JPS6255089B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in optical temperature sensors that utilize the temperature dependence of birefringent crystals.

A conventional optical temperature sensor of this type is shown in Figure 1a.
I found the one shown here. In this figure, 5,1
4 and 23 are birefringent crystals whose length ratio is 4:2:1. Reference numerals 4 and 6 denote a polarizer and an analyzer, respectively, whose light transmitting axes are perpendicular to each other, and the C axis of the birefringent crystal 5 forms an angle of 45° with these transmission axes. Polarizer and analyzer 1 placed between birefringent crystals 14 and 23
The relationship between the C axis and the transmission axis of 3, 15 and 22, 24 is also the same as above. 1, 10 and 19 are lights, 2, 11, 20 are optical fibers, 3, 12, 2
1 is a lens for collimating light, 7, 16
and 25 are lenses for condensing light; 8, 17, and 26 are optical fibers; 9, 18, and 27 are light detectors; and 40, 41, and 42 are comparators.

Next, the operation will be explained.

An optical fiber 2 guides the light 1 to the location where the temperature is to be measured. The light from the optical fiber 1 is focused by the lens 3, and the light is transmitted through a polarizer 4, a birefringent crystal 5, and an analyzer 6. Next, the light is focused onto an optical fiber 8 by a lens 7 and photoelectrically converted by a detector 9. Light 10 with the same wavelength as light 1
and 19 are similarly connected to optical fibers 11 and 2, respectively.
0, lenses 12, 21, polarizers 13, 22, birefringent crystals 14, 23, analyzers 15, 24, lenses 16, 25, optical fibers 17, 26, and are photoelectrically converted by detectors 18, 27. Ru.

Now, the intensity I of light transmitted in the order of polarizer, birefringent crystal, and analyzer can be expressed as follows.

I=I ₀ /2(1-cosδ) …(1) δ=2π(ne−n ₀ )/λl …(2) Here, ne and n ₀ are the extraordinary refractive index and ordinary refractive index of the birefringent crystal. where l is the length of the crystal and λ is the wavelength of light. Moreover, I ₀ is the intensity of light incident on the crystal.

By the way, the retardation δ expressed by equation (2) changes in proportion to the temperature. Therefore the detector 9,
The outputs of 18 and 27 change as shown in FIG. 1 b, c, and d when the horizontal axis represents temperature. If the comparators 42, 41, and 40 compare the level of 1/2 of the maximum of these outputs, the detector 2
The outputs from 7, 18, and 9 correspond to the first, second, and third digits of the binary code, respectively, and represent the temperature from T ₁ to T ₂ digitally. Of course, accuracy can be improved by increasing this number of digits. The temperatures of T ₁ and T ₂ are set by choosing the length of the crystal.

Since the conventional optical temperature sensor is constructed as described above, a long birefringent crystal is required to improve accuracy.

This invention was made in order to eliminate the drawbacks of the conventional ones as described above, and by repeatedly reflecting light in the crystal, a long propagation distance can be obtained with a short crystal, thereby reducing the cost. The purpose of this invention is to provide a highly accurate optical temperature sensor.

The optical temperature sensor according to the present invention has a light projecting part that generates a light beam, and a ratio of the optical path length of the light when the incident light from the light projecting part is sequentially reflected on the end face of the crystal and taken out to the outside. :2:...: ²ⁿ A birefringent crystal provided with a partial reflection film or a total reflection film at a required position, a light receiving part that receives light emitted from the birefringent crystal, and the light receiving part. The sensor is equipped with a detection section that measures the intensity of the light that passes through it.

An embodiment of the present invention will be described below with reference to FIG.

FIG. 2 shows an optical temperature sensor according to an embodiment of the present invention. In FIG. 2a, the same reference numerals as in FIG. 1a indicate the same or equivalent elements, but in this embodiment, a single thin plate-like birefringent crystal 5 is used. Further, 3 is a lens serving as a light projection unit that condenses the light 1 and projects a monochromatic light beam, and 4 is a polarizer that extracts only the linearly polarized component of the monochromatic light beam from the lens 3. In this example, the polarizer 4 has two functions: a polarizer and an analyzer.
At predetermined positions on the end faces 31 and 32 of the birefringent crystal 5, beam splitters 28 and 2 made of partially reflective films are installed.
9 and a total reflection film 30 are provided. The thickness direction of the birefringent crystal is the C-axis, and the polarizer has a transmission axis at 45 degrees to this axis.
The transmission axis of is perpendicular to this. 6 is an analyzer for analyzing the light transmitted through the partially reflective film 28; 7 and 16 are lenses serving as light receiving parts that receive the light transmitted through the analyzer 6 or 4, respectively; 25 is an analyzer for the light reflected by the total reflection film 30; Of these, a lens serves as a light receiving section that receives the light that has passed through the analyzer 4.

