JPH10293223A - Light guide element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent unnecessary polarized light in a substrate from being coupled with a light guide again, to obtain characteristics with a high extinction ratio, and to facilitate the manufacture and handling. SOLUTION: This light guide element has >=1 Y-shaped branch part and linear parts connecting to it on the main surface of a substrate 10 with electrooptic effect and also has electrodes 16 formed on the substrate surface in parallel to the light guides of the linear parts. A removing means for unnecessary light emitted from the light guides is provided not on the light guides. As this removing means, there are a proton exchange light guide film 14 for a case wherein a titanium heat diffusion light guide 12 is used, a light absorbing film and a titanium heat diffusion light guide film of metal for a case wherein a proton exchange light guide is used, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide device for use in optical measurement and an optical communication system. More specifically, an optical waveguide having a Y-shaped branch portion is provided on a substrate having an electro-optical effect. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide element provided with unnecessary light removing means for removing unnecessary light leaking out of an optical waveguide from light propagating in a wave path and preventing unnecessary light from returning to the optical waveguide. This optical waveguide device is useful, for example, for an optical integrated circuit for an optical fiber gyro.

[0002]

2. Description of the Related Art As a sensing technique using an optical fiber, a light for detecting a rotational angular velocity utilizing a Sagnac effect in which a phase difference proportional to rotation is generated between two light waves that propagate in a closed optical loop in opposite directions. There is a fiber gyro. There are several types of optical fiber gyros, one of which is called an interference type, which is a long polarization-maintaining fiber or a single-mode optical fiber wound in a coil shape for sensing. A phase difference (Sagnac shift) between two light waves that propagates in the loop in the opposite direction
Is a method of detecting interference. As a technique for effectively detecting the phase difference, for example, there is a phase modulation method.

FIG. 9 shows the basic configuration of this type of optical fiber gyro. Light emitted from a light source (for example, SLD: superluminescent diode) 80 is equally divided by a 3 dB coupler 81, one of the lights is linearly polarized by a polarizer 82, and branched by a light branching / coupling unit 83. The light enters from both ends of the fiber coil 84. The other light split equally is discarded or used as monitor light. The light that has turned around the optical fiber coil 84 in the opposite direction
3 and pass through the polarizer 82,
Equally distributed, one of which is a light receiver (PD: photodiode)
85 and is detected as an electric signal. A phase modulation unit 8 is provided between the optical branching / coupling unit 83 and the optical fiber coil 84.
6 is provided so as to apply phase modulation to the light wave.

Here, the light branching / coupling portion 83 is an optical waveguide having a Y-shaped branch portion formed on a substrate 88 having an electro-optic effect, and the phase modulation portion 86 is formed of a linear portion formed on the same substrate 88. (In this case, four electrodes) provided in parallel to the optical waveguides. As the substrate material, for example, lithium niobate (LiNbO ₃ ) crystal or the like is used. The optical waveguide is formed by a titanium heat diffusion method or a proton exchange method. Normally, this type of optical waveguide element has linear polarization, because it has polarization dependence. In particular, in the case of an optical waveguide element for an optical fiber gyro,
Due to the required characteristics, the polarizer 82 is provided with a polarization adjusting function so that linearly polarized light having a high extinction ratio propagates.

The Sagnac shift is detected, for example, as follows. A sine wave is applied from the sine wave oscillating circuit 91 to the electrodes of the phase modulating section 86 for modulation, and a saw-tooth wave is supplied from the saw-tooth wave generating circuit 92 and superimposed. Setting the amplitude of the sawtooth wave to an appropriate value can be equivalent to shifting the light wave frequency by that frequency. In this way, both lights passing through the two optical waveguides are transmitted to the optical fiber coil 84.
The light propagates through the inside at different frequencies, and the photodetector 85 has a phase difference. Using the light receiving signal from the light receiving device 85 and the reference signal from the sine wave oscillating circuit 91, the demodulator 93 feeds back the frequency of the sawtooth wave so as to cancel the Sagnac shift by the phase difference. As a result, a frequency proportional to the input rotational angular velocity is obtained, which becomes a gyro output.

[0006]

In the conventional optical waveguide device, since only single polarized light is guided by the optical waveguide, the separated unnecessary polarized light component is radiated as radiation into the substrate. . If the emitted light interferes with each other in the substrate and is coupled again to the optical waveguide or the optical fiber to be connected, it becomes noise and degrades the extinction ratio. For example, when the present invention is applied to an optical fiber gyro, noise is added to the gyro signal, and the measurement accuracy is deteriorated.

