JPS62151717A - Light applied sensor - Google Patents

Abstract

PURPOSE:To reduce the cost of a light applied sensor, to increase its accuracy, and to improve the easiness in use by obtaining two light outputs required to remove light intensity and loss variations by using a polarization beam splitter, etc. CONSTITUTION:Light emitted by a light emitting element 1 is incident on an opto-elastic material 51 through the polarization beam splitter 41, etc. The light passed through the material 51 has its optical path bent by 80 deg. through a right-angled prism 61, but its two orthogonal polarized components become out of phase. The light passed through this prism 61 is incident on the splitter 41 again through the material 51. This splitter 41 operates as an analyzer. The incident on the splitter 41 is separated into two polarized components; one component is guided to an optical fiber 10 through a microlens 8 and the other polarized component is guided to an optical fiber 11 through a right-angled prism 61 and a microlens 9. The light components incident on the fibers 10 and 11 reach photodetecting elements 12 and 13 respectively, whose outputs are processed electrically to obtain an accurate pressure measured value.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a so-called optical sensor that detects a physical quantity using an optical method. [For more details, see 0 regarding optical sensors using photoelastic materials whose birefringence value changes according to physical quantities in the outside world. <Prior art> Optical sensors using birefringence detect by using polarized light. It has the characteristic of being able to detect with high sensitivity by converting the physical quantity to be detected into a minute difference in the refractive index of light in two orthogonal directions. Among them, optical sensors that utilize temporary birefringence based on the photoelastic effect are capable of detecting various mechanical quantities, such as pressure, load, acceleration, strain, and sound.

An important consideration in the production of optical sensors is to configure the sensor so that detection errors do not occur even if the intensity of light reaching a photodetector such as a light receiving element changes due to disturbance. Here, the disturbance may be a fluctuation in the output intensity of the light emitting element, a coupling loss between the light emitting element and an optical fiber, an optical fiber and various optical components, or other factors.

FIG. 4 shows a conventionally used configuration of an optical sensor that suppresses errors against the above-mentioned disturbances. The light output from the light emitting element 1 is transmitted through the optical fiber 2.
, and enters the first polarizing beam splitter 4 through the microlens 3. Here, only light having a single polarization component passes through the photoelastic material 5 and the 1/4 wavelength plate 6, and reaches the second polarization beam splitter 7. Here, the photoelastic material 5 has the effect of changing the ratio of two orthogonal polarization components according to its internal stress, and the polarization beam splitter 7
It has the effect of separating two polarized light components into separate optical paths.

One of the two separated polarized lights passes through the microlens 8 and the optical fiber 10 and reaches one of the light receiving elements 12, and the other passes through the microlens 9 and the optical fiber 11 and reaches the other light receiving element 13. The outputs of the light receiving elements 12 and 13 are
Since the output increases or decreases at the same rate with respect to the disturbance before the polarizing beam splitter 7, by taking the ratio of these two outputs, it is possible to suppress the detection error due to the disturbance to a low level.

Here, the output is proportional to the birefringence of the photoelastic material 5 and depends on the phase difference θ between two orthogonal polarization components. When the photoelastic material 5 has no intrinsic birefringence, θ changes from 00 as the physical quantity in the outside world changes from 0. By the way, θ
The change in the amount of light for a constant change in is maximum when θ=
Since the angle is 90°, an optical element that provides a 90° phase bias is usually inserted separately from the sensor material 2 in order to optimize the sensitivity. A quarter-wave plate 6 is generally used for this purpose.The quarter-wave plate 6 is made of a material with inherent birefringence and has a predetermined thickness. . As specific materials, quartz plates, mica plates, calcite plates, rutile plates, etc. are often used as high-precision ones, and stretched plastic plates are often used as inexpensive ones.

Now, the quarter wavelength plate 6 as in the above example has the following drawbacks. That is, quartz plates, mica plates, etc. are single-crystal, and not only are the materials extremely expensive, but the thickness must be determined accurately, and the processing cost is also high. Stretched plastic plates are inexpensive, but have large variations and are not suitable for precision applications.

On the other hand, a Fresnel rhomboid is known as an element that has the same function as the I/4 wavelength plate that uses birefringence and is completely different in principle. This has a configuration as shown in FIG. 5, and utilizes the fact that a phase difference occurs between two orthogonal polarized light components during total reflection. Here, the rhomboid 65
The angle formed by the laser beam has an extremely complicated value determined by the refractive index of the medium, making it difficult to process and inconvenient in use because the emitted light moves parallel to the incident light. Because of the above-mentioned difficulties, Fresnel rhomboids have rarely been used in optical sensors.

<Objective of the Invention> The present invention has been made in view of the above points, and uses a quarter-wave plate or Fresnel plate as a means for providing a phase bias to optimize the sensitivity of an optical sensor. By using total reflection in a right-angled prism instead of a rhomboid, the purpose is to reduce the cost, increase precision, reduce size, and improve ease of use of optical sensors.