Next, the operation will be explained.

The light 1 is guided by an optical fiber 2 to a temperature measuring position, and the light emitted from the optical fiber 2 is collimated by a lens 3. This collimated light beam enters the birefringent crystal 5 through the polarizer 4, and when this incident light reaches the end face 32,
Half of the light is reflected by the beam splitter 28, and the remaining half of the light passes through the analyzer 6, is focused by the lens 7, and is guided by the optical fiber 8 to the detector 9. .

Now, the light reflected by the beam splitter 28 and traveling through the birefringent crystal 5 again reaches the end face 31, where the beam splitter 2
Half of the light is transmitted by the detector 9, passes through the analyzer 4, passes through the lens 16 and the optical fiber 17, and is then detected by the detector 18. The remaining half of the light is reflected by the beam splitter 29,
This reflected light is further totally reflected by the total reflection film 30 provided on the end face 32, passes through the end face 31, passes through the analyzer 4, passes through the lens 25 and the optical fiber 26, and is detected by the detector 27. be done.

As described above, the light reaching the detectors 9, 18, 27 passes through the birefringent crystal plate 5 by 1:
It propagates with an optical path length ratio of 2:4. However, once the light passes through the beam splitter, the intensity of the light becomes
Therefore, the outputs of the detectors 9, 18, and 27 become as shown in FIG. 2b, c, and d. Therefore, if the comparators 42, 41, and 40 compare the levels based on the half of the maximum value of each output,
Its output provides a temperature measurement expressed in binary code.

In this embodiment, even if the birefringent crystal is short, the optical path length of the extracted light can be increased, thereby improving measurement accuracy. Furthermore, since the number of optical fibers can be reduced, the cost of the device can be reduced.

In the above embodiment, the temperature detection output is 3 bits, that is, the number of detectors is 3, but this number of bits can be increased by appropriately adding a reflecting mirror and a beam splitter. It is clear that the measurement accuracy can be easily improved.

As described above, according to the optical temperature sensor according to the present invention, light is repeatedly reflected within the birefringent crystal,
Since the ratio of the optical path length of the light is 1:2:...: ²ⁿ , the light is taken out of the crystal, so even if the birefringent crystal is short, a long optical path can be formed.
Furthermore, since a single crystal is used, measurement accuracy can be easily improved, and the number of optical fibers can be reduced, which has the effect of making the device inexpensive and simple in structure.

[Brief explanation of the drawing]

Figure 1a is a configuration diagram of a conventional optical temperature sensor.
Figures b, c, and d are all output characteristic diagrams of the detectors and comparators in Figure 1a, Figure 2a is a configuration diagram showing an optical temperature sensor according to an embodiment of the present invention, Figure 2b,
Both c and d are output characteristic diagrams of each detector and comparator in FIG. 2a. 4... Polarizer, 5... Birefringent crystal, 6... Analyzer,
28, 29... Beam splitter, 30... Total reflection film, 19, 18, 27... Detector. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] 1. In an optical temperature sensor, light incident on a crystal is sequentially reflected at the end face of the crystal to form a zigzag optical path, and the ratio of optical path lengths within the crystal is 1:2. :...: ²ⁿ (n is an integer of 2 or more) A birefringent crystal in which a partial reflection film and a total reflection film are provided at required positions on the end face of the crystal so that light is sequentially emitted at positions such that n (n is an integer of 2 or more); a light projector that projects light onto the birefringent crystal via a polarizer; a light receiver that is provided to receive the light emitted from the birefringent crystal via an analyzer; What is claimed is: 1. An optical temperature sensor comprising: a detection section that measures the intensity of light; and temperature detection is performed from the output of the detection section. 2. The optical temperature sensor according to claim 1, wherein the polarizer also serves as an analyzer for a side of the crystal of the polarizer.