As a countermeasure for solving such a problem, a technique has been proposed in which a groove is provided in the substrate at right angles to the optical waveguide to block light propagating inside the substrate (Japanese Patent Application Laid-Open No. H7-1995).
181405). However, the thickness of the substrate is usually as thin as about 1 mm, and it is cumbersome and difficult to form a groove having such a depth as to block the propagation of light. It will be easier.

An object of the present invention is to provide an optical waveguide device which prevents unnecessary polarized light in a substrate from being coupled to the optical waveguide again, realizes a characteristic having a high extinction ratio, and is easy to manufacture and handle. is there. Another object of the present invention is to remove unnecessary light so that unnecessary light emitted from the optical waveguide is returned as light and is not superimposed on the gyro signal as noise so that highly accurate measurement of angular velocity can be realized. An object of the present invention is to provide an optical waveguide device for an optical fiber gyro having a function.

[0009]

According to the present invention, an optical waveguide having at least one Y-shaped branch portion and a plurality of linear portions connected thereto is formed on a main surface of a substrate having an electro-optical effect. , An optical waveguide element in which an electrode is formed on a substrate surface in parallel with one or more of the linear waveguides. Here, in the present invention, means for removing unnecessary light radiated from the optical waveguide is provided in a portion other than the optical waveguide. Examples of the removing means include a proton exchange optical waveguide film and a metal light absorption film when using a titanium heat diffusion optical waveguide, a metal light absorption film and a titanium heat diffusion optical waveguide film when using a proton exchange optical waveguide. There is.

As a substrate having an electro-optical effect, for example, lithium niobate (LiNbO ₃ ) crystal or lithium tantalate (LiTaO ₃ ) crystal is preferable. The polarization incident on the optical waveguide device is about 25 dB. For example, in order for the optical fiber gyro to function with high accuracy, 50 d is required.
It is necessary to produce B-polarized light. In the present invention, a high extinction ratio that can satisfy required performance is realized by removing unnecessary light.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention is shown in FIGS. Figure 1 is a plan view, A is its x ₁ -x ₁ cross-sectional view of FIG. 2, B is x ₂ -x ₂ cross-sectional view of the optical waveguide device. This is because the main surface of the substrate 10 having the electro-optic effect is formed on the optical waveguide 1 having the Y-shaped branch portion and the linear portion by the titanium thermal diffusion method.
The two-dimensional optical waveguide region 1 for leaking light from the optical waveguide is formed on a part or all of the other main surface of the substrate in proximity to the titanium thermal diffusion optical waveguide by a proton exchange method.
4 (indicated by dots in FIGS. 1 and 2). A plurality (four in this case) of modulation electrodes 16 are formed in parallel with the linear portion of the optical waveguide 12. Here, the two-dimensional optical waveguide region 14 formed by the proton exchange method is separated from the titanium heat diffusion optical waveguide 12 on the input / output end side, but is formed in a pattern that is in contact with the titanium heat diffusion optical waveguide 12 in most other portions. I do. In addition, the substrate 10 has a shape in which both end surfaces are beveled at an appropriate angle to prevent reflected return light at an end when light enters and exits.

The titanium heat diffusion optical waveguide 12 formed on the X-cut LiNbO ₃ substrate 10 transmits incident light (TE mode light and TM mode light) having a low extinction ratio as it is, while the proton exchange optical waveguide region 14 Propagates only TE mode light. Among the light incident from one end of the titanium heat diffusion optical waveguide 12, the TM mode light propagates as it is according to the pattern of the titanium heat diffusion optical waveguide 12, while the TE mode light transmits the proton exchange optical waveguide region formed adjacently. Due to the influence of 14, the light is radiated out of the titanium heat diffusion optical waveguide 12 without any lateral confinement. Then, the light propagates through the proton exchange optical waveguide region 14 and is emitted outside the substrate 10 without being coupled to the optical fiber. For this reason,
The proton exchange optical waveguide region 14 functions as a polarizer and performs a function of removing unnecessary light. As described above, the proton exchange optical waveguide region 14 is formed so as to be separated from the titanium heat diffusion optical waveguide 12 near the input / output end of the titanium heat diffusion optical waveguide 12 because the proton exchange optical waveguide region 14 is This is to prevent the propagating light from being coupled to the optical fiber.

The TM mode light is transmitted through a titanium heat diffusion optical waveguide 1.
2, the propagation loss is smaller than the propagation in the proton exchange optical waveguide region 14, and there is no decrease in the material properties (electro-optic constant) as seen in the proton exchange method. Driving at a low voltage becomes possible. In addition, since the entire length of the optical waveguide functions as a polarizer, the device can be downsized. When a Z-cut substrate is used, TE mode light is used.

