JPS6347602A - Wave guide type optical displacement sensor - Google Patents

Abstract

PURPOSE:To discriminate not only the displacement quantity of a body to be measured but also the displacement direction thereof, by forming two branched wave guides to a substrate having optical anisotropy as ones for reference light and signal light and providing definite phase difference between interference lights respectively generated in a TE mode and a TM mode. CONSTITUTION:A Ti-film becoming a wave guide pattern is deposited on a substance composed of a Z-cut LiNbO3 crystal and thermally diffused in a wet O2-atmosphere to form a wave guide for input/output 12, a wave guide 13 for reference light, a wave guide 14 for signal light and a Y-branch wave guide 11. Next, a reflecting part 2 is provided on the terminal of the wave guide 13 and a mirror 4 is provided on the terminal of the wave guide 14 through a rod lens 3 and a plane-of-polarization preserving optical fiber 21 of a single mode is connected to the incident end of the wave guide 12. by this constitution, both TE and TM mode lights propagating through the fiber 21 are allowed to propagate through the wave guides 12, 11, 13, 14 to obtain interference light having intensity corresponding to the intensity difference between reference light and signal light.

Description

DETAILED DESCRIPTION OF THE INVENTION Summary of the Invention Two optical waveguides branched into two are fabricated on a substrate having optical anisotropy, and these are used for reference light and signal light. This method takes advantage of the fact that TE mode light and TM mode light have different optical path lengths because they have different refractive indices.

By creating a certain phase difference between the interference light between the reference light and signal light in TE mode and the interference light between reference light and signal light in TM mode, not only the total displacement but also the direction of displacement can be determined. I did it like that.

BACKGROUND OF THE INVENTION Technical Field This invention fabricates a Michelson interferometer using an optical waveguide on a substrate, and detects light reflected from a reflective surface on the substrate and a signal reflected from a reflective surface on an object to be measured outside the substrate. The present invention relates to a waveguide type optical displacement sensor that measures the amount of displacement of a fixed object based on changes in light intensity due to interference with light.

The applicant of the prior invention has already made several proposals regarding this type of waveguide type optical displacement sensor. for example.

Patent Application No. 1983-142837, Patent Application No. 14263-1983
No. 8, Japanese Patent Application No. 6O-142H9, etc. especially,
The latter two patent applications disclose a waveguide type optical displacement sensor that can determine not only the amount of displacement of an object to be measured but also the direction of displacement. Two interference lights are required to determine the direction of displacement of the object to be measured.

Furthermore, the phases of these interference lights must be shifted from each other by a certain angle (for example, π-/4 or π/2). Therefore, it is necessary to fabricate two systems of optical interferometers on the substrate, and two optical fibers are required to extract the two interference lights. One more optical fiber is required to introduce the input light into the substrate, so at least three optical fibers in total are required.

SUMMARY OF THE INVENTION The present invention provides a waveguide type optical displacement sensor with a simpler configuration.

The waveguide type optical displacement sensor according to the present invention includes an optical waveguide for a reference light having a reflective surface formed on the end surface on a substrate having optical anisotropy, and a mirror surface that is emitted from the @ surface and interlocks with the object to be measured. an optical waveguide for the signal light projected onto the optical waveguide, a branching optical waveguide for branching the input light into the reference light and signal light optical waveguides, and a reference light and the reference light returned from the reference and signal light optical waveguides. Interference optical waveguides that interfere with the signal light are fabricated so that the refractive index for the TE mode light and the 7M mode light propagating through these optical waveguides are different, and the intensity of the TE mode interference light propagating through the interference optical waveguide is It is characterized in that the lengths of the reference light optical waveguide and the signal light optical waveguide are made different so that a certain phase difference occurs between the intensity change and the intensity change of the 7M mode interference light.

When the various optical waveguides described above are fabricated on a substrate using a Y-branch optical waveguide, the nine-branch optical waveguide and the interference optical waveguide will have common parts.

The various optical waveguides described above can also be fabricated on a substrate using a so-called asymmetric X-branch optical waveguide.

A polarization maintaining optical fiber may be used to introduce the TE mode light and the 7M mode light into the branched optical waveguide on the substrate. In addition, the light emitted from the interference optical waveguide is a polarized beam.
It is sufficient to separate the light into TE mode light and 7M mode light using a splitter.

The separated TE mode light and 7M mode light are received by separate photoelectric conversion elements, and changes in the light intensity are converted into electrical signals. By binarizing these electrical signals and counting the number of binarized pulses, the amount of displacement of the object to be measured can be detected.

Furthermore, since there is a certain phase difference between the light intensity changes of the TE mode light and the light intensity changes of the 7M mode light, these two
The direction of displacement of the constant object can be determined based on the order of changes in the two received light signals.

According to this invention, TE mode light that does not interfere with each other and 7
Since M-mode light is used, just by installing one optical interferometer on the board, it is possible to not only measure the amount of displacement but also determine the direction of displacement, and the configuration of the optical waveguide on the board becomes simple. When using a Y-branch optical waveguide, a single optical fiber can introduce the input light into the substrate and extract the interference light, and when using an asymmetrical X-branch optical waveguide, a total of two optical fibers can be used. Since an optical fiber is sufficient, optical axis adjustment etc. are also facilitated. Furthermore, the phase difference between the TE mode interference light and the 7M mode interference light can be provided by simply setting the lengths of the reference light optical waveguide and the signal light optical waveguide to be different. The configuration can also be simplified.

Description of examples FIG. 1 shows the main parts of a waveguide type optical displacement sensor.

A Z-cut LiNbo3 crystal is used as the substrate 1, and after forming an optical waveguide pattern Ti film (thickness: 300 mm) on this substrate 1, it is exposed to a wet 02 atmosphere.
The optical waveguide is formed by thermally diffusing Ti into the substrate 1 at 100° C. for 5 hours. This optical waveguide includes nine optical waveguides 12 for human power and output (for optical interference), and an optical waveguide 13 for reference light.

It is composed of a signal light optical waveguide 14 and a Y-branch optical waveguide 11, and the optical waveguide 12 is connected to reference light and signal light optical waveguides 13 and 14 by the Y-branch optical waveguide 11.

The reference light optical waveguide 13 is shorter in distance than the signal light optical waveguide 14, and a reflection section 2 is provided at the end of the reference light optical waveguide 13. This reflecting section 2.

It can be constructed by forming a slot at the end position of the optical waveguide 13 on the substrate 1 by dry etching and depositing metal on the inner wall surface of the slot.

Fixed to the object to be measured. Alternatively, a rot lens 3 is provided between the mirror coupled to the mirror and the terminal end of the signal light optical waveguide 14 on the substrate 1 (the end surface of the substrate 1). This lens 3 is for collimating the light emitted from the optical waveguide 14, projecting it onto the mirror 4, condensing the reflected light, and introducing it into the optical waveguide L4. Due to the presence of this lens 3, a substantially constant amount of signal light (reflected light) always returns to the optical waveguide 14 and toward the Y-branch optical waveguide 11, regardless of the distance between the end of the optical waveguide 14 and the mirror 4. The light then propagates through the optical waveguide 14.

The intensity of the reference light that is reflected by the reflection unit 2 and returns to the Y-branch optical waveguide 11 is approximately equal to the intensity of the signal light that is reflected by the mirror 4, propagates through the signal light optical waveguide 14, and returns to the Y-branch optical waveguide 11. It is preferable to adjust the reflectance of the 1 reflecting section 2 or the reflectance of the mirror 4 so that the angle .

A single mode polarization maintaining optical fiber (single polarization fiber) is installed at the end face of the optical waveguide 12.
21 is connected. As shown in an enlarged view in FIG. 2, this polarization-maintaining optical fiber 21 has stress-generating glass fibers 24 inserted into its support 23 at upper and lower positions in the X-axis direction of the central core 22. Became TE
Mode (X-axis direction) and 7M mode (X-axis direction) light can be propagated while maintaining the plane of polarization in one direction. Further, the optical fiber 21 is arranged in such a state that the above-mentioned X and Y axis directions' coincide with the Z and Y axis directions of the substrate crystal 1.
It is connected to the optical waveguide 12.

Therefore, both the TE mode and 7M mode light propagated in the optical fiber 21 are introduced into the optical waveguide 12 and propagate through the optical waveguides 11, 13, 14, etc.

Regarding light in one mode, for example, TE mode light, this light travels from the optical waveguide 12 to the Y-branch optical waveguide 11, where it is split with equal intensity and passes through the optical waveguides 13 and 1.
Proceed to 4. The light propagating through the reference light optical waveguide 13 is reflected by the reflection section 2, travels through the optical waveguide 13 in the opposite direction, and returns to the Y-branch optical waveguide 11 (reference light). The light that has proceeded to the signal light optical waveguide 14 exits from this optical waveguide 14, passes through the rod lens 3, and heads toward the mirror 4. The light reflected by the mirror 4 passes through the rod lens 3 again and enters the optical waveguide 14, propagates through this optical waveguide 14 in the opposite direction and enters the Y-branch optical waveguide 11.
Return to (signal light). These reference light and signal light travel from the Y-branch optical waveguide 11 to the optical waveguide 12, where they interfere with each other, and interference light having an intensity corresponding to the phase difference between the reference light and the signal light is obtained. This interference light (output light) travels from the optical waveguide 2 to the destination fiber 21 and is sent to a measurement system to be described later. The phase difference between the reference light and the signal light depends on the optical path difference between these two lights, that is, the amount of displacement of the mirror 4.

FIG. 4 shows changes in the output light intensity with respect to the amount of displacement (optical path difference) (for example, waveforms No. and I). The output light intensity changes sinusoidally with a period of λ (light wavelength) with respect to changes in the optical path difference. The signal light is transmitted between the output end of the optical waveguide 14 and the mirror 4.
The optical path difference is equal to twice the amount of displacement of the mirror 4.

Similarly, for the TE mode light, interference light (output light) having an intensity representing the amount of displacement of the mirror 4 is obtained.

By the way, since the Z-cut LiNbO3 substrate 1 gives different refractive indexes to the TE mode light and the 7M mode light, the reference light optical waveguide 13 and the signal light optical waveguide 14 are made to have different lengths. Since the interference light of the TE mode light and the interference light of the TM mode light are set to 1 to 1 in FIG. No, 2
As shown in , a certain phase difference will occur. Two interference lights No. 1 to which a certain phase shift is given to each other. The direction of displacement of the mirror 4 can be determined based on the order of changes in No. and 2.

The difference in length between the optical waveguides 13 and 14 can be set as follows.

He-Ne laser (λ-0,833am) as light
At room temperature, the refractive index of the optical waveguide is
For TM and TE mode light, the following formula % formula % [[ If there is a difference of L in the optical waveguide length, TE mode light and 7
The optical path length of the M mode light varies accordingly.

The phase difference Δ between these two modes of light is.

Δ−2πL ([No] − [Nel)/λ= 2
πL x 0.1233 (λ-0,833 μm).

When the phase difference Δ is 2π/8, in the configuration shown in Fig. 1, the light travels back and forth through the optical waveguide, so interference light (No.
) is 2π/4.

L in this case is.

Δ−2π/8 -2πLx 0.1233 Than L = 1.014μm becomes.

If the phase difference between the two interference lights is 2π/4, it is possible to determine the displacement direction of the object to be measured.

When the phase difference between the two interference lights is 2π, the graphs of the intensities of these interference lights overlap, making it impossible to determine the direction of displacement. When the phase difference is 2π.

L = 8.113 μm.

Therefore, the difference in optical waveguide length for the phase difference between the two interference lights to be 2π/4 is finally given by the following equation.

It will be understood that L-1,014+mX 8.113 (μm) m is a positive integer with an optical waveguide fabrication accuracy of about ±0.2 μm, which makes it possible to sufficiently determine the displacement direction.

Now, the overall operation will be explained with reference to FIGS. 3 and 4.

