JPH07301537A - Fiber optic gyro - Google Patents

Abstract

(57) [Summary] [Purpose] For the optical IC 14 that constitutes the optical branching / coupling unit
Leaked light is reduced from entering the fiber optic coil.
Then, the gyro bias error is reduced. [Structure] Side 2 parallel to optical waveguide 15 of optical IC 14
3, 24 and the back surface 25 with the abrasive grain # 1000 as the rough surface 31
To do. Alternatively, the side surfaces 23 and 24 and the back surface 25 are formed of SiO. ₂Na
Which anti-reflection film 31 or titanium, aluminum etc.
Cover with a metal light absorbing film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention splits and supplies light from a light source to a sensing optical fiber coil as right-handed light and left-handed light or supplies right-handed light propagating through an optical fiber coil.
The present invention relates to an optical fiber gyro equipped with an optical IC used to interfere counterclockwise light, and particularly to an optical fiber gyro with reduced bias error based on the optical IC.

[0002]

2. Description of the Related Art An optical fiber gyro applies a clockwise light and a counterclockwise light to a coil formed by an optical fiber, and when a rotational angular velocity about its axis is applied to the optical fiber coil, both the clockwise and counterclockwise light beams are applied. It is a gyroscope that uses the Sagnac effect that a phase difference occurs between them.

FIG. 2 shows a simplified configuration of a conventional optical fiber gyro using an optical IC. The light emitted from the light source 11 is an optical fiber 12 and an optical coupler 1 including the optical fiber.
It is introduced into one end of the optical waveguide 15 formed in the optical IC (optical integrated circuit) 14 through the optical waveguide 3. The light introduced into the optical waveguide 15 is branched into the optical waveguides 16 and 17 in the middle of the optical IC 14, reaches the other end of the optical IC 14, and is incident on both ends of the optical fiber coil 18 as clockwise light and counterclockwise light. To be done. The light propagating through the optical fiber coil 18 returns to the optical IC again, is combined in the optical waveguide 15 and interferes, and the interference light exits the optical IC 14 and is branched and guided to the light receiver 19 by the optical coupler 13.

Now, when the rotational angular velocity about the axis is applied to the optical fiber coil 18, a phase difference due to the Sagnac effect occurs in both left and right-handed light, and the amount of interference light reaching the light receiver 19 fluctuates. Occurs. The output of the gyro can be obtained by converting the fluctuation of the light quantity into the rotational angular velocity. However, the amount of change in the phase difference between left and right-handed light is usually extremely small, so either the right-handed light or the left-handed light is phase-modulated and the synchronization component is detected to improve detection sensitivity. I am letting you. Since this point is not related to the contents of the present invention, detailed description thereof will be omitted.

The optical IC 14 is manufactured, for example, by forming a waveguide having a slightly higher refractive index in a lithium niobate crystal (LiNbO ₃ ). The optical waveguide is mainly produced by the Ti diffusion method and the proton exchange method.
There are different types, the former is thermal diffusion of titanium to dope into the crystal, and the latter is to exchange H ⁺ and Li ⁺ with a benzoic acid solution to obtain a refractive index difference.

[0006]

According to the structure of the conventional optical fiber gyro shown in FIG. 3, the optical fiber coupler 1 is used.
The coupling loss is about 0.5 dB due to misalignment between the optical fiber and the optical waveguide and the difference in the shapes of the optical fiber mode and the optical waveguide mode at the coupling portion 21 which is incident on the optical waveguide 15 from 3. Light 22 is light I
The light is radiated into the crystal of C14, propagates in the optical IC crystal, and emits light I.
Both side surfaces 23 and 24 parallel to the C crystal optical waveguide and the back surface 25
While repeatedly reflected at, reach the coupling portions 26, 27 on the emission side, that is, the optical fiber coil 18 side, and mix into the optical fiber of the optical fiber coil 18.

Since the light mixed in the optical fiber coil 18 has a different optical path from the original light guided through the optical waveguide 15, it causes a gyro bias error. That is, the optical waveguide 1
This is because there is a phase difference between the guided light from No. 5 and the leaked light due to the difference in optical path length. In particular, there are two types of polarization modes, TE mode and TM mode, which can propagate in the optical waveguide. However, when the polarization state of leaked light is different from that of guided light, the refractive index that affects each of these is different. The temperature coefficients are different, which causes periodic fluctuations with respect to temperature changes.

It is an object of the present invention to provide an optical fiber gyro which prevents such recombination of leaked light and has a small bias fluctuation. For example, when a lithium niobate crystal is used as the optical IC 14, the refractive index of the crystal is n = 2.1.
7 and the refractive index n ₀ = 1 of air have a large difference in refractive index, and the critical angle θ _c is larger than sin ⁻¹ n ₀ / n≈27 ° at the side surfaces 23 and 24 and the back surface 25 of the IC 14. The leaked light 22 having an angle θ is totally reflected. Since the side surfaces 23 and 24 and the back surface 25 of the conventional optical IC 14 are smooth as if they were optically polished, the crystal itself of the optical IC 14 functions as an optical waveguide, and the leaked light 22 is guided by the inside of the crystal. Fiber coil 1
It will be mixed in 8.

