JPH0582889B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention is directed to propagating linearly polarized light to a sensor section and changing the polarization state in response to changes in physical quantities during the propagation and during the propagation of reflected return light. The present invention relates to a reflective optical sensor that measures the change when the change occurs.

[Conventional technology]

FIG. 5 shows the configuration of a conventional reflective optical sensor.
In this reflective optical sensor, first, laser light emitted from a laser light source 1 passes through a polarizer 2 and becomes linearly polarized light. This linearly polarized light enters the incident end of the sensor section 4 through the half mirror 3, propagates through the sensor section 4, and is subject to changes in some physical quantity to be detected, such as temperature, magnetic field, electric field, pressure, etc. , the polarization state of the propagating light changes. Furthermore, the return light reflected by the reflecting mirror 5 propagates through the sensor section 4 again, the polarization state changes here as well, and the light is emitted from the output end of the sensor section 4 and enters the half mirror 3. This half mirror 3 transmits and reflects approximately half of the amount of the returned light. One of these days,
The reflected light is separated by an analyzer 6 consisting of a Wollaston prism whose orientation matches that of the polarizer 2,
guided to photodetectors 7 and 8, respectively. The photodetectors 7 and 8 obtain light intensity signals corresponding to changes in the physical quantities described above.

[Problem that the invention seeks to solve]

However, in the above-mentioned reflective optical sensor,
Half mirror 3 returns the returned light that has undergone a change in polarization state.
As a result, the intensity of the light incident on the photodetectors 7 and 8 is greatly attenuated compared to the intensity of the laser light from the laser light source 1. For example, the light intensity of the laser beam is I ₀ , the optical loss in the polarizer 2 is 0%, the optical loss in the half mirror 3 is 0%, the transmittance is 50%, the reflectance is 50%, and the optical loss in the sensor part 4. Assuming that the reflectance at the reflector 5 is 0% and the reflectance at the reflector 5 is 100%, the photodetector 7
The total amount of light I ₁ received by and 8 is as follows: I ₁ ≈I ₀ ×0.5 × 1 × 0.5 = 0.25I ₀ (1). As a result, the measurement accuracy of the optical sensor deteriorates.

In addition, in the above-mentioned reflective optical sensor, about half of the returned light passes through the half mirror 3,
Since the light returns to the laser light source 1 through the polarizer 2, there is a drawback that the intensity of the laser light from the laser light source 1 fluctuates. As a countermeasure to this, it is possible to insert an optical isolator between the laser light source 1 and the half mirror 3, but the structure becomes complicated.

The present invention was made in view of the above-mentioned drawbacks, and its purpose is to increase the amount of light received by a photodetector to improve measurement accuracy, and to eliminate return light from a laser light source to improve laser light. to stabilize and further,
It is an object of the present invention to provide a reflective optical sensor that avoids complicating the structure.

[Means for solving problems]

In order to achieve such an objective, the present invention
Instead of the conventional polarizer 2, half mirror 3, and analyzer 6, a first polarizer and a polarization plane substantially
A Faraday rotation element that rotates the polarization plane by 45 degrees and a second polarizer having an orientation that matches (preferably coincides with) the rotation angle of 45 degrees of the plane of polarization by the Faraday rotation element with respect to the first polarizer are provided.

Typical examples of the first and second polarizers include a polarizing beam splitter and a Fran-Taylor prism. The basic structure of a polarizing beam splitter is to prepare two triangular prisms each having an isosceles triangular cross section, and to connect the two prisms with their largest sides facing each other via a polarizing film. The Glan-Taylor prism is composed of two birefringent prisms (material examples: calcite, quartz) with an air layer or a thin film of an isotropic material interposed therebetween.

The first polarizer converts the incident light into linearly polarized light and outputs the linearly polarized light in the forward direction, and reflects the returned light whose polarization plane has been rotated by substantially 90° through the second polarizer and the Faraday rotation element in the reverse direction. The second polarizer has the above-described orientation in the forward direction and transmits the incident light, and in the reverse direction transmits the component of the returned light in the direction parallel to the direction of the second polarizer, and in the direction perpendicular to the direction of the second polarizer. Reflects the azimuth component. The reflected light from the first and second polarizers is each guided to a photodetector.

