JPH0684884B2 - Waveguide optical displacement sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Summary of the Invention Two branched optical waveguides are formed on a substrate having optical anisotropy, and these are used for reference light and signal light. Since the TE mode light and the TM mode light have different refractive indexes, the fact that the optical path lengths differ makes it possible to use the interference light between the TE mode reference light and the signal light and the interference light between the TM mode reference light and the signal light. By providing a constant phase difference between and, the displacement direction as well as the displacement direction can be identified.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Michelson interferometer manufactured by using an optical waveguide on a substrate, and a reference light reflected by a reflection surface on the substrate and a signal reflected by a reflection surface on an object to be measured outside the substrate. The present invention relates to a waveguide type optical displacement sensor that measures a displacement amount of an object to be measured based on a light intensity change due to interference with light.

The applicant of the present invention has already made some proposals for this type of waveguide type optical displacement sensor. For example, Japanese Patent Application Sho 60-14263
No. 7, Japanese Patent Application No. 60-142638 and Japanese Patent Application No. 60-142639. In particular, the latter two patent applications disclose a waveguide type optical displacement sensor capable of discriminating not only the displacement amount of the object to be measured but also its displacement direction. Two interference lights are required to determine the displacement direction of the object to be measured, and the phases of these interference lights must be deviated from each other by a certain angle (eg π / 4 or π / 2). .
Therefore, it is necessary to fabricate two optical interferometers on the substrate, and two optical fibers are required to extract two interference lights. Since another optical fiber is required to introduce the input light into the substrate, at least three optical fibers in total are required.

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

The waveguide type optical displacement sensor according to the present invention includes a reference light optical waveguide having a reflection surface formed on an end surface of a substrate having optical anisotropy, and a mirror surface which is emitted from the end surface and interlocks with an object to be measured. The optical waveguide for the projected signal light, the branch optical waveguide for branching the input light into these optical waveguides for reference light and signal light, and the reference light and the signal returned from the optical waveguides for reference light and signal light An optical waveguide for interference that interferes with light is fabricated so that the TE mode light propagating in these optical waveguides and the TM mode light have different refractive indices, and the intensity change of the TE mode interference light propagating in the optical waveguide for interference It is characterized in that the reference light optical waveguide and the signal light optical waveguide have different lengths so that a constant phase difference is generated between the intensity of the TM mode interference light and the intensity change of the TM mode interference light.

When the above-mentioned various optical waveguides are manufactured on the substrate by using the Y-branch optical waveguide, the branch optical waveguide and the interference optical waveguide will have a common part. The above-mentioned various optical waveguides can be manufactured on a substrate by 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 TM mode light into the branched optical waveguide on the substrate.
The light emitted from the interference optical waveguide may be separated into TE mode light and TM mode light by a polarization beam splitter.

The separated TE mode light and TM mode light are received by separate photoelectric conversion elements, and the change in light intensity is converted into an electrical signal. The amount of displacement of the measured object can be detected by binarizing these electric signals and counting the number of binarized pulses. In addition, since there is a constant phase difference between the light intensity change of TE mode light and the light intensity change of TM mode light, it is possible to determine the displacement direction of the measured object by the change order of these two received light signals. Becomes

According to the present invention, since TE mode light and TM mode light that do not interfere with each other are used, it is possible to not only measure the displacement amount but also determine the displacement direction by providing only one optical interferometer on the substrate. Therefore, the structure of the optical waveguide on the substrate can be simplified. When the Y-branch optical waveguide is used, it is possible to introduce the input light into the substrate and to derive the interference light with one optical fiber. Even when the asymmetric X-branch optical waveguide is used, a total of two optical fibers can be used. Since an optical fiber is sufficient, it is easy to adjust the optical axis. Further, the phase difference between the TE mode interference light and the TM mode interference light can be given by simply setting the lengths of the reference light optical waveguide and the signal light optical waveguide differently. Also, the configuration can be simplified.

Description of Embodiments FIG. 1 shows a main part of a waveguide type optical displacement sensor.

Z-cut LiNbO ₃ crystal is used as the substrate 1,
Ti film of optical waveguide pattern (thickness 300Å) on this substrate 1
After forming the wet O ₂ atmosphere for 5 hours at 1000 ° C. Ti
An optical waveguide is formed by thermally diffusing the light into the substrate 1. This optical waveguide is composed of an optical waveguide 12 for input and output (for optical interference), an optical waveguide 13 for reference light, an optical waveguide 14 for signal light, and a Y-branch optical waveguide 11. Are connected to the reference light and signal light optical waveguides 13 and 14 by the Y branch optical waveguide 11.

The reference light optical waveguide 13 has a distance L from the signal light optical waveguide 14.
However, the reflection portion 2 is provided at the end of the reference light optical waveguide 13. This reflection part 2 is an optical waveguide 13 on the substrate 1.
It can be configured by forming a slot by dry etching at the end position of and then depositing metal on the inner wall surface of this slot.

A lot lens 3 is provided between a mirror fixed to the object to be measured or coupled so as to interlock with the object and the end of the optical waveguide 14 for signal light on the substrate 1 (the end face of the substrate 1). ing. The lens 3 is for collimating the light emitted from the optical waveguide 14 to project it on the mirror 4, and collecting the reflected light to introduce it into the optical waveguide 14. Due to the presence of this lens 3, the end of the optical waveguide 14 and the mirror 4
Irrespective of the distance between the Y-branch optical waveguide 11 and the Y-branch optical waveguide 11
To propagate through the optical waveguide 14.

The intensity of the reference light reflected by the reflector 2 and returned to the Y-branch optical waveguide 11 and the intensity of the signal light reflected by the mirror 4 and propagated through the signal-light optical waveguide 14 and returned to the Y-branched optical waveguide 11 are substantially equal. It is preferable to adjust the reflectance of the reflecting portion 2 or the reflectance of the mirror 4 so that

A single-mode polarization-maintaining optical fiber (single polarization fiber) 21 on the end face of the optical waveguide 12.
Is connected. As shown in the enlarged view of FIG. 2, this polarization-maintaining optical fiber 21 has
Glass fibers 24 for stress generation are inserted in the support 23 at the upper and lower positions in the axial direction, and TE mode (y-axis direction) and TM mode (x-axis direction) light is polarized in one direction. Can be saved and propagated. Further, the optical fiber 21 is connected to the optical waveguide 12 in a state where the x and y axis directions thereof coincide with the Z and Y axis directions of the substrate crystal 1.

Therefore, the TE mode, T that has propagated in the optical fiber 21
Both M-mode light is introduced into the optical waveguide 12 and propagates through the optical waveguides 11, 13, 14 and the like.

Regarding light of one mode, for example, TE mode light, this light travels from the optical waveguide 12 to the Y-branch optical waveguide 11,
Here, the light beams are branched with equal intensity and proceed to the optical waveguides 13 and 14. The light propagating through the reference light optical waveguide 13 is reflected by the reflecting portion 2, travels in 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 is emitted from this optical waveguide 14 and passes through the rod lens 3 and the mirror 4
Head to. The light reflected by the mirror 4 is again the rod lens 3
After passing through the optical waveguide 14, the light enters the optical waveguide 14, propagates in the optical waveguide 14 in the opposite direction, and returns to the Y branch optical waveguide 11 (signal light). These reference light and signal light travel from the Y-branch optical waveguide 11 through 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 12 to the optical fiber 21 and is sent to the measurement system 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. The change of the output light intensity with respect to the displacement amount (optical path difference) is shown in FIG. 4 (for example, No. 1 waveform). The output light intensity changes sinusoidally with a cycle of λ (light wavelength) with respect to a change in optical path difference. Since the signal light reciprocates between the output end of the optical waveguide 14 and the mirror 4, the optical path difference is equal to twice the displacement amount of the mirror 4.

Similarly, with respect to the TE mode light, interference light (output light) having an intensity representing the displacement amount of the mirror 4 can be obtained.

By the way, since the Z-cut LiNbO ₃ substrate 1 gives different refractive indexes to the TE mode light and the TM mode light, the reference light optical waveguide 13 and the signal light optical waveguide 14 differ by L in their lengths. Since these are set as
Between the interference light of TE mode light and the interference light of TM mode light,
As indicated by No. 1 and No. 2 in Fig. 4, a certain phase difference will occur. The displacement direction of the mirror 4 can be discriminated by the change order of the two interference lights No. 1 and No. 2 to which a constant phase shift is given to each other.

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

The He-Ne laser (λ = 0.633 μm) is used as light, and the refractive index of the optical waveguide is given by the following equations for TM and TE mode light under normal temperature conditions.

Ne + ΔNe = 2.200 + 0.0175 = 2.2175 = [Ne] No + ΔNo = 2.286 + 0.0095 = 2.2955 = [No] If the optical waveguide lengths differ by L, the optical path lengths of TE mode light and TM mode light will be adjusted accordingly. Different.

The phase difference Δ between the lights in these two modes is Δ = 2πL ([No] − [Ne]) / λ = 2πL × 0.1233 (λ = 0.633 μm).

When the phase difference Δ is set to 2π / 8, the phase difference between the interference light (No. 1 and No. 2) is 2π / 4 because the light reciprocates in the optical waveguide in the configuration of FIG.

In this case, L is Δ = 2π / 8 = 2πL × 0.1233 and L = 1.014 μm.

If the phase difference between the two interference lights is 2π / 4, the displacement direction of the measured object can be determined.

When the phase difference between the two interference lights is 2π, the graphs of the interference light intensities overlap each other, and the displacement direction cannot be determined. When the phase difference is 2π, L = 8.113 μm.

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

L = 1.014 + m × 8.113 (μm) m is a positive integer ± 2μm It can be seen that the displacement direction can be sufficiently discriminated with an optical waveguide fabrication accuracy of about 0.2μm.

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

A He-Ne laser 31 is used as a light source. This laser light passes through the isolator 32 and the beam splitter 33, and enters the polarization-maintaining optical fiber 21 by the lens 34. The isolator 32 is for preventing the reflected light from returning to the laser 31. When the linearly polarized laser light is introduced into the optical fiber 21 with its polarization plane inclined by 45 ° with respect to the x axis and the y axis of the optical fiber 21, TE mode light and TM mode light propagating in the optical fiber 21 Of equal intensity. TE mode light and TM mode light are sent to the optical waveguide on the substrate 1 through the optical fiber 21.

The directions of the TE mode interference light and the TM mode interference light returning from the optical fiber 21 are converted by the optical beam splitter 33. These two interference lights having mutually orthogonal polarization planes are split by the polarization beam splitter 35 and received by the light receiving elements 36A and 36B, respectively.

These interference light signals are converted into electric signals by the light receiving elements 36A and 36B, respectively, and then level-discriminated and binarized by the circuits 37A and 37B having appropriate threshold levels. The rising and / or the falling of the binarized signal are counted by the counters 38A and 38B. Therefore, the displacement amount of the mirror 4 is measured in units of λ / 2 or λ / 4. λ
In the case of / 4, the amount of displacement can be measured with a resolution of about 0.16 μm. It goes without saying that the displacement amount can be measured using only one of the No. 1 or No. 2 (TE mode or TM mode) binarized signals shown in FIG.

On the other hand, the rising and / or falling of the binarized signal is detected by the differentiating circuits 39A and 39B, and the differentiated signal is input to the direction discriminating circuit 40.

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

In the above embodiment, the optical waveguide is formed on the substrate with the Y-branch optical waveguide as the center, but the present invention can be realized by using the asymmetric X-branch optical waveguide as shown in the above-mentioned prior application. .

[Brief description of drawings]

FIG. 1 shows an embodiment of the present invention, FIG. 1 is a perspective view showing a main part of a waveguide type optical displacement sensor, and FIG. 2 shows an arrangement of coupling between a polarization-maintaining optical fiber and an optical waveguide on a substrate. 3 and FIG. 3 are a configuration and a block diagram showing the configuration of the entire optical displacement measuring system, and FIG. 4 is a waveform diagram showing an interference light intensity signal and a binarized signal generated based on the interference light intensity signal. 1 ... Substrate, 2 ... Reflector, 4 ... Mirror interlocked with the object to be measured, 11 ... Y branch optical waveguide, 12 ... Input and output (for optical interference) optical waveguides, see 13 ... Optical waveguide for light, 14 ... Optical waveguide for signal light.

Claims (1)

[Claims] 1. A reference light optical waveguide having a reflection surface formed on an end surface of a substrate having optical anisotropy, and a signal light emitted from the end surface and projected on a mirror surface interlocking with an object to be measured. Optical waveguide, a branch optical waveguide that splits the input light into these reference light and signal light optical waveguides, and interference that interferes with the reference light and the signal light returned from the reference light and signal light optical waveguides. The optical waveguide for the optical waveguide was fabricated so that the TE mode light and the TM mode light propagating in these optical waveguides had different refractive indices, and the intensity change of the TE mode interference light propagating in the interference optical waveguide
A waveguide-type optical displacement sensor, characterized in that the reference light optical waveguide and the signal light optical waveguide have different lengths so that a constant phase difference is generated between the TM-mode interference light and the intensity change. .