Another embodiment of the present invention is shown in FIGS. Figure 3 is a plan view of the optical waveguide device, A in FIG. 4 is the x ₃
-X ₃ cross-sectional view, B is x ₄ -x ₄ cross-sectional view. this is,
An optical waveguide 22 having a Y-shaped branch portion and a plurality of linear portions continuous with the Y-shaped branch portion is formed on the main surface of the substrate 20 having an electro-optic effect by a titanium heat diffusion method or a proton exchange method, and light absorption by a metal thin film is performed. This is an optical waveguide device in which the film 24 (shown by hatching in FIGS. 3 and 4) also serves as an electrode. The light absorbing film 24 is provided so as to cover most of the main surface of the substrate 20. The modulation electrode is provided on almost the entire surface from one end to the Y-shaped branch portion, and the linear portion (up to the other end) is excluding the vicinity of the linear portion. So that it can function as

As for the elliptically polarized light incident from the polarization maintaining fiber, only the TE mode light propagates in the proton exchange optical waveguide 22 formed on the X-cut LiNbO ₃ substrate 20. Unnecessary light leaked from the optical waveguide 22 is absorbed by the light absorbing film 24 made of a metal thin film. This is because the loss of propagating light in the optical waveguide on which the metal thin film is formed is reduced by TM mode light and T
This is because, when comparing the E mode light, the propagation loss of the TM mode light is significantly larger than the propagation loss of the TE mode light. In this way, the TM mode light is absorbed and removed,
It functions as a polarizer, and unnecessary light does not become stray light, and therefore does not return to the optical waveguide or optical fiber. Note that Z
Similarly, when a cut substrate is used, TE mode light is used.

The light absorbing film made of a metal thin film may be formed only on the same main surface as the optical waveguide as shown in FIG. 4, or may be provided on both main surfaces as shown in FIG. As shown in FIG. In each figure, the light absorbing film is indicated by reference numeral 24.

Here, as the metal thin film, for example, gold is preferable. Considering the electromagnetic field distribution (leakage) of the light wave out of the optical waveguide, the light absorption is saturated at about 1000 °,
A film thickness of about 1000 ° or more is sufficient. A chromium thin film may be provided as a base of the gold thin film. These metal thin films can be formed by a vapor deposition method, a sputtering method, or the like.

FIGS. 7 and 8 show still another embodiment of the present invention.
Shown in Figure 7 is a plan view of the optical waveguide device, A is its x ₅ -x ₅ cross-sectional view of FIG. 8, B is x ₆ -x ₆ cross-sectional view. That is, an optical waveguide 32 having a Y-shaped branch portion and a plurality of linear portions continuous with the Y-shaped branch portion is formed on the main surface of a substrate 30 having an electro-optical effect by the proton exchange method.
The optical waveguide region 34 for leaked light is separated by a distance that does not couple energetically with the propagating light inside,
(Represented by dots in FIGS. 7 and 8). A phase modulation electrode 36 is formed near the linear portion of the optical waveguide 32.

With respect to the elliptically polarized light incident from the polarization-maintaining fiber, only the TE mode light propagates through the optical waveguide 32 by the proton exchange optical waveguide 32 formed on the X-cut LiNbO ₃ substrate 30. Unnecessary light (TM mode light) leaking from the optical waveguide 32 is transmitted to the titanium heat diffusion optical waveguide region 34.
Trapped in This is for TM mode light.
The presence or absence of the proton exchange optical waveguide does not affect the propagation, and when it leaks into the substrate 30 and reaches the titanium heat diffusion optical waveguide region 34, it is a preferable region for the TM mode light to propagate. This is to prevent leakage. In this way, the TM mode light is removed and functions as a polarizer, and unnecessary light is confined and does not return to the optical waveguide or the optical fiber. When a Z-cut substrate is used, TM mode light is used.

The gap between the proton exchange optical waveguide 32 and the titanium heat diffusion optical waveguide region 34 depends on the wavelength used.
Usually, it may be set to about 10 to 20 μm. For the wavelength used (for example, 0.8 to 1.55 μm), 3 to 5
About twice or more is necessary.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical waveguide device having the structure shown in FIGS.
It was produced in the following procedure. An X-cut LiNbO ₃ wafer having a thickness of 1 mm was prepared as a substrate having an electro-optic effect. Using a photolithography technique, a titanium thin film having a width of 3 μm and a thickness of 500 ° is formed on the optical waveguide pattern, and the wafer is accommodated in an electric furnace at a temperature of 1000 ° C. for a holding time of 6 hours. By heat diffusion, a titanium heat diffusion optical waveguide was formed. Next, using a photolithography technique, the portion of the titanium heat diffusion optical waveguide, and almost the entire surface except for the input / output end faces of the optical waveguide,
Proton exchange was performed at 250 ° C. for 2 hours using a benzoic acid solution, and then annealing was performed at 400 ° C. for 2 hours to form a proton-exchange optical waveguide region. Thereafter, two pairs of gold electrodes were formed by vapor deposition in parallel with the linear portions of the titanium heat diffusion optical waveguide.

