JPH0432336B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a method for measuring minute temperature changes using an optical rod and a laser beam, which are known to be expensive due to optical interference.

[Conventional technology]

Two glass plates called Fabry-Perot glass plates with anti-reflection coatings arranged in parallel are known as etalon plates. When white light with a broadband wavelength is incident on this etalon plate and multiple reflections occur between the glass plates, only light with a specific wavelength is reinforced by light interference and emitted.

FIG. 1 is an explanatory diagram of the operation of the etalon plate. In the figure, 1 is a light source that emits white light, 2 is an optical fiber for guiding this light to a focusing lens 3, and 4 is an etalon plate into which the light from the focusing lens 3 is incident. This etalon plate 4 is made by sandwiching two glass plates 4a and 4b between spacers 4c and 4d and facing each other with high parallelism.
In addition to polishing the surface of b, the spacer is also polished to improve dimensional accuracy and obtain an accurate gap g. Furthermore, an antireflection film is formed on the surfaces of the glass plates 4a and 4b so that uniform characteristics can be obtained over a wide range of wavelengths of white light.

Here, the light incident on the etalon plate 4 undergoes multiple reflections between the gap between the glass plates 4a and 4b, and only the light with a specific wavelength λg determined by the physical dimension (g) of the gap is intensified by interference and emitted. Ru. The pattern in which this emitted light appears is that among the light that is multiple-reflected between the gaps, the light of the wavelength that is emitted to the glass plate 4a side (shown by the dotted line) has a phase opposite to that of the incident light, so they cancel each other out and disappear. It almost never comes out.

On the other hand, light (indicated by a solid line) that has been strengthened due to interference is emitted from the glass plate 4b side, but the spectrum of this light becomes an integer multiple of a half wavelength depending on the gap distance, assuming that the medium in the gap is the same. Only the light that passes through it will pass through. Therefore, if this emitted light is focused by a focusing lens 5 and supplied to an optical wavelength analyzer 7 via an optical fiber 6, the gap can be measured from the detected wavelength. If the gap interval is made to change with temperature, the amount of change can be read from the spectroscopy.

[Problem to be solved by the invention]

However, in such an etalon plate, 2
In order to construct a highly accurate parallel gap using a single sheet of glass, manufacturing requires advanced technology and is expensive, and it is extremely difficult to make the gap change in parallel. Furthermore, a large bulge is formed in the middle of the thin optical fiber.

Measuring devices using etalon plates are constructed entirely of insulators, so they are suitable for measuring the internal temperature of high-voltage heavy electrical equipment, power cables, and traverse busbars, etc., in energized and energized states. When inserting into a limited space such as an oil passage and arranging it at a target location, such a bulge makes it difficult to arrange it in a narrow space, and also reduces measurement accuracy. Furthermore, it is not preferable to embed it in equipment from the beginning due to high voltage phenomena such as corona discharge.

On the other hand, in the cylindrical waveguide developed for optical communication, in other words, a core having a refractive index n ₁ in the center is provided with a cladding part having a refractive index n ₂ smaller than n ₁ on the outside, and an outer jacket etc. It is also conceivable to use the provided optical fiber (a high-grade optical fiber in a single optical mode) as a temperature sensor by utilizing the Fabry-Perot effect. Although such a device can be made smaller in size and solve the space problem, it is too sensitive to environmental factors such as pressure, bending, and vibration, making it difficult to put it into practical use.

Such optical fibers are constructed to transmit or multiple-reflect incident light between their end faces, but in order to make them artificially long, Rayleigh scattering and the temperature coefficient at the core-clad boundary must be avoided. Small irregularities, undulations, and bends in the fibers occur due to the tension caused by the differences. In an optical fiber, light entering from one end face is reflected and propagated to the other end face using the difference in refractive index between the core and cladding, so if there are irregularities or bends as described above, coupling between the propagation positions may occur. , the characteristics become unstable due to the effects of radiation loss, etc., and the reproducibility and environmental friendliness, which are important for measurement, deteriorate significantly.

The present invention was made in order to solve these conventional problems, and an object of the present invention is to provide a method that makes it possible to downsize the measuring device and to measure stable and highly accurate temperature changes. That is.

[Means to solve the problem]

In order to achieve such an objective, the present invention
An optical rod 18, which is placed in the temperature measurement area and has both cylindrical end faces polished parallel and flat, has a laser emitter 13 on one end face that oscillates in a plurality of postures including main oscillation and incidental oscillation. The emitted laser light is input through the optical fiber 16, and the transmitted light emitted from the other end of the optical rod is converted into an electrical signal by the photoelectric converter 26, and the converted electrical signal and the change in the length of the optical rod are The amount of temperature change in the temperature-measuring area is measured based on the characteristics of changes in transmitted light intensity in which the main peak P based on the main oscillation of the laser beam and the secondary peak B based on the incidental oscillation occur periodically in response to the laser beam. The direction of temperature change in the temperature-measuring area is detected from the order in which the main and secondary peaks of light intensity occur.

