JPH0319937B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical pressure sensor that detects changes in pressure by converting them into changes in light intensity.

Conventionally, mechanical measuring devices using metal diaphragms and Bourdon tubes, as well as electrical measuring devices using various strain gauges or utilizing the pressure change characteristics of semiconductor pn junctions, have been widely used for pressure measurement. has been done. However, mechanical pressure measuring devices have a drawback in that they have hysteresis and cannot perform highly accurate measurements. For electrical measurements, pressure sensors, in which a diaphragm is formed by anisotropic etching on a silicon single crystal, and a strain gauge is created on the diaphragm by diffusion, are often used as highly accurate and hysteresis-free sensors. I'm getting used to it. However, this semiconductor sensor has problems such as reduced measurement accuracy in an environment where electromagnetic noise is present, the risk of fire or explosion due to electric sparks, and the tendency for metal conductors to corrode depending on the atmosphere. Therefore, measurement devices that optically measure pressure changes have been in the spotlight as measurement devices that solve these problems. As a pressure sensor using light, a silicon chip forming a diaphragm and an optical fiber are arranged facing each other, and pressure changes are converted into displacement of the silicon diaphragm.
Furthermore, a method is known in which this positional change is converted into a change in the amount of reflection of light emitted from the optical fiber, and the light is propagated in the opposite direction through the fiber again. However, in this optical pressure sensor, the relative positional relationship between the silicon chip and the optical fiber must be maintained constant and with high precision, making assembly extremely difficult, and the drawback is that the change in the amount of light in response to pressure changes is small. be.

It is an object of the present invention to provide an optical pressure sensor that can obtain a large change in the amount of light even in response to a slight change in pressure and can perform highly accurate pressure measurement, and that is simple in structure and easy to manufacture.

The optical pressure sensor according to the present invention includes at least one input/output single mode optical waveguide and a pair of pressure detection single mode optical waveguides each having one end coupled to one end of the input/output optical waveguide. means for respectively reflecting the light waves propagating through the pressure detection optical waveguide at the other end thereof, and applying a pressure to be detected onto at least one of the pair of pressure detection optical waveguides to change the phase constant of the optical waveguide. It is characterized by consisting of means.

When the pressure to be detected is applied to one of the pressure detection optical waveguides, the phase constant of the optical waveguide changes in the portion where the pressure is applied. The light waves input to the input/output optical waveguide are branched to a pair of pressure detection optical waveguides, propagated, reflected by the reflecting means, propagated in the opposite direction, and then enter the input/output optical waveguide again where they are superimposed. Ru. Since the phase constant of one of the pressure detection optical waveguides changes depending on the pressure to be detected, a phase difference occurs between the light waves propagating through the pair of pressure detection optical waveguides. Since the intensity of the light emitted from the input/output optical waveguide changes depending on this phase difference, the pressure to be detected can be determined by measuring the intensity of the emitted light.

It is also possible to detect the difference between these pressures by applying pressures of different magnitudes to both pressure detection optical waveguides.

In this invention, since the change in phase constant due to pressure in the optical waveguide is utilized, pressure can be detected with high accuracy. Moreover, the structure is simple, and manufacturing is facilitated by simply providing an optical waveguide structure formed on a substrate, a means for pressurizing a part of the structure, and a reflection means.

Before explaining the embodiments of the present invention, a special optical waveguide structure used in the embodiments will be explained.

In FIG. 1, a pair of single mode optical waveguides 1
and 2 intersect at one end at a small angle θ1. These optical waveguides 1 and 2 have the same width W1,
W2, and therefore the phase constants are set equal. There is another pair of single mode optical waveguides 3 and 4, which also intersect at one end at a small angle θ2. Optical waveguide 3
The width of the optical waveguide 4 is different from the widths W3 and W4 of the optical waveguide 4.
W4 is narrower than the width W3 of the optical waveguide 3.
Therefore, the phase constants of optical waveguides 3 and 4 are different, and optical waveguide 3 is larger. The optical waveguides 1 and 2 and the optical waveguides 3 and 4 are coupled at their respective intersections so that these optical waveguides are substantially straight. This joint portion is designated by the reference numeral 5.
For convenience of explanation, XYZ coordinate axes are taken, with the direction from the optical waveguides 1 and 2 toward the optical waveguides 3 and 4 being the Z axis, and the direction perpendicular to the plane of the paper being the X axis. Moreover, the optical waveguides 1 and 2 are called the symmetric side, and the optical waveguides 3 and 4 are called the asymmetric side.

For simplicity, consider a two-dimensional structure with no change in the X direction. In addition, both of the two crossing angles θ11 and θ2 are sufficiently small, the light wave travels approximately in the Z direction, and the distance between optical waveguides 1 and 2 and the distance between optical waveguides 3 and 4 changes with respect to minute changes in the Z direction. Changes are assumed to be negligible. That is, when considering a minute section excluding the coupling portion 5, it can be considered that there are two parallel optical waveguides and a uniform five-layer structure is formed in the Y direction. In such a case, a local normal mode analysis method can be applied.