The optical waveguide device thus manufactured is
It exhibited excellent characteristics with a propagation loss of 0.1 dB / cm and an extinction ratio of 50 dB.

[0023]

Since the present invention is an optical waveguide device configured as described above, only single polarized light is guided by the optical waveguide, and unnecessary polarized light components are emitted as radiation light into the substrate. Since it is possible to prevent the unnecessary polarized light from being removed and returning to the optical waveguide again, characteristics with a high extinction ratio can be realized.

Further, when the optical waveguide device of the present invention is applied to an optical fiber gyro, the radiated light from the optical waveguide to the substrate does not couple again to the optical waveguide or the optical fiber to be connected. Is not superimposed as noise, and the angular velocity can be measured with high accuracy.

[Brief description of the drawings]

FIG. 1 is a plan view showing an embodiment of an optical waveguide device according to the present invention.

FIG. _{2 is a} cross-sectional view of x ₁ -x ₁ and x ₂ -x ₂ thereof.

FIG. 3 is a plan view showing another embodiment of the optical waveguide device according to the present invention.

FIG. _{4 is a} sectional view taken along lines x ₃ -x ₃ and x ₄ -x ₄ .

FIG. 5 is a sectional view showing another embodiment of the optical waveguide device according to the present invention.

FIG. 6 is a cross-sectional view showing another embodiment of the optical waveguide device according to the present invention.

FIG. 7 is a plan view showing still another embodiment of the optical waveguide device according to the present invention.

[8] The x ₅ -x ₅ and x ₆ -x ₆ cross-sectional view.

FIG. 9 is an explanatory view showing an example of an optical fiber gyro.

[Explanation of symbols]

 Reference Signs List 10 Substrate having electro-optic effect 12 Titanium heat diffusion optical waveguide 14 Proton exchange optical waveguide region 16 Modulation electrode

Claims (6)

[Claims] An optical waveguide having at least one Y-shaped branch portion and a plurality of linear portions connected thereto is formed on a main surface of a substrate having an electro-optic effect, and at least one straight line is formed. In an optical waveguide device having an electrode formed on the surface of a substrate in parallel with the optical waveguide of the Y-shaped portion, the Y-shaped branch portion and the optical waveguide of the linear portion are formed by a titanium thermal diffusion method, and the main surface of the substrate other than the optical waveguide is formed. A part or all of the two-dimensional optical waveguide region for leaking light from the optical waveguide is formed by a proton exchange method in a pattern adjacent to the optical waveguide and away from the input / output end thereof. An optical waveguide element. 2. An optical waveguide having at least one Y-shaped branch portion and a plurality of linear portions continuous therewith is formed on a main surface of a substrate having an electro-optic effect, and at least one straight line is formed. In an optical waveguide element having an electrode formed on the surface of a substrate in parallel with the optical waveguide of the shape, a light absorbing film made of metal is applied to the electrode so as to cover at least most of the main surface of the substrate on which the optical waveguide is formed. An optical waveguide element, which is also formed and absorbs light leaked from the optical waveguide. 3. The optical waveguide device according to claim 2, wherein the optical waveguide is formed by a titanium heat diffusion method or a proton exchange method. 4. An optical waveguide having at least one Y-shaped branch portion and a plurality of linear portions connected thereto is formed on a main surface of a substrate having an electro-optic effect, and at least one straight line is formed. In an optical waveguide device in which an electrode is formed on the substrate surface in parallel with the optical waveguide of the shape portion, the optical waveguide is formed by a proton exchange method, and a part or all of the main surface of the substrate other than the optical waveguide is provided inside the optical waveguide. A two-dimensional optical waveguide for leaking light from said optical waveguide formed by a titanium thermal diffusion method, separated by a distance that does not couple energy with said propagating light. 5. The method according to claim 1, wherein the substrate is LiNbO ₃ or LiTaO _3.
5. The optical waveguide device according to claim 1, comprising: 6. A phase modulation electrode is provided in parallel with an optical waveguide of a linear portion connected to both branch branches of a Y-shaped branch portion of an optical waveguide, and an optical fiber loop is provided between both ends of the linear portion. 6. The optical waveguide device according to claim 1, wherein the optical waveguide device is used for an optical fiber gyro configured by connecting both terminals.