JPH08261713A - Optical waveguide type displacement sensor - Google Patents

Abstract

PURPOSE: To make possible the attenuation of measurement light by improving the focusing characteristics of measurement light and without providing a reflection prevention film on the boundary surface between a substrate and a boundary surface in an optical waveguide type displacement sensor with an optical waveguide substrate and a refractive index distribution type lens. CONSTITUTION: One end face of an optical waveguide substrate A is formed as an inclined surface (1d) and an input/output terminal for measurement light is formed at the inclined surface (1d). One end face 6a of a refractive index distribution type lens 6 which is arranged so that a light axis center line matches a longer direction X of the optical waveguide substrate A is formed in parallel with the inclined surface (1d) and the other end face 6b is formed vertical to the light axis center line. A reflection prevention film 7 is formed on the end face 6b of the refractive index distribution type lens.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art As one of the methods for measuring the displacement of an object to be measured using an optical interferometer, there is an optical waveguide type displacement sensor utilizing the principle of Michelson interferometer. Forming a Michelson interferometer on an electro-optic crystal substrate such as lithium niobate (LiNbO ₃ ) or lithium tantalate (LiTaO ₃ ) eliminates the need for complicated optical system alignment, and allows for a small deformation meter. Can be realized. In the Michelson interferometer, when a boundary surface perpendicular to the optical axis of the measurement light exists, the reflected light from the boundary surface is mixed with the reflected light from the object to be measured, and the measurement accuracy is reduced.

Therefore, in the optical waveguide type displacement sensor, it is necessary to sufficiently suppress the reflected light from the boundary surface by making the input / output end surface for measurement light an inclined surface. However, the light emitted from the optical waveguide is not parallel to the longitudinal direction of the optical waveguide substrate due to refraction, and when the condenser lens is aligned with the optical axis of the emitted light, the incident direction of the measurement light on the DUT is increased. Does not coincide with the longitudinal direction of the optical waveguide substrate, making assembly difficult and positioning the optical waveguide displacement sensor with respect to the object to be measured.

In order to solve this problem, there is a method in which one end surface of the gradient index lens is obliquely polished. As such a technique, for example, the applicant of the present invention has a patent application 6-1
There is one proposed in 6544.

[0005]

However, in the above-mentioned method, when the sloped end surface of the gradient index lens is the surface facing the object to be measured, the aberration of the lens becomes large, and However, there is a problem in that the light condensing property of is deteriorated. The deterioration of the light converging property means that the lateral resolution at the time of measuring the displacement of the measured object cannot be obtained. Further, in the case where the inclined end surface of the gradient index lens is a surface facing the optical waveguide substrate, in order to suppress the aberration of the gradient index lens and keep the condensing characteristics good, It is necessary to match the optical axis with the optical axis center line of the gradient index lens.

For that purpose, a case is produced in which the optical waveguide substrate and the gradient index lens can be precisely aligned in three directions, that is, the longitudinal direction of the optical waveguide substrate, the direction perpendicular thereto, and the rotating direction of both. Therefore, there is a problem that the manufacturing cost of the case is increased.

Further, conventionally, in order to reduce the attenuation of the measurement light, it was necessary to provide an antireflection film on both the end faces of the optical waveguide and the gradient index lens. The formation of the antireflection film has a problem of increasing the cost of the product.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an optical waveguide type displacement sensor having an optical waveguide substrate and a gradient index lens, and a gradient index distribution type sensor. An optical fiber that is excellent in light-condensing characteristics of the measurement light by the lens and that can reduce the attenuation of the measurement light without providing an antireflection film on both slopes of the optical waveguide substrate and the gradient index lens. It is to provide a waveguide type displacement sensor.

[0009]

In order to achieve this object, an optical waveguide type displacement sensor according to the present invention has at least one end face in the longitudinal direction as an inclined face with respect to the longitudinal direction. And an optical waveguide substrate having a measuring light input / output end formed therein, and a gradient index lens arranged so that light can be transmitted between the measuring light input / output end of the optical waveguide substrate. The gradient index lens is arranged so that the optical axis center line thereof coincides with the longitudinal direction of the optical waveguide substrate. Further, of the end faces of the gradient index lens, the end face facing the optical waveguide substrate is parallel to the slope of the optical waveguide substrate, and the other end face is with respect to the optical axis center line of the gradient index lens. Is formed vertically, and an antireflection film is formed on the other end surface of the gradient index lens.

In a preferred embodiment of the present invention, the measuring light input / output optical waveguide makes an angle θ _W with respect to the normal line of the end face formed as the inclined surface of the optical waveguide substrate. Is formed as. n _W sin (θ _W ) = n ₁ sin (θ _t ) (1)

In the equation (1), n ₁ is the refractive index of the gradient index lens, n _W is the refractive index of the optical waveguide substrate,
θ _t is an angle formed by the normal line of the end face formed as the inclined surface of the optical waveguide substrate and the longitudinal direction of the optical waveguide substrate.

Further, it is preferable that the optical waveguide substrate and the gradient index lens are bonded with a transparent adhesive. Further, at least an optical waveguide and a directional coupler are formed on the surface of the optical waveguide substrate, and it is an inclined surface on which the measurement light input / output end is formed among both end faces in the longitudinal direction of the optical waveguide substrate. A reference light reflecting mirror is formed on the end face, and a light source light input end and an interference signal light output end are formed on the other end face.

[0013]

The light emitted from the light source is branched into two optical waveguides by the directional coupler in the optical waveguide substrate, and is guided to the end face formed as an inclined surface with respect to the longitudinal direction of the optical waveguide substrate. One of the lights is reflected by a reference light reflecting mirror provided on this end face, and returns to the directional coupler again as reference light. The other light is refracted at the end face formed as an inclined surface and is incident on the gradient index lens.

At this time, the angle θ _t formed by the normal line of the end face formed as the inclined surface of the optical waveguide substrate and the longitudinal direction of the optical waveguide substrate, and the optical waveguide for inputting / outputting the measuring light is the optical waveguide substrate. The angle θ _W formed with the normal to the end face formed as a slope is formed so as to satisfy the equation (1),
And, if the angle between the normal line of the end face of the gradient index lens facing the optical waveguide substrate end face and the longitudinal direction of the gradient index lens is equal to the angle θ _t ,
The measurement light obliquely emitted from the longitudinal direction of the optical waveguide substrate can be refracted at the end surface of the gradient index lens, and the traveling direction of the measurement light can be changed to a direction parallel to the longitudinal direction of the optical waveguide substrate. .

Further, the angle formed by the longitudinal direction of the optical waveguide substrate and the normal line of the end face (slope) of the optical waveguide substrate is such that the longitudinal direction of the gradient index lens and the end face of the gradient lens (slope).
Since it is the same as the angle formed by the normal line, the optical waveguide substrate and the gradient index lens can be closely adhered to each other. In this case, if a transparent resin such as an ultraviolet curable resin is used as the adhesive, the adhesive itself has a refractive index close to that of a gradient index lens, so that reflection is suppressed and both the optical waveguide substrate and the gradient index lens are suppressed. Even if the antireflection film on the slope is omitted, the transmittance can be kept high.

[0016] Here, the reflected return light intensity from the optical waveguide substrate end face adversely affect the displacement measurement accuracy largely depends on the angle theta _W of the optical waveguide and the optical waveguide substrate end face (slope), the angle theta _W The larger the value, the more effectively the reflected return light is attenuated. When the intensity of the reflected return light is measured by changing the angle θ _W with the optical waveguide formed on the lithium niobate substrate by the proton exchange method, the relationship as shown in FIG. 3 is obtained.
As can be seen from FIG. 3, the angle θ _W is preferably 5 degrees or more so that the reflected return light intensity can be suppressed to −50 dB or less, which does not adversely affect the displacement measurement accuracy.

However, according to the equation (1), the angle θ
_{As W} increases, the angle θ _t formed by the normal to the longitudinal direction of the optical waveguide substrate and the optical waveguide substrate end face (slope) also increases. When the angle θ _t becomes extremely large, the aberration becomes remarkable, the condensing characteristic of the measurement light L is deteriorated, and the resolution in the lateral direction is deteriorated. Therefore, it is desirable that the angle θ _t be the minimum necessary angle. A prism (having the same shape as the gradient index lens and having no refractive index distribution) that gives an angle θ _w and an angle θ _t that satisfies Expression (1), and the measurement light emitted from the prism are The aberration due to prism hardening of the gradient index lens can be evaluated from the blur of the focus when an aberration-free lens for focusing is assumed. Then, when a computer simulation by the ray tracing method was performed assuming such an optical system, the result as shown in FIG. 4 was obtained. However, the divergence angle of the measurement light emitted from the end face of the optical waveguide substrate is half-valued to 0.
The distance from the end face of the optical waveguide substrate to the prism was 1 mm, and the refractive index of the prism was 1.557. For example, in order to reduce the blur to 1 μm or less under such a condition, the angle θ _t must be 13 degrees or less. Also,
Here, the smaller the distance from the end face (slope) of the optical waveguide substrate to the prism, the smaller the aberration. Therefore, the adhesion of the optical waveguide substrate and the gradient index lens is effective from the viewpoint of aberration.

The measuring light reaching the other end surface of the gradient index lens is emitted toward the object to be measured, and the light reflected from the object to be measured passes through a path opposite to the above-mentioned path to the optical waveguide substrate. Incident. The measurement light returned to the optical waveguide substrate interferes with the reference light, and the interference light is emitted from the optical waveguide substrate toward the light receiving element. The displacement of the object to be measured can be measured by detecting the intensity change of the interference light.

[0019]

1 and 2 show an embodiment of an optical waveguide type displacement sensor according to the present invention. The optical waveguide substrate A in this embodiment is made of lithium niobate (LiNbO ₃ ) or lithium tantalate (L
It is composed of an electro-optic crystal substrate 1 such as iTaO ₃ ) and optical waveguides 2 and 3 are formed on the substrate 1. A modulation electrode 8 is provided on the optical waveguide 3. The optical waveguide substrate A has two end faces 1a and 1b parallel to the longitudinal direction X and two end faces 1c and 1d that are obliquely polished with respect to the longitudinal direction X. The optical waveguide 3 reaches the end face 1d at a right angle to the end face 1d, and the end face 1d is provided with a reflecting mirror 5 made of a metal or a dielectric capable of efficiently reflecting the light of the optical waveguide 3. . The optical waveguide 2 is formed at an angle with respect to the end face 1d. A directional coupler 4 is arranged in the optical waveguides 2 and 3.

In this embodiment, a SELFOX lens (trade name) 6 is used as the gradient index lens. The SELFOX lens 6 is arranged such that its optical axis center line coincides with the longitudinal direction X of the optical waveguide substrate A, and the SELFOX lens 6 is bonded to the optical waveguide substrate A with a transparent adhesive. The cell fox lens 6 has an end face 6a having the same inclination angle as the end face 1d (slope) of the optical waveguide substrate A and an end face 6b perpendicular to the optical axis. An antireflection film 7 is formed on the end surface 6b of the SELFOX lens 6. The on-axis refractive index of the SELFOX lens 6 in this example is 1.557, and the refractive index distribution constant is 0.059049.

The refractive index of the lithium niobate substrate is about 2.2.
The SELFOC lens 6, which is a kind of gradient index lens, has a refractive index of about 1.6. According to the equation (1), when the angle θ _W is 5 degrees or more, the angle θ _t is 6.88 degrees or more, and when the angle θ _t is 13 degrees or less, the angle θ _W is 9.4.
Although it may be set to 2 degrees or less, the angle θ _t is 8 degrees and the angle θ _W is 5 degrees in consideration of the ease of processing the end faces 1d and 6a of the optical waveguide substrate A and the SELFOC lens 6. 8
Set in degrees. When the measurement light L is emitted from the optical waveguide substrate A having the end face 1d set in this way, the condensing characteristics when refracting and condensing with the SELFOC lens 6 were examined by computer simulation by the ray tracing method. .

FIG. 5 shows the vicinity of the focal point when the angle of the incident side end face 6a which is the end face of the SELFOC lens 6 on the optical waveguide substrate side is 0 ° and the angle of the output side end face 6b which is the DUT side is 8 °. FIG. 6 shows light rays, and FIG. 6 shows light rays near the focal point when the angle of the incident side end face 6a is 8 degrees and the angle of the outgoing side end face 6b is 0 degree. However, in this simulation, the divergence angle of the measurement light L emitted from the end face of the optical waveguide is set to 0.2 rad instead of the measured value of 0.05 rad for ease of calculation, but the tendency of the simulation result by the above-described operation is It doesn't change. The displacement of the best image plane in FIG. 6 is 50 μm or less in the optical axis direction, and the size of the beam waist on which the measurement light is most condensed is 3 μm or less, whereas in FIG. 3 in direction
There is a variation of about 00 μm, and the size of the beam waist spreads to 10 μm or more. This result shows the usefulness of the method of processing the incident side end surface of the gradient index lens element into a sloped surface.

As described above, the SELFOX lens 6
Since the emission side end face 6b of is formed perpendicular to the optical axis,
An antireflection film 7 is formed on the emission side end face 6b. Reflected return light of about 0.2% (-27 dB) is generated from the emission side end face 6b due to the limit of the characteristics of the antireflection film 7. If the detection limit of the signal light is −50 dB with respect to the reference light,
When all of this reflected return light amount is coupled to the optical waveguide, the measurement accuracy is significantly deteriorated, but the measurement light L is condensed from the exit end face 6b with a working distance of about 0.6 to 1.4 mm. If a self-focusing lens 6 having a pitch (the meandering period of light in the lens) of 0.4 to 0.45 is used in the condensing system, the reflected return light is reflected in the self-focusing lens 6 as shown in FIG. After being condensed once, it reaches the end face of the optical waveguide as diffused light.
The amount of reflected return light coupled to 3 is suppressed to the extent that it does not affect the measurement.

Both end surfaces 1 of the optical waveguide substrate A in the longitudinal direction X
Parts c and 1d for protecting the end surfaces called end surface blocks were loaded and polished so that both end surfaces 1c and 1d had an inclination angle of 8 degrees. An optical fiber for inputting and outputting an optical signal was connected to the end face 1c. When the LD light source was made incident on this input optical fiber and the light intensity emitted from the end face 1d having an inclination angle of 8 degrees was measured, a measured light output of about 50 μW could be confirmed.

Next, the cellfox lens 6 having a length of 11 mm (pitch 0.42) is attached to the end surface 1d of the optical waveguide substrate A.
It was adhered (at an inclination angle of 8 degrees). Self fox lens 6
Although the positioning was performed by observing with a microscope, the positioning in the direction perpendicular to the longitudinal direction X is sufficient to bring the optical waveguide substrate A and the SELFOX lens 6 into close contact with each other. An ultraviolet curable resin was used as the adhesive. The refractive index of the ultraviolet curable resin is about 1.5, and the reflection at the interface between the adhesive and the lens has a small difference in the refractive index and can be almost ignored. The transmittance T at the interface between the optical waveguide substrate A and the adhesive can be theoretically calculated by the equation (2), and is about 96.4%.
Becomes _{T = 1- (n w -n a} ) 2 / (n w + n a) 2 (2) In the equation (2), n _w is the refractive index of the optical waveguide substrate, n _a
Is the refractive index of the ultraviolet curable resin. The transmittance of the end surface of the lithium niobate substrate without the antireflection film 7 is about 86.
%, So that the optical waveguide substrate A and the SELFOX lens 6 are brought into close contact with each other, and are adhered with a transparent adhesive,
An improvement of about 10% can be achieved. Actually, since the measurement light L reciprocates and passes through the boundary surface between the optical waveguide substrate A and the adhesive, an improvement of about 20% can be achieved.

Laser light is input to the optical waveguide 2 desired on the end face 1c of the optical waveguide substrate A by an optical fiber, while
The interference light is guided from the optical waveguide 3 to the measuring device by an optical fiber. Next, the modulation electrode 8 was electrically connected by bonding. Furthermore, the optical waveguide substrate A
The optical waveguide substrate A and the SELFOX lens 6 as a whole were housed in a stainless case in order to protect the SELFOX lens 6 from mechanical impact. In this case, since only one direction is required to bring the two into close contact with each other, it is sufficient to use an inexpensive case that does not require high processing accuracy.

When the displacement of the object to be measured was measured by performing phase modulation with the optical waveguide type displacement sensor thus manufactured,
The measurement accuracy was as good as several nm or less. This suggests experimentally that unnecessary reflected return light is sufficiently suppressed. Further, in order to evaluate the light condensing characteristics, the line / space formed on the lithium niobate substrate was measured, and it was confirmed that there was a lateral resolution of 4 μm or less. This indicates that the measurement light is effectively condensed to about 4 μm, and that the present invention has realized the lateral resolution characteristic of 4 μm or less.

[0028]

As described above, according to the present invention, it is possible to provide a highly accurate optical waveguide type displacement sensor having excellent light condensing characteristics, that is, lateral resolution. Furthermore, in the optical waveguide displacement sensor according to the present invention, it is not necessary to provide an antireflection film on both slopes of the optical waveguide substrate and the gradient index lens, and the attenuation of the measurement light can be reduced at a low manufacturing cost. it can.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of an embodiment of an optical waveguide type displacement sensor according to the present invention.

FIG. 2 is an enlarged explanatory view of a joint portion between the optical waveguide substrate and the gradient index lens in FIG.

FIG. 3 is a graph showing a change in reflected return light when an angle θ _{W of the} optical waveguide with respect to a normal line of the end surface (slope) of the optical waveguide substrate is changed.

FIG. 4 is a graph showing aberration due to a prism effect of a gradient index lens.

FIG. 5 is a graph showing a computer simulation by a ray tracing method of a condensing characteristic of a light beam obliquely emitted from an optical waveguide when the emission surface of the SELFOX lens is polished to 8 degrees.

FIG. 6 is a graph showing a computer simulation by a ray tracing method of a condensing characteristic of a light beam obliquely emitted from an optical waveguide when the incident surface of the SELFOX lens is polished to 8 degrees.

FIG. 7 is a graph showing a computer simulation by a ray tracing method of reflected return light from the end face of the SELFOX lens perpendicular to the optical axis.

[Explanation of symbols]

 A optical waveguide substrate 1 electro-optical crystal substrate 1a, 1b, 1c, 1d optical waveguide substrate end face 2,3 optical waveguide 4 directional coupler 5 reflecting mirror 6 cellfox lens 6a, 6b cellfox lens 7 antireflection film L measurement light

Claims (4)

[Claims] 1. An optical waveguide substrate in which at least one end face in the longitudinal direction is inclined with respect to the longitudinal direction, and a measuring light input / output end is formed on the inclined face, and the measurement of the optical waveguide substrate. A gradient index lens arranged so that light can be transmitted to and from the light input / output end, and the optical axis center line of the gradient index lens coincides with the longitudinal direction of the optical waveguide substrate. The end surface of the gradient index lens facing the optical waveguide substrate is parallel to the slope of the optical waveguide substrate, and the other end surface is the optical axis of the gradient index lens. An optical waveguide type displacement sensor, which is formed perpendicularly to a center line, and an antireflection film is formed on the other end surface of the gradient index lens. 2. An angle θ _t formed by a normal line of an end face formed as a sloped surface of the optical waveguide substrate and a longitudinal direction of the optical waveguide substrate, and an optical waveguide for inputting / outputting measurement light is a sloped surface of the optical waveguide substrate. Angle θ with the normal to the end face formed as
_W is formed so as to satisfy the expression (1), and the normal line of the end face of the gradient index lens facing the end face of the optical waveguide substrate and the longitudinal direction of the gradient index lens are formed. The optical waveguide displacement sensor according to claim 1, wherein the angle is equal to the angle θ _t . n _W sin (θ _W ) = n ₁ sin (θ _t ) (1) where n ₁ is the refractive index of the gradient index lens and n _W is the refractive index of the optical waveguide substrate. 3. The optical waveguide type displacement sensor according to claim 1, wherein the optical waveguide substrate and the gradient index lens are bonded to each other with a transparent adhesive. 4. An optical waveguide and a directional coupler are formed on at least a surface of the optical waveguide substrate, and a measurement light input / output end is formed on both end faces in the longitudinal direction of the optical waveguide substrate. 4. The reference light reflecting mirror is formed on the inclined end surface, and the light source light input end and the interference signal light output end are formed on the other end surface. An optical waveguide type displacement sensor according to item 1.