<Example> The present invention will be described in detail below with reference to FIGS. 1 to 3.

FIG. 1 is a schematic configuration diagram of a pressure sensor showing an embodiment of the present invention, in which light emitted from a light emitting element I is transmitted through an optical fiber 2.
．． Rod lens 3. After passing through the polarizing beam splitter 41,
The light is incident on the photoelastic material 51.

Here, a pressure P is uniformly applied to the photoelastic material 51 as shown by the arrow in FIG. 1 by pressure applied to an external diaphragm. However, the external diaphragm is omitted in FIG.

The optical path of the light passing through the photoelastic material 51 is bent by 180 degrees by the right angle prism 61, but at this time a phase difference occurs between the two orthogonal polarized components. In order to obtain this phase difference, the arrangement relationship described later is set between the polarizing beam splitter 41 and the right angle prism 6I. right angle prism 61
The light passes through the photoelastic material 51 again and enters the polarizing beam splitter 41. This polarizing beam splitter acts as an analyzer. The light incident on the polarizing beam splitter 41 is separated into two polarized components, one of which is guided to the optical fiber 10 via the microlens 8, and the other polarized component is simply passed through a right-angle prism arranged to convert the optical path. 62, the light is guided to the optical fiber 11 through the microlens 9. Optical fiber 10°1! The light introduced into the sensor reaches the light receiving elements 12 and 13, respectively, and by electrically processing the output, an accurate pressure measurement value can be obtained.

In this embodiment, optical glass was used as the photoelastic material 51, but other types of glass such as quartz glass may also be used.
aP, LiNbO3, LiTaO3 and other optical crystals or optical ceramics, phenolic resin, celluloid, epoxy resin, diallyl phthalate polymer, styrene polyester copolymer, :) Methyl methacrylate polymer, polycarbonate, gelatin, polyurethane rubber, epoxy rubber, Silicone resin, polycyclohexyl methacrylate, polyacrylonitrile, polyvinyl chloride, unsaturated polyester photosensitive resin, and other polymeric materials can be used.

Now, both the photoelastic material 51 and the right angle prism 61 function to provide a phase difference between two orthogonal polarized lights. In addition, the polarizing beam splinter 41 functions to match the phases of two orthogonal polarized lights at the time of incidence, and at the time of output, the light beam has an intensity corresponding to the phase difference given by the photoelastic material 51 and the right-angle prism 61. It works to emit .

The effect of the right angle prism 61 regarding the phase difference between two orthogonal polarized lights and the effect of the optical output will be described below with reference to FIG. 2. FIG. 2 shows the arrangement of the constituent elements 41, 51, 6], which is determined by taking into account the positional relationship between the polarized light and each element.

For convenience of explanation, the traveling direction of light at each point on the optical path is expressed as z, z
The direction perpendicular to the direction and parallel to the reflecting surface is defined as y, and the direction perpendicular to 2 and y is defined as X, as shown in FIG. As the light is reflected, the orientation of the 2 and X directions with respect to absolute space will change.

Furthermore, for simplicity, the light beam guided to the light receiving element 13 in FIG. 1 is omitted in FIG. 2.

A stress is applied to the photoelastic material 51 in the y direction as shown in FIG. Therefore, the light passing through the photoelastic material 51 has a phase difference θ0 proportional to the stress between the X-polarized light and the y-polarized light.
is applied. This effect is the photoelastic effect.

On the other hand, as will be described later, in the rectangular prism 61, a phase difference due to total reflection is applied between polarized light whose vibration direction lies within the total reflection plane and polarized light whose vibration direction is perpendicular to the polarized light. In the case of the arrangement shown in FIG. 2, the polarized light within the total reflection plane corresponds to y-polarized light, and the polarized light orthogonal thereto corresponds to X-polarized light.

As mentioned above, if the phase difference due to the photoelastic material 51 is 00°, and the phase difference due to two total reflections in the right angle prism is 2δ, then both side phase differences are the phase differences between the X-polarized light and the y-polarized light. Can be added. That is, the added phase difference θ is θ=θ0+2δ. The phase difference caused by total reflection is called a phase bias because a constant value is always added to the phase difference caused by photoelasticity, similar to the phase difference caused by the I74 wavelength plate.

Now, if the electric field amplitude of the linearly polarized light just before it passes through the polarizer 4I and enters the photoelastic material 51 is Eo, and its angle with the X direction is φ shown in FIG. 2, then the polarizer 4! The emitted light intensity I
A is given by the following equation.

θ IA c(lEo l (cas22φ+s stone 22φ bit 2 completion) With respect to the change in θ, the amount of change in the output 1out becomes maximum when the direction φ of the axis of polarizer +4 is 45° or 1
35°, and the output at this time is given by the following equation.