〔Example〕

FIG. 1 is a block diagram showing the main parts of the present invention.
Laser light from He-Ne laser light source 1 (wavelength:
633 nm) enters the first polarizing beam splitter 9 and becomes linearly polarized light. This linearly polarized light is incident on the Faraday rotation element 10. Faraday rotation element 1
0 is a magneto-optical medium (FR-5: manufactured by HOYA Corporation) 1
1, and a magnet 12 for applying a magnetic field to the magneto-optic medium 11. Since the magneto-optic medium of this example is a paramagnetic material, the linearly polarized light transmitted into the magneto-optic medium 11 rotates its plane of polarization substantially by 45 degrees counterclockwise with respect to the direction of travel of the light. and emit light. Faraday rotation element 10
The emitted light from the second polarizing beam splitter 9 has an orientation substantially rotated counterclockwise by 45 degrees with respect to the first polarizing beam splitter 9.
The light enters the polarized beam splitter 13 and is transmitted without loss. Note that the second polarizing beam splitter 1
The azimuth angle of 3 is 45°, which is the Faraday rotation angle of 45° mentioned above.
Therefore, if there is a slight deviation in the Faraday rotation angle, the amount corresponding to the deviation must be adjusted (hereinafter, the same applies to the adjustment of the amount equivalent to the deviation). As shown in FIG. 1, the transmitted light is transmitted to a sensor section installed between a second polarizing beam splitter 13 and a reflecting mirror 5 that reflects the light emitted from the second polarizing beam splitter 13 in the opposite direction. 4, the polarization state changes due to changes in physical quantities to be detected, such as temperature, magnetic field, electric field, and pressure, and the returned light is emitted.The returned light is reflected by the reflector 5 and propagates to the sensor section 4 again in the same manner. After undergoing a change in the polarization state, the light beam enters the second polarization beam splitter 13 in the opposite direction. The second polarizing beam splitter 13 transmits light having an azimuth component parallel to the azimuth of the second polarizing beam splitter 13 among the incident light due to the return light, and transmits the light having an azimuth component parallel to the azimuth of the second polarizing beam splitter 13. Reflects light with a vertical azimuth component. Second
The light reflected by the polarized beam splitter 13 is the first
Photodetector 7 (silicon PIN photodiode)
The light is received by the Second polarizing beam splitter 13
The transmitted light propagates in the opposite direction to the magneto-optic medium 11 of the Faraday rotation element 10, and is output after being rotated substantially 45° clockwise with respect to the direction in which the light travels. Then, the emitted light enters the first polarizing beam splitter 9 in the opposite direction, and the orientation of the linearly polarized plane is substantially rotated by 90 degrees with respect to the orientation of the first polarizing beam splitter 9. Therefore, it is reflected by the first polarizing beam splitter. This reflected light is received by the second photodetector 8 (silicon PIN photodiode). Therefore, the photodetectors 7 and 8 measure the amount of light corresponding to the change in the physical quantity to be detected described above.

In the present invention, each of the first and second polarizing beam splitters 9 and ₁₃ and the magneto-optic medium 11 of the Faraday rotation element 10 is Assuming that the optical loss is 0% and other conditions are the same as in the conventional example, the total amount of light received by photodetectors 7 and 8 is
I ₁ becomes I ₁ ≒ I ₀ (2), and compared to the conventional example of 0.25I ₀ , a light amount equivalent to four times is obtained, and as a result, the measurement accuracy (S/N
ratio) can be improved.

Furthermore, since the return light reflected by the reflecting mirror 5 is reflected by the first and second polarizing beam splitters 9 and 13 and does not return to the laser light source 1, it is possible to stabilize the laser light emitted by the laser light source. can be secured.

FIGS. 2 to 4 show examples of the configuration of the sensor section 4 and its surroundings.