As is well known, the eigenmodes of a five-layer optical waveguide consisting of two single mode optical waveguides are:
There are two types: even mode and odd mode. Figure 2a,
Part b shows the propagation states of even mode and odd mode in this five-layer optical waveguide structure.
FIG. 2c shows how the phase constants of the even and odd modes of this five-layer optical waveguide structure change.
On the symmetric side consisting of the optical waveguides 1 and 2, at a position sufficiently far from the coupling part 5 and where the distance between the optical waveguides 1 and 2 is wide, the coupling between the optical waveguides 1 and 2 can be ignored, so the two eigenmodes are degenerated. However, the phase constants of both modes are equal. As the coupling portion 5 is approached, the degeneracy is broken and the difference in phase constants of both modes becomes larger.
In the coupling part 5, the two optical waveguides become one, and the 3
Since it has a layered optical waveguide structure, the even mode shifts to the fundamental mode (the one with the larger phase constant) of the three-layered optical waveguide, and the odd mode shifts to the primary mode (the one with the smaller phase constant). After passing through the coupling part 5, the optical waveguide 3
When the optical waveguides 3 and 4 enter the asymmetric side consisting of The phase constants of each asymptote to different values. In this example, since the width of the optical waveguide 3 is wider than the width of the optical waveguide 4, the phase constant of the optical waveguide 3 is larger. Therefore,
Even mode light wave power is concentrated in the optical waveguide 3, and odd mode light wave power is concentrated in the optical waveguide 4.

The above explanation is for the case where the light propagates from the symmetric side to the asymmetric side, but when the light propagates from the asymmetric side to the symmetric side, the above explanation can be followed in reverse.

FIG. 3 shows the light wave output obtained when various light waves are input into the above-mentioned optical waveguide from the symmetrical side. FIG. 3a shows two optical waveguides 1 on the symmetric side,
This is a case where light waves of the same phase are input to 2. The even mode is excited and propagates on the symmetrical side, and changes to the fundamental mode at the coupling portion 5, and to the even mode again on the asymmetrical side. Since the optical wave power of the even mode on the asymmetric side is concentrated in the optical waveguide 3, the output optical wave is obtained from the optical waveguide 3. FIG. 3b shows a case where light waves having mutually opposite phases are input into two optical waveguides 1 and 2 on the symmetrical side. The odd mode is excited and propagates on the symmetrical side, and changes to the primary mode at the coupling portion 5, and to the odd mode again on the asymmetrical side. Since the light wave power of the odd mode on the asymmetric side is concentrated in the optical waveguide 4, the output light wave is obtained from the optical waveguide 4. FIG. 3c shows a case where a light wave is input only to the optical waveguide 1. In this case, it is considered that the even mode and the odd mode are excited with equal power on the symmetric side, so the superposition of a and b in Fig. 3 occurs, and light waves with equal power are output to optical waveguides 3 and 4. .

It is also possible to input light waves from the asymmetric side, in which case the reverse process described above is followed. for example,
When a light wave is input to the optical waveguide 3, light waves in the same phase are outputted from both optical waveguides 1 and 2 on the symmetric side (indicated by broken line arrows in FIG. 3a). You can think about other things in the same way. From the above considerations, it will be understood that this optical waveguide structure has a function equivalent to a normal optical beam splitter (for example, a half mirror).

FIG. 4 shows an embodiment. Z-cut
By thermally diffusing Ti onto one surface of the LiNbO ³ crystal substrate 10, a pair of optical waveguides 1 and 2, a pair of optical waveguides 3 and 4, and a coupling portion 5 at the intersection of these are formed as shown in FIG. It is formed. Parallel optical waveguides 11, 12, 13, and 14 are continuous at the end portions of these optical waveguides 1 to 4 opposite to the coupling portion 5, respectively. Optical waveguides 3 and 13 are for input, optical waveguides 4 and 14 are for output, and optical waveguides 1, 2, 11, 12.
is for pressure detection. This kind of usage is the third
This is the same as inputting light waves from the asymmetric side, as shown by the broken line in Figure a. The crossing angle between optical waveguides 1 and 2, and the crossing angles θ1 and θ2 between optical waveguides 3 and 4 are both
It is set to a small value of 1.2° or more. A reflective film 8 is formed on the end face of the substrate 10 on the side of the optical waveguides 11 and 12 by depositing Al. Furthermore, SiO ₂ thin films 6 and 7 of the same length are formed on the optical waveguides 11 and 12, respectively, so as to cover them. Then, a rod 9 is placed on one SiO ₂ film 6.
The pressure to be detected is applied by. If the pressure to be detected is generated by a mechanical mechanism, this rod 9 is connected directly or indirectly to that mechanism. When the pressure to be detected is fluid pressure, this rod 9 is connected to the piston of the cylinder into which the fluid is introduced. Instead of the rod 9, a cylinder may be placed on the SiO ₂ film 6, and the fluid to be detected may be introduced directly into this cylinder.

