JP2692551B2 - Optical waveguide substrate for displacement sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical waveguide substrate used as a displacement sensor for measuring displacement of an object to be measured by optical interference.

[0002]

2. Description of the Related Art With the development of lasers, electronics, microcomputers, sensor elements and the like, displacement sensors for minute portions due to optical interference of laser light, which are excellent in coherence, have become widespread. Since a displacement sensor using laser light is non-contact and highly accurate, it is used for measuring the surface roughness of crystal wafers, optical components, X-ray mirrors, and the like.

[0003] the displacement sensor using the laser beam, there is one using the principle of Michelson interferometer. This is because an optical waveguide is formed on the surface of an electro-optical crystal substrate such as lithium niobate (LiNbO ₃ ) or lithium tantalate (LiTaO ₃ ) and the measurement light reflected by the DUT and the reference light reflected by the reflecting mirror are formed. Is made to interfere with a directional coupler to measure a minute displacement of the object to be measured.

In the displacement sensor, when laser light passes through the interface of different substances such as optical waveguide-optical fiber and optical waveguide-air, Fresnel reflection occurs due to the difference in refractive index between them. In particular, when lithium niobate is used for the electro-optic crystal substrate, the refractive index of this material is 2.2, which is larger than the refractive index of the optical fiber of 1.45 and the refractive index of air of 1.0, and Fresnel reflection occurs remarkably. . This Fresnel reflection becomes noise, which is an obstacle to accurate displacement measurement especially when the reflectance of the object to be measured is low and the intensity of the measurement light is weak.

To solve this problem, an optical waveguide
There is a method of forming an antireflection film on the end surface or a method of obliquely polishing the desired end surface of the waveguide connected to the optical fiber to prevent Fresnel reflection noise . The latter is an optical fiber
-Applied to optical waveguide for gyro (eg PCT
/ US90 / 00989).

According to the former, the Fresnel reflection noise is maximized.
It can be suppressed up to 30 dB. However, the color and surface of the DUT
Depending on conditions such as conditions, the reflected return light may have Fresnel reflection noise.
In some cases, it will be about the same. As such, the amount of reflected light is small
In order to measure the DUT, more Fresnel reflection
It is necessary to reduce noise. Waveguide for displacement sensor
It is applied to the circuit board, and the fiber connection end and the measurement light input / output end are
If it is cut diagonally, Fresnel reflection can be suppressed to about 60 dB.
Can be controlled. However, with this method, a reference beam is obtained
Therefore, the amount of light reflected from the reflecting mirror is also attenuated .

Further, there is a method of forming the reflecting mirror in the middle of the optical waveguide instead of forming it on the end face of the optical waveguide substrate. However, in order to realize this, an extremely complicated process is required, and it is not a practical method. Absent.

[0008]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an optical waveguide substrate used for a displacement sensor which prevents noise generation due to Fresnel reflection and enables accurate displacement measurement.

[0009]

[MEANS FOR SOLVING THE PROBLEMS] To achieve the above object
The optical waveguide substrate for a displacement sensor according to claim 1,
For example, optical waveguides and directional couplings on the electro-optic crystal substrate surface.
And an optical waveguide substrate for a displacement sensor, in which a reflecting mirror is formed
A plate, the light guide from the directional coupler to the reflector
The waveguide is formed obliquely to the side surface of the substrate,
The end face of the substrate forming the reflecting mirror is inclined in the oblique direction.
The feature is that it is a vertical surface.

[0010]

1 is a conceptual view showing an embodiment of the optical waveguide substrate of the present invention. The laser light L1 incident on the optical waveguide substrate 1 passes through the directional coupler 3 from the optical waveguide 2a and passes through the directional coupler 2.
One of them is divided into the measurement light L2 through the optical waveguide 2c, and the other becomes the reference light L3 through the optical waveguide 2d. The measurement light L2 hits the object to be measured 5, is reflected, reaches the directional coupler 3 again from the optical waveguide 2c, and the reference light L3 is reflected by the reflecting mirror 4, and reaches the directional coupler 3 again from the optical waveguide 2d. In the directional coupler 3, the measurement light L2 and the reference light L3 interfere with each other, and a part of the interference light L4 is output from the optical waveguide 2b. By measuring the intensity of the interference light L4, the minute displacement of the surface of the DUT 5 can be measured.

In the optical waveguide substrate for a displacement sensor of the present invention, since the optical waveguide extending from the directional coupler 3 to the reflecting mirror is formed in an oblique direction with respect to the substrate, the return light is generated even if Fresnel reflection occurs. It does not become. Moreover, since the end surface of the optical waveguide substrate forming the reflecting mirror is a surface perpendicular to the oblique direction, the measurement light can return to the optical waveguide substrate.

A crystal of LiNbO ₃ or LiTaO ₃ can be used for the electro-optic crystal substrate, and the optical waveguide is formed by proton exchange on the substrate surface. A metal or oxide vapor-deposited one can be used for the reflecting mirror.

[0013]

EXAMPLE An optical waveguide substrate having the structure shown in FIG. 1 was prepared and used as a displacement sensor. The electro-optic crystal substrate 1 has a thickness of 1 m
Using LiNbO ₃ crystal having m, width of 2.8 mm, and length of 30 mm, the end faces 1a and 1c were slanted parallel to each other and inclined at an angle of 80 ° with respect to the end face 1b. The optical waveguide 2 and the directional coupler 3 were formed by exchanging protons on the surface of the substrate 1. Of these, the optical waveguide 2d is bent outward from the directional coupler 3 with a radius of curvature of 60 mm, and the end face 1 is formed near the substrate end face 1a.
A straight line having an angle of 10 ° with respect to b was used.

The reflecting mirror 4 is an optical waveguide 2d on the end face 1a of the substrate.
It was formed by vapor-depositing Ti to a thickness of 20 nm and Al to a thickness of 300 nm on a 1 mm square portion of the end portion. Therefore, the optical waveguide 2d is perpendicular to the substrate end face 1a and the surface of the reflecting mirror 4.

The return light due to Fresnel reflection was reduced to 60 dB, and the fluctuation of the intensity of the interference light 2b became 1/30.
Therefore, this optical waveguide substrate can be sufficiently used for a displacement sensor.

[0016]

EFFECT OF THE INVENTION There is provided an optical waveguide substrate used in a displacement sensor capable of preventing generation of noise due to Fresnel reflection and enabling accurate displacement measurement.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing an embodiment of an optical waveguide substrate of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Electro-optic crystal substrate 1a End face for forming a reflection mirror of electro-optic crystal substrate 1b Side face of electro-optic crystal substrate 1c Input / output light side end face of electro-optic crystal substrate 2 Optical waveguide 2a Incident light optical waveguide 2b Interfering light optical waveguide 2c Optical Waveguide for Measurement Light 2d Optical Waveguide for Reference Light 3 Directional Coupler 4 Reflector 5 Object to be Measured L1 Laser Light L2 Measurement Light L3 Reference Light L4 Interference Light

Claims (1)

(57) [Claims] 1. An optical waveguide substrate for a displacement sensor, wherein an optical waveguide, a directional coupler, and a reflecting mirror are formed on a surface of an electro-optic crystal substrate, the optical waveguide substrate extending from the directional coupler to the reflecting mirror. For a displacement sensor, wherein an optical waveguide is formed in an oblique direction with respect to a side surface of the substrate, and an end surface of the substrate forming the reflecting mirror is a surface perpendicular to the oblique direction. Optical waveguide substrate.