First, Figure 2 shows calcite, quartz,
A birefringent material 14 such as mica or polarization-maintaining optical fiber, specifically a retardation plate such as a 1/4 or 1/8 wavelength plate, is used, and the main axis direction of the birefringence is directed to the second polarized beam beam. The plane of polarization of the output light emitted from the pritter 13 in the forward direction is substantially inclined at 45 degrees.
In this example, the incident light propagates through the birefringent material 14 at a constant temperature, and the return light reflected by the reflecting mirror 5 propagates through the birefringent material 14 again, becoming elliptically polarized light. The light beam enters the polarized beam splitter 13 of No. 2. Therefore, when the temperature changes, the elliptically polarized state described above changes. In this example, the main axis of birefringence of the birefringent substance 14 is substantially
By tilting it at 45 degrees, the change in the elliptical polarization state due to temperature change is utilized. The polarization state of propagating light can be changed just by tilting the principal axis.

FIG. 3 shows an embodiment of a reflective optical sensor for temperature using a polarization-maintaining optical fiber that can propagate light having planes of polarization orthogonal to each other without substantially mutual interference. In this example, the main axes of birefringence (X-axis and Y-axis), the directions of one of the principal axes are matched (preferably coincident), and the incident light is made to enter the polarization-maintaining optical fiber lead section 15. This incident light propagates inside the lead portion 15 while maintaining its polarization plane. This propagating light passes through the optical fiber rotating polarization mode coupler 16 and passes through the above-mentioned two
The two principal axes (X' and Y axes) are rotated by 45° and are orthogonal to each other.
The polarization mode light is split into polarization mode light in the (Y' axis) direction and propagated through the next polarization preserving optical fiber sensor section 17. Here, as the optical fiber rotating type polarization mode coupler 16, the main axis (X
The main axes (X'-axis and Y'-axis) of the sensor section 17 are connected so that they are substantially different from each other by 45 degrees. Although the length of the coupling portion is a finite length shorter than the beat length of the lead portion 15 and the sensor portion 17, the connection may be made by a coupling portion having zero length. In addition, a space with a finite length or an isotropic material can be connected to two lead parts with different main axes and the sensor part 1.
It may be arranged as a joint between 7 and 7.

The sensor section 17 is wound around the side surface of the bobbin 18 in order to be more susceptible to temperature changes. The bobbin 18 has a circular cross section and is made of a metal whose thermal expansion coefficient is larger than that of the lead portion 15 and the sensor portion 17. The polarized mode light passes through the sensor section 17, is reflected by the reflecting mirror 5 (or the reflective film if a reflective film is attached to the end surface of the sensor section 17), and is coupled through the sensor section 17 again. Return to vessel 16. That is, the sensor section 17 includes a second polarizer 13 and a reflecting surface (a reflective film attached to the reflective mirror 5 or the end surface of the sensor section 17) that reflects the emitted light of the second polarizer 13 in the opposite direction. installed between. At this time, in the sensor unit 17, each polarization mode light in the two principal axes (X' axis and Y' axis) undergoes a different phase change due to the temperature change, and as a result, there is a difference between the two polarization modes due to the temperature change. A phase difference occurs. The polarization mode light in the directions of the two mutually orthogonal principal axes (X' axis and Y' axis) due to this returned light is
are combined through a coupler 16 and are orthogonal to each other.
The light propagates through the lead portion 15 as polarization mode light in two principal axes (X-axis and Y-axis) directions. Then, this polarization mode light is emitted from the output end of the lead section 15,
The light enters the second polarized beam splitter 13 in the opposite direction.

According to this example, the influence of disturbance on the lead portion 15 can be removed, so a highly sensitive optical sensor can be realized. Even if the lead part 15 is lengthened, 2
Since no phase difference is generated between the two polarization modes, remote sensing becomes possible.

FIG. 4 shows an embodiment of a reflective optical sensor for magnetic fields using a polarization-maintaining optical fiber similar to the previous embodiment. In this example, two principal axes of birefringence (X-axis and the Y-axis) by substantially 45°, and the incident light is made to enter the polarization-maintaining optical fiber 19. When this incident light enters the polarization-maintaining optical fiber 19, it becomes polarized components of two main axes (X-axis and Y-axis), and the optical fiber 19 maintains the plane of polarization.
propagate inside. At the end of the optical fiber 19, it is reflected by the reflecting mirror 5 and returns to the optical fiber 19, but
In between, a quarter-wave plate 20 as a retardation plate and a magneto-optical material 21 to which a magnetic field H is applied (for example, a paramagnetic material (FR-5: manufactured by HOYA Corporation), a diamagnetic material (FD-
5: the same product) and a ferromagnetic material (such as YIG (Y ₃ Fe ₅ O ₁₂ ))), the plane of polarization is essentially rotated by 90°, and due to the non-reciprocal magneto-optic effect, both polarizations are A phase difference occurs between the wave modes in accordance with changes in the magnetic field H.
The returned light is then emitted from the output end of the optical fiber 19 and enters the second polarizing beam splitter 13 in the opposite direction. This example also produces the same effects as the example shown in FIG.