A suitable light source (not shown) is provided for the input optical waveguide 13.
Light is incident from the source through an optical system or optical fiber. The output optical waveguide 14 is coupled to a photoelectric detector (not shown) via an optical system or an optical fiber. When no pressure is applied by the rod 9, the phase constants of the optical waveguides 1, 11 and 2, 12 are equal. Therefore, the light incident from the optical waveguide 13 is equally divided into both optical waveguides 1 and 2 and propagated, and after being reflected by the reflective film 8 returns to the optical waveguide 13 again, and no output light is obtained from the optical waveguide 14. (See Figure 3a). When pressure is applied through the rod 9, the thickness of the optical waveguide 11 changes due to this pressure, and the refractive index of the optical waveguide 11 changes due to the strain caused by the pressure. , the phase constant of the optical waveguide 11 changes. Therefore, the optical waveguides 1 and 11 and the optical waveguides 2 and 12
A phase difference occurs in the light waves propagating through the optical waveguide 14, and light with an intensity corresponding to this phase difference is emitted from the optical waveguide 14. Pressure can be detected by measuring the intensity of light output from the optical waveguide 14. The intensity change of the output light of the optical waveguide 14 with respect to the applied pressure is the fifth
As shown in the figure.

In FIG. 4, light may be input from the optical waveguide 14 and detection light may be obtained from the optical waveguide 13. Alternatively, pressure may be applied to the SiO ₂ film 7. By applying different pressures to both the SiO ₂ films 6 and 7, the difference between these pressures can be detected. Furthermore, optical waveguides 1, 11, 2,
It is also possible to use the optical waveguides 12 for input/output and the optical waveguides 3, 13, 4, and 14 for pressure detection.

Furthermore, if the light reflection positions in the optical waveguides 11 and 12 are set to differ by λ/4, where the wavelength of the light is λ, the maximum intensity output light can be obtained from the optical waveguide 14 when no pressure is applied. The light reflecting means may be provided outside the substrate 10 instead of being formed on the substrate 10. In addition to the reflecting mirror, for example, an end face of an optical fiber may be used as the reflecting means.

FIG. 6 shows another embodiment of the invention.
By dispersing silver ions in an electric field on the glass substrate 20, a single mode optical waveguide 24 for input/output, a Y-shaped optical branch 23 of this optical waveguide 24, and a pair of optical branches each connected to the Y-shaped optical branch 23 are formed. Parallel single mode optical waveguides 21 and 22 for pressure detection
is formed, and a reflective film 25 is formed by depositing Al on the end face of the substrate 20 on the side of the optical waveguides 21 and 22. A rod 9 for applying a pressure to be detected to one of the pair of optical waveguides 21 and 22 is arranged. Light is input from the optical waveguide 24. When no pressure is applied, there is no phase difference between the light waves propagating through the pair of optical waveguides 21 and 22, so the light waves from both waveguides 21 and 22 emitted from the optical waveguide 24 are superimposed. The light shown is the maximum value. When pressure is applied to the optical waveguide 21 by the rod 9, the phase constant of the optical waveguide 21 changes, so that the intensity of the light output from the optical waveguide 24 is modulated by the pressure.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the operating principle of a waveguide optical beam splitter, Figures 2a and b are diagrams showing how the eigenmode propagates in this beam splitter, and Figure 2c is a diagram showing the phase constant. Graphs showing changes; FIG. 3 is a diagram showing various relationships between the input and output of light waves to this beam splitter; FIG. 4 is a perspective view showing an embodiment of the present invention; FIG.
A graph showing changes in the intensity of output light due to pressure, and FIG. 6 is a perspective view showing another embodiment of the present invention. 1, 2, 11, 12, 21, 22...Single mode optical waveguide for pressure detection, 3, 4, 13, 14, 24
... Single mode optical waveguide for input/output, 10, 20... Substrate, 8, 25... Reflection film, 9... Rod for applying pressure to be detected.

Claims (1)

[Claims] 1. A substrate having at least one input/output single mode optical waveguide and a pair of pressure detection single mode optical waveguides each having one end coupled to one end of the input/output optical waveguide. , means for reflecting the light waves propagating through the pressure detection optical waveguide at the other end, and means for applying a detected pressure onto at least one of the pressure detection optical waveguides to change the phase constant of the optical waveguide. Optical pressure sensor. 2. A means for inputting a light wave into the input/output optical waveguide from the other end, and a means for detecting the intensity of the light emitted from the other end of the input/output optical waveguide.
An optical pressure sensor according to claim 1.