[0009]

According to the invention of claim 1, both side surfaces and the back surface of the optical IC are roughened so as to scatter light. In the invention of claim 2, both side surfaces and the back surface of the optical IC are covered with an antireflection film for reducing reflected light or a light absorbing film for absorbing light.

[0010]

According to the configuration of the first aspect of the present invention, the leaked light is scattered on the rough surface, and the leaked light recombined with the optical fiber coil is reduced. According to the configuration of the second aspect of the invention, the leaked light does not act as an optical waveguide against the leaked light due to the presence of the reflected light prevention film or the light absorption film.

In both the invention of claim 1 and the invention of claim 2, the amount of recombination of leaked light into the optical fiber coil is significantly reduced, and the bias error of the gyro is reduced.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention as a whole has a configuration similar to that of the conventional one shown in FIG. 2, for example, but is characterized by an optical IC 14, an example of which (corresponding to the invention of claim 1). Is shown in FIG. 1A. In this embodiment, both side surfaces 23 and 24 of the optical IC 14 and the back surface 25 are rough surfaces 31 that scatter light. The rough surface 31 is formed, for example, by polishing the IC crystal with # 1000 abrasive grains for 2 to 3 minutes by a normal polishing method.

Due to this structure, of the light incident on the optical IC 14 from the optical fiber coupler 13, the light 22 leaked from the optical waveguide 15 is the side surfaces 23, 24 and the back surface 2.
Each time it enters the rough surface 31 of No. 5, it is scattered, and the amount of recombination with the optical fiber coil 18 is reduced. FIG. 1B shows a cross section of the optical IC 14, which is a main part of the embodiment of the invention of claim 2. In this embodiment, the antireflection film 32 is formed so as to cover the side surfaces 23 and 24 and the back surface 25.

In the case of the single-layer antireflection film 32, the reflection can be made zero by setting the refractive index n _a and the film thickness d as follows. Refractive index _{n a = (n C n o} ) 1/2 n c of the antireflection film 32: IC 14 crystal refractive index of, n _o: the air refractive index film thickness d = (2m + 1) λ / (4n a cosθ) m = 0, 1, 2, ..., λ: wavelength, θ: incident angle For example, as a crystal of the optical IC 14, lithium niobate (n _C
= 2.17) and λ = 0.83 μm as the wavelength used,
When the incident angle of the leaked light 22 is θ = 80 °, n _a = 1.4
7, d = 0.81 μm.

As a refractive index material near n _a = 1.47, there is SiO ₂ (n = 1.45), for example, which can be easily formed as the antireflection film 32 by vapor deposition or the like.
In order to suppress the reflection more precisely, the antireflection film 32 may have a multilayer film structure. However, the incident angle of the leaked light 22 is 8
Since there are angles other than 0 °, it is difficult to completely reduce the light of all angular waves to zero.

As another material of the antireflection film 32, for example, a material having a refractive index of about 1.47 is an ultraviolet curable adhesive,
Other coating materials can also be used. Instead of the antireflection film 32, a light absorption film that absorbs light may be formed. This light absorbing film can be formed by evaporating a metal such as titanium or aluminum. The thickness of the light absorption film is, for example, 1000Å to 3000Å
The degree can be considered. The light absorption film attenuates the leaked light 22 considerably, and the amount of light that recombines with the optical fiber coil 18 is significantly reduced.

[0017]

As described above, according to the invention of claim 1, since the side surface and the back surface of the optical IC are roughened,
The leakage light is scattered and the recombination into the fiber optic coil is significantly reduced. According to the invention of claim 2, since the side surface and the back surface of the optical IC are covered with the antireflection film or the light absorption film, reflection of leakage light is prevented or leakage light is absorbed,
Again, contamination of the fiber optic coil is reduced.

Therefore, in both the first and second aspects of the invention, the bias error of the gyro can be reduced.

[Brief description of drawings]

FIG. 1A is an optical part I which is an essential part of an embodiment of the invention of claim 1;
FIG. 9 is a sectional view of C, and B is a sectional view of an optical IC, which is a main part of an embodiment of the present invention.

FIG. 2 is a diagram showing an example of a conventional optical fiber gyro.

Claims (2)

[Claims] 1. An optical fiber gyro using an optical IC, characterized in that two side surfaces and a back surface of the optical IC parallel to the optical waveguide are rough surfaces that scatter light. Fiber gyro. 2. In an optical fiber gyro using an optical IC, two side surfaces and a back surface of the optical IC, which are parallel to the optical waveguide, are an antireflection film for reducing light reflection or a light absorbing film for absorbing light. A fiber optic gyro characterized by being covered.