JPH09251146A - Optical waveguide element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical waveguide element in which resonance due to vibration can be eliminated and no change in the sensitivity is caused by changes in the frequency, by disposing an absorber for vibration on the backside of a substrate on which a branched interference type optical waveguide and a modulation electrode are formed. SOLUTION: An optical waveguide 2 is formed on a lithium niobate crystal substrate 1 and a modulation electrode 3 is formed near the waveguide. An indium layer of 1 to 5 μm thickness is formed by vapor deposition all over the back surface of the substrate 1, namely, on the opposite surface of the substrate 1 to the surface where the optical waveguide 2 and the modulation electrode 3 are formed. The indium film acts as an absorber 4 for vibration. The optical waveguide element is fixed with the indium film 4 to a glass plate as a base body and then assembled in a package. Or, a lead or indium-lead alloy layer may be formed to 1 to 5μm thickness as an absorber 4 for vibration all over the back surface of the substrate 1. As for the base body, a sheet of lead or indium-lead alloy having about 2mm thickness comprising lead or indium-lead alloy may be used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the structure of an optical waveguide device used in an optical modulator or the like.

[0002]

2. Description of the Related Art Optical waveguide devices are often used in sensor heads of optical modulators and electric field sensors. Many of them use a material having an electro-optical effect, such as lithium niobate crystal, as a substrate, and are composed of an optical waveguide pattern and a modulation electrode formed on the surface of the substrate as shown in FIG. In FIG. 3, (a) is a plan view of the optical waveguide device,
(B) is a figure which shows the cross section. The optical waveguide pattern usually constitutes a branch interferometer. The modulation electrode 3 is
In the case of the optical electric field sensor, which is formed in the vicinity of the phase shift optical waveguide 2 of the branch interference waveguide, the electric field and the received signal received by the antenna are generated in the two phase shift optical waveguides 2 in opposite directions. Due to the change in the refractive index of each phase shift optical waveguide 2 due to the electric field, a phase difference occurs between the light passing therethrough, and light interference occurs at the point where the two lights merge.
The electric field sensor uses the principle of modulating the electric field into light intensity and detecting the light intensity.

[0003]

The signal received by the antenna is applied with high frequency electric fields in opposite directions to the two phase shift optical waveguides through the modulation electrodes. for that reason,
The respective phase shift optical waveguides are displaced in opposite directions due to the piezo effect, and in-plane vibration is generated in the substrate. In the case of an X-cut substrate, this in-plane vibration propagates as a transverse wave in the thickness direction of the substrate. In this phenomenon, the traveling wave of the vibration and the reflected wave from the facing surface act on each other at a frequency at which the thickness of the substrate is an integral multiple of a half wavelength of the vibration, and a resonance state occurs.

When such a resonance frequency is included in the frequency band used, the optical waveguide element appears as a change in sensitivity to the electric field strength due to resonance. For example, in the case of an optical waveguide device using a lithium niobate crystal as a substrate, a phenomenon was observed in which the sensitivity characteristic to an electric field of constant strength drastically changes at an interval of about 5 MHz at frequencies from 170 MHz to 230 MHz. This resonance period is defined by the thickness of lithium niobate-resonance frequency constant 2400.
Based on the value of MHz · μm, the resonance frequency is 4.8 MHz, which is obtained for the substrate thickness of 500 μm, and it is confirmed that this sensitivity change is due to the resonance phenomenon. In the case of an electric field sensor using an optical waveguide element, such a change in sensitivity causes a harmful error. In general, the optical waveguide element is adhered to a glass plate or the like via an adhesive in order to increase its mechanical strength. The glass plate or the like serves as a pedestal when it is incorporated in the package. As described above, the glass plate or the like as the pedestal has only a mechanical holding function, and does not exhibit an effective function of suppressing or reducing the resonance phenomenon.

An object of the present invention is to provide an optical waveguide device which eliminates resonance due to vibration and does not cause sensitivity change due to frequency change.

[0006]

In order to solve the above-mentioned problems, the present invention forms a branch interference type optical waveguide on the surface of a crystal substrate exhibiting an electro-optical effect, and a modulation electrode in the vicinity of the branch interference type optical waveguide. In the optical waveguide element configured as described above, a vibration absorber made of indium or lead is provided on the back surface of the substrate on which the branch interference type optical waveguide and the like are formed.

Further, it is practical and more effective if the optical waveguide element is joined to the support through the vibration absorber. The vibration absorber and the support may be the same and integrated.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The constitution of the present invention will be specifically described below with reference to Examples.

(Embodiment 1) FIG. 1 is a diagram showing a cross section of a structure of an optical waveguide device according to an embodiment of the present invention. The substrate 1 is an X substrate of lithium niobate crystal, and the optical waveguide 2 is Y
Formed in the direction. A modulation electrode 3 is formed near the two phase shift optical waveguides 2. The back surface of the substrate 1, that is, the other surface of the substrate 1 on which the optical waveguide 2 and the modulation electrode 3 are formed has a thickness of 1 to 5 μm over the entire surface.
Of indium was formed by vapor deposition. The indium film serves as a vibration absorber 4. The optical waveguide device was fixed to a glass plate as a support via an indium film as a pedestal and incorporated into a package. According to this embodiment, about 3
An improvement in dB was obtained.

(Embodiment 2) FIG. 2 is a diagram showing a cross section of the structure of an optical waveguide device according to another embodiment of the present invention. The optical waveguide device obtained in Example 1 is provided with a thickness of 2 m having a width equal to or larger than that of the optical waveguide device through the indium film of the absorber 4.
A lead sheet of m was used as the support 5 and was fixed thereto by an adhesive means. Here, the support 5 serves as a pedestal when the optical waveguide device is incorporated in the package. An improvement of about 7 dB was obtained by this example.

(Embodiment 3) In this embodiment, an X substrate of lithium niobate crystal is used as a substrate and an optical waveguide is formed in the Z direction. An indium sheet having a thickness of 2 mm was adhered and fixed on the entire back surface of the substrate. The indium sheet serves as both a vibration absorber and a support. About 6 dB according to this embodiment
Improved.

As a result of measuring the electric field detection sensitivity of each optical waveguide element having each of the above-mentioned constitutions from 90 MHz to 800 MHz, no harmful sensitivity change was observed.

In addition to each of the above embodiments, lead or indium-lead alloy may be formed as a vibration absorber over the entire back surface of the substrate to a thickness of 1 to 5 μm, and as a support, lead or indium may be used. -Thickness of lead alloy 2
A sheet of about mm may be used.

[0014]

As described above, it is possible to dispose a vibration absorber made of an appropriate material or a support therethrough on the back surface of the substrate on which the branch interference type optical waveguide and the modulation electrode are formed. It was shown that the vibration that causes the deterioration of the frequency characteristic of the sensitivity is suppressed by. Therefore, the present invention can provide an optical waveguide element that eliminates resonance due to vibration and does not cause sensitivity change due to frequency change.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a configuration of an optical waveguide device that is an embodiment of the present invention.

FIG. 2 is a sectional view showing the structure of an optical waveguide device according to another embodiment of the present invention.

FIG. 3 is an explanatory diagram showing a configuration of a conventional optical waveguide device. FIG.
3A is a plan view and FIG. 3B is a cross-sectional view.

[Explanation of symbols]

 1 substrate 2 optical waveguide 3 modulation electrode 4 absorber 5 support

Claims (5)

[Claims] 1. An optical waveguide element constituted by forming a branch interference type optical waveguide and a modulation electrode in the vicinity of the branch interference type optical waveguide on a surface of a substrate exhibiting an electro-optical effect, wherein An optical waveguide device having a vibration absorber on the surface of the optical waveguide. 2. The optical waveguide element according to claim 1, wherein the optical waveguide element is bonded to a support through the vibration absorber. 3. The vibration absorber is integrated with the support,
The optical waveguide device according to claim 1, wherein the optical waveguide devices are the same. 4. The optical waveguide device according to claim 1, wherein the vibration absorber contains at least one of indium and lead. 5. The optical waveguide device according to claim 2, wherein the support contains at least one of indium and lead.