to effectively applying a constant bias voltage between the electrodes. In reality, the stress adjustment film 13 has a more complicated relationship due to the influence of the buffer layer 10. However, in the present invention, the stress adjustment film 13 is checked while measuring the applied voltage-light output characteristics of the light control element.
Change the thickness of. That is, the stress adjustment film 13 is gradually deposited while checking the characteristics each time, or the stress adjustment film 1 is etched by etching while checking the characteristics each time.
The thickness of 3 is gradually reduced. Thereby, the stress applied to the substrate 1 can be adjusted to adjust the magnitude of the polarization Pz, and the voltage applied between the electrodes can be adjusted effectively to change the optical path length of the optical waveguide. . That is, the operating point of the light control element can be arbitrarily set according to the driving condition by adjusting the thickness of the stress adjusting film.

The operation of the optical modulator shown in FIG. 1 is as shown in FIG.
The description is omitted here because it is the same as that shown in FIG.

FIG. 3 is a diagram showing a Mach-Zehnder type optical modulation element for explaining another embodiment of the optical control element according to the present invention. FIG. 3A is a plan view, and FIG. 3B is its B-
It is a B line sectional view.

The structure of the light control element is similar to that of the example of FIG. 1, except that stress adjusting grooves 14 and 15 extending in the longitudinal direction of the optical waveguides 2 and 3 are provided on the back surface of the LiNbO ₃ substrate 1. It is provided. This stress adjusting groove 14
And 15 by changing the width and depth of LiNbO
₃ The amount of warpage of the substrate 1 can be adjusted. As a result, the magnitude of the polarization Pz generated by the piezoelectric effect can be adjusted by adjusting the stress applied to the substrate 1, and the voltage applied between the electrodes can be effectively adjusted to change the optical path length of the optical waveguide. it can. That is, by adjusting the width and depth of the stress adjustment groove 14, the operating point of the light control element can be arbitrarily set according to the driving conditions. The stress adjusting groove may be formed on the surface of the substrate as long as it does not affect the optical waveguide.

The operation of the optical modulator shown in FIG. 3 is as shown in FIG.
The description is omitted here because it is the same as that shown in FIG.

Similarly, in the above-mentioned embodiment, the thickness of the stress adjusting film or the width and depth of the stress adjusting groove is adjusted.
Area, pattern shape, arrangement or material of stress adjustment film,
Alternatively, the same effect can be achieved by adjusting the area, pattern shape or arrangement of the stress adjusting groove.

As described above, according to the present invention, it is possible to set the phase difference in the multiplexing section arbitrarily and set the operating point of the optical modulator to a desired position. For example, FIG. 5B shows a state before adjusting the thickness of the stress adjusting film or the depth of the stress adjusting groove, and from this state, the thickness of the stress adjusting film or the depth of the stress adjusting groove. It is possible to obtain the state of FIG. 5A or FIG. That is, as shown in FIG. 5A, the light output can be set to be maximum when the applied voltage is 0, or, as shown in FIG. 5C, the light output can be set when the applied voltage is 0. It is also possible to set it to be half the maximum value.

As described above, according to the present invention, the operating point of the optical modulator can be arbitrarily set according to the driving condition, and the optical modulator having good characteristics can be manufactured with a high yield.

Further, according to the present invention, when the optical modulator is driven, the operating point of the optical modulator can be set so that the effective DC voltage component applied becomes 0. Therefore, LiNbO ₃
It is possible to manufacture an element without the DC drift which is a problem in the light control element using the ferroelectric crystal material such as.

The present invention is not limited to the above embodiments, but any type of light control element utilizing the control of the phase of a light wave, such as a balance bridge type light intensity modulator or a directional coupler type switch, is used. Can also be applied to.
The material of the substrate, the shape of the optical waveguide, the shape of the electrodes and the like used in the present invention are not limited to those in the above-mentioned embodiment, and for example, a ferroelectric crystal such as LiTaO ₃ can be used as the substrate material. A ridge type optical waveguide or the like can be used as the optical waveguide. As the electrode shape, an electrode having a traveling waveform suitable for increasing the speed can be used. Further, the stress adjusting film is not limited to the above-mentioned embodiment, but a dielectric thin film such as a silicon nitride film, a metal film, or a composite film of these can be used.

[0033]

As described above, according to the present invention, the operating point of the light control element can be arbitrarily set by the stress adjusting film and the stress adjusting groove according to the driving condition.

[Brief description of drawings]

FIG. 1 shows an embodiment of a light control element of the present invention, (a)
Is a plan view and (b) is a sectional view taken along line AA.

FIG. 2 is an enlarged cross-sectional view showing distortion of the substrate in the light control element of the present invention.

FIG. 3 shows another embodiment of the light control element of the present invention,
(A) is a top view and (b) is the BB sectional view taken on the line.

4A and 4B show an example of a conventional light control element, FIG. 4A is a plan view, and FIG. 4B is a sectional view taken along the line CC.

FIG. 5 is a characteristic diagram of applied voltage vs. optical output when a voltage is applied to electrodes in a Mach-Zehnder interferometer type optical modulator.

[Explanation of symbols]

 1 substrate 2,3 optical waveguide 4,5 Y branch optical waveguide 6,7 input / output optical waveguide 10 buffer layer 11,12 electrode 13 stress adjusting film 14 stress adjusting groove

Claims (2)

[Claims] 1. A light control device comprising: a substrate having at least one optical waveguide and having an electro-optic effect; a buffer layer formed on the substrate; and an electrode arranged on the buffer layer. An optical control element, comprising a stress adjusting film for applying stress to the substrate on the back surface side of the substrate. 2. An optical control comprising a substrate having at least one optical waveguide and having an electro-optical effect, a buffer layer formed on the substrate, and an electrode arranged on the buffer layer. In the element, the light control element is characterized in that a stress adjustment groove for applying stress to the substrate is formed in the substrate.