IA　glEo　12sin” The form of this output is shown in FIG.

Based on the above considerations, in this example, the angle between the axial direction of the polarizer and the X direction was set to φ=45°. In addition,
It can be seen that when φ=0° or φ=90°, the output is constant regardless of θ, and it cannot be used as a sensor that utilizes birefringence.

Here, a supplementary explanation will be given regarding the phase difference accompanying total reflection.

The phase difference δ caused by one total reflection is expressed by the following equation.

Here, n is the refractive index of the right-angle prism, and ψ is the angle formed by the incident light with respect to the perpendicular to the reflective surface.

In this embodiment, ψ=45°, and the phase difference δ in this case is expressed by the following equation.

The reason for setting the phase difference 21 due to two total reflections is to use the region where the slope of the output curve shown in FIG. 3 is the largest. Here, the point where the slope is maximum is when the phase difference 2δ is 90°, and the refractive index of the sensor material that satisfies this is n=1.
It is 554. Although the phase difference 2δ does not necessarily have to be 90°, it is desirable to use a material with a refractive index as close to this value as possible. Further, ψ does not necessarily have to be 45°.

In the above discussion, the amount of light IB reaching the light receiving element I3 in FIG. 1 has been ignored. Let I be the light intensity just before it enters the photoelastic element 51 through the microlens 3 and the polarizing beam splitter 41 in FIG. 1, and assuming that there is no optical loss, I = IA from the energy invariance
+IB. From this, IB=I chamber 2L Here, by taking (IA-IB)/(IA+IB) as the output, the influence on the fluctuation of ■ is eliminated.

In other words, ■ changes with the emission intensity, coupling loss due to the use of optical fibers, and other absorption and scattering losses, so ■
It is important to obtain an output that is independent of Note that after rA and IB are separated by the polarizing beam splitter 41, each light undergoes different loss fluctuations, so even with this method, the post-separation fluctuations cannot be removed, but this effect is generally not so serious.

Note that the present invention can perform the following substitutions to this embodiment. As long as the right-angle prism 6I is located between the polarizer and the analyzer, the front-and-back relationship with the photoelastic material 51 is not defined. The right-angle prism 61 may be divided into two right-angle prisms, each of which may be totally reflected once. That is, the right-angle prism may have a trapezoidal shape or the like, and it is sufficient that the angle formed by the two total reflection surfaces is 90°. The polarizing beam splitter may be replaced with a Glan-Thompson prism or the like. Although uniform stress was applied to the photoelastic material 51, stress associated with bending moment may be applied.

The optical application sensor using the photoelastic material 5I according to the present invention is not limited to a pressure sensor, but can also be used as an acoustic, strain, or displacement sensor. Further, the photoelastic material 51 can be combined with materials having different coefficients of thermal expansion to form a temperature sensor, combined with an electrostrictive material to form a voltage sensor, and combined with a magnetostrictive material to form a current sensor or magnetic sensor.

By performing such a combination, it can be applied to almost all physical quantities.

<Effects of the Invention> As described above, the optical sensor of the present invention eliminates total reflection in a right-angle prism in order to obtain the two optical outputs necessary to eliminate fluctuations in light intensity and loss using a polarizing beam splitter or the like. The feature is that it is used to set the phase bias and also reverse the optical path. As a result, different functions such as a mirror and a quarter-wave plate can be obtained with one element, so the structure is simple and compact, and it is easy to handle, and there is less variation in phase bias compared to a quarter-wave plate. Since a small total reflection member is used, it has the advantages of high precision and low cost. This advantage is extremely useful for widespread use of optical sensors.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram of an optical sensor showing an embodiment of the present invention, Fig. 2 is a perspective view for explaining the arrangement of the main parts in Fig. 1, and Fig. 3 is an explanation for explaining the output of the sensor. Figure 4 is a configuration diagram showing a conventional optical sensor, and Figure 5 is a conventional optical sensor with microlenses 3, 8, 9...
4・7・41...Polarizing beam splitter 6...1
/4 wavelength plate 5.51...Photoelastic material 12.13.
... Light receiving element 61 ... Right angle prism 65 ... Fresnel's rhomboid agent Patent attorney Yoshihiko Fukushi (and 2 others) Go

Claims (1)

[Claims] 1. A light source, a polarizer disposed on the light traveling path of the light source, a photoelastic material to which stress is applied according to the physical quantity of the external world, and 2. The light path is reversed and perpendicular to the polarizer. a rectangular prism disposed at a position to set a phase bias between polarized light components, an analyzer that separates two orthogonal polarized light components, and first and second photodetectors that respectively detect the two polarized light components. 1. An optical application sensor, characterized in that the outputs of the first and second photodetectors correspond to the physical quantities.