A He-Ne laser 31 is used as a light source. This laser light passes through an isolator 32 and a beam splitter 33, and enters the polarization maintaining optical fiber 22 through a lens 34. The purpose of the isolator 32 is to prevent reflected light from returning to the laser 31. When a linearly polarized laser beam is introduced into the optical fiber 21 with its plane of polarization inclined by 45@ with respect to the X-axis and y-axis of the optical fiber 21, TE mode light and 7M mode light propagating through the optical fiber 21 are generated. The intensities of will be equal. TE mode light and 7M mode light are sent to the optical waveguide on the substrate 1 through this optical fiber 21.

The TE mode interference light and T
The direction of the M-mode interference light is changed by the optical beam splitter 33. These two interference lights with mutually orthogonal polarization planes are separated by a polarizing beam splitter 35,
The light is received by light receiving elements 38A and 38B, respectively.

These interfering optical signals are converted into electric signals by light receiving elements 38A and 36B, respectively, and then level-discriminated and binarized by circuits 37A and 37B having appropriate threshold levels, respectively. The rising edge and/or falling edge of this binary signal is detected by the counter 38A.

It is counted by 3813. Therefore, the amount of displacement of the mirror 4 is determined in units of λ/2 or λ/4.

In the case of λ/4, the amount of displacement can be measured with a resolution of about 0.16 μm. N001 or N shown in Figure 4
It goes without saying that the amount of displacement /IPj can be determined using only one of the binary signals of 0 and 2 (TE mode or 7M mode).

On the other hand, the rise and/or fall of the above binary signal is detected by differentiating circuits 39A and 39B, and this differentiated signal is input to direction determining circuit 40.

When the mirror 4 moves in the direction away from the substrate 1, for example, the rising edge of the output signal of the threshold circuit 37A (
The falling edge) is the same day as the rising edge (falling edge) of the output signal of path 37B.
, and when the mirror 4 moves in the opposite direction, the order of change of these two signals is reversed. A determining circuit 40 determines the moving direction of the mirror 4 based on the order in which changes in the outputs of the two differentiating circuits 39A and 39B appear. The direction determination circuit 40 can also be configured by a CPU.

In the embodiment described in , the Y-branch optical waveguide is the main focus. ! Although a plated optical waveguide is formed, the present invention can also be realized using an asymmetrical X-branched optical waveguide as shown in the above-mentioned prior application.

[Brief explanation of the drawing]

The drawings show embodiments of the present invention, with Fig. 1 being a perspective view showing the main parts of a waveguide type optical displacement sensor, and Fig. 2 showing the arrangement of coupling between a polarization-maintaining optical fiber and an optical waveguide on a substrate. 3 is a configuration and block diagram showing the overall configuration of the optical displacement measurement system, and FIG. 4 is a waveform diagram showing an interference light intensity signal and a binary signal created based on the interference light intensity signal. 1... Board. 2... Reflective part. 4...Mirror linked to the object to be measured. 11...Y branch optical waveguide. 12... Optical waveguide for input and output (for optical interference). 13... Optical waveguide for reference light. 14... Optical waveguide for signal light. Patent applicant Tateishi Electric Co., Ltd. Representative Patent attorney Kenji Ushiku (1 other person)

Claims (1)

[Scope of Claims] An optical waveguide for a reference light having a reflective surface formed on an end face on a substrate having optical anisotropy, and a signal light emitted from the end face and projected onto a mirror surface interlocking with a measured object. A branching optical waveguide that branches the input light into these optical waveguides for reference light and signal light, and a branching optical waveguide that causes the reference light and signal light returned from the optical waveguides for reference light and signal light to interfere with each other. Interference optical waveguides are fabricated to have different refractive indexes for TE mode light and TM mode light propagating through these optical waveguides, and intensity changes of TE mode interference light and TM mode interference light propagating through the interference optical waveguides are A waveguide type optical displacement sensor, characterized in that a reference light optical waveguide and a signal light optical waveguide have different lengths so that a certain phase difference occurs between the intensity change of the reference light and the signal light.