As a modification of the present invention, first, the first and second
Glan-Taylor prisms may be used as polarizers instead of the polarizing beam splitters 9 and 13, respectively. In addition, in FIG. 3, the coupler 16
Instead, a polarizer having an orientation substantially tilted by 45 degrees with respect to the two principal axes of birefringence (X-axis and Y-axis) of the polarization-maintaining optical fiber lead section 15 is used; Instead of the optical fiber sensor section 17 (including the bobbin section 18), the above-mentioned birefringent material, KDP (KH ₂ PO ₄ ), lithium niobate (LiNbO ₃ ), BSO (Bi ₁₂ SiO), or other electro-optical material can be used. A substance (physical quantity to be detected: electric field) or the like may be used.

〔Effect of the invention〕

As described above, according to the reflective optical sensor of the present invention, the amount of light received by the photodetector can be increased four times compared to the conventional example, improving measurement accuracy. Most of the reflected return light can be made incident on the photodetector to prevent it from leaking to the laser light source side, and furthermore, the overall structure can be simplified.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing the main parts of the present invention;
3 and 4 are block diagrams showing embodiments of the sensor section and its surroundings, respectively, and FIG. 5 is a block diagram showing a conventional reflective optical sensor. DESCRIPTION OF SYMBOLS 1... Laser light source, 4... Sensor section, 5... Reflector, 7, 8... Photodetector, 9... First polarization beam splitter, 10... Faraday rotation element,
11...Magneto-optical medium, 12...Magnet, 13...
Second polarizing beam splitter, 14...birefringent material, 15...polarization maintaining optical fiber lead section, 1
6... Optical fiber rotating polarization mode coupler, 17
...Polarization preserving optical fiber sensor section, 18...Bobbin, 19...Polarization preserving optical fiber, 20...1/4
Wave plate, 21...magneto-optical medium.

Claims (1)

[Scope of Claims] 1. A first polarizer that converts incident light into linearly polarized light and emits it and reflects the returning light from the opposite direction; A Faraday rotation element that rotates the polarization direction of the light by substantially 45° in a magnetic field due to the Faraday effect and outputs the light; and a Faraday rotation element that adapts the orientation to the polarization direction of the output light of the Faraday rotation element and transmits the output light and outputs it. a second polarizer that reflects the returning light from the opposite direction; and a reflective surface that reflects the emitted light of the second polarizer in the opposite direction; The output light of the second polarizer is propagated, and the returned light that is reflected by the reflective surface is propagated in the opposite direction, and the polarization state of the propagated light is changed according to the change in the physical quantity to be detected. a first photodetector that makes the return light propagated through the sensor section incident on the second polarizer from the opposite direction and detects the intensity of the partially reflected reflected light; The remaining returned light that has passed through the second polarizer is transmitted from the opposite direction to the Faraday rotation element, and the transmitted light is further rotated substantially by 45 degrees with respect to the traveling direction of the light, and the transmitted light is then transmitted from the opposite direction to the first polarizer. A second photodetector detects the intensity of the reflected light that is incident on the polarizer and reflected.
A reflective optical sensor comprising: a photodetector. 2. The reflective optical sensor according to claim 1, wherein the sensor portion is a refractable material. 3. The reflective optical sensor according to claim 1, wherein the sensor section is a polarization-maintaining optical fiber. 4. The reflective optical sensor according to claim 1, wherein the sensor portion is made of a magneto-optical material. 5. The reflective optical sensor according to claim 1, wherein the sensor portion is an electro-optic material.