[Effect]

Changes in rod length, that is, temperature changes, can be measured from the electric signal from the photoelectric converter and the waveform characteristics of the relationship between rod length and intensity of transmitted light as shown in FIG.

The main oscillation light of the laser emitter has a large amplitude and has a main peak like P. The incidental oscillation light has a frequency close to that of the main oscillation light, and its amplitude is small, forming a secondary peak of the peak b. Since the length of the optical rod on the horizontal axis is proportional to temperature, minute changes in temperature can be seen from the waveform by measuring the intensity of transmitted light.

If only the main peak P is used, the waveform will be symmetrical and it will be difficult to tell whether the temperature change is rising or falling, but since there is a secondary peak next to the main peak (on the left in this example), the temperature Arrow A in the order that changes occur
It can be seen whether the temperature is rising as shown by arrow B or decreasing as shown by arrow B.

〔Example〕

Hereinafter, the present invention will be explained in detail with reference to Examples.

FIG. 2 is a configuration diagram of an embodiment of a thermometer implementing the temperature change measuring method of the present invention.

In the figure, broadband white light emitted from a light source 10 is focused by a focusing lens 11 and emitted through a beam splitter 12 made of a semi-transparent film, and a single color laser beam is emitted from a laser emitter 13. is reflected by the beam splitter 12 through the polarizer 14 and the wavelength plate 15, and is emitted.

In addition to the main oscillation, this laser emitter 13 oscillates in a plurality of postures, including an incidental oscillation whose frequency is slightly different from the main oscillation and whose amplitude is small, and as described later, the light of the main oscillation and the incidental oscillation is used to determine the direction of temperature change.

The white light and laser light emitted from the beam splitter 12 pass through an optical fiber 16, are condensed by a focusing lens 17, and enter an optical rod 18 which acts as a temperature sensor.

This optical rod 18 is placed in the temperature measurement area, and has a predetermined length l (3 to 75cm) determined by the temperature measurement range.
mm), and both end faces are polished to a flatness of less than 1/20 of the wavelength of the light used and a parallelism of less than 16 seconds at an angle, and a dielectric multilayer is coated on that surface. 18a and 18b are coated. Further, the outer peripheral surface of the optical rod 18 is coated with a film 18c having a predetermined refractive index as necessary to confine light.

Due to the Fabry-Perot effect, white light has a specific wavelength that is multiple-reflected on both end faces and is intensified and emitted from the light rod 18 as light no.
The laser light passes through as it is and is emitted as light mo. The emitted lights no and mo pass through the converging lens 19
The light passes through an optical fiber 20 and enters a beam splitter 21, where it is split into two lights. and enters the photodiode array IC board 24. The photodiode array IC board 24 outputs an electrical signal corresponding to the wavelength depending on the position of the photodiode that receives the light no.

On the other hand, the mo that is separated by the beam splitter 21
passes through a narrow band optical filter 25 to a photoelectric converter 26
where it is converted into an electrical signal.

Note that this narrow band optical filter 25 is necessary for both the main laser beam of wavelength λ ₁ and the auxiliary laser beam of wavelength λ ₂ adjacent to this main laser beam and in an incidental oscillation mode with a smaller amplitude. It is a filter with a pass frequency band that sufficiently passes up to a certain bandwidth, but excludes laser light in other incidental oscillation forms with wavelengths other than that and stray light that becomes noise.

Here, white light n ₁ incident on the light rod 18
As described above, the light is multiple-reflected between the end faces of the films 18a and 18b, and only the light of a specific wavelength with respect to the length l is strengthened by interference and emitted from the film 18b. In the spectrum of the emitted light no, only light having an integral multiple of a half wavelength passes through, depending on the length l of the optical rod 18. Incidentally, since the light having the wavelength emitted to the film 18a side is inverted in phase with the incident light, they cancel each other out and disappear, so that almost no light is emitted.

Further, the electrical signals converted from light to electricity by the photodiode array IC board 24 and the photoelectric converter 26 are signals having information on physical changes (temperature) inside the optical rod 18, and these electrical signals is processed by the microprocessor 27, decoded as an actual temperature value, and output as a temperature signal. Note that 28 is a storage device. The temperature signal is displayed in analog form by display 29 and digitally by display 30.

FIG. 3 is a diagram of observed waveforms of spectra of specific wavelengths, in which a shows the distribution when the length of the optical rod is l=l ₁ and b shows the distribution when the length of the optical rod is l=l ₂ .

The relationship between l ₁ and l ₂ is l ₂ = l ₁ + 1000 Å. When the length of the optical rod is l ₁ , the resonant wavelengths are λb ₁ , λd ₁ , λe ₁ , and when the length of the optical rod is l ₂ , the resonant wavelengths are sharp λa ₂ , λb ₂ , λc ₂ ,
λd ₂ and λe ₂ . λb ₁ is opposed to λb ₂ , λd ₁ is opposed to λd ₂ , and λe ₁ is opposed to λe ₂ . Therefore, if you know the shrinkage/expansion rate depending on the temperature determined by a certain material (glass or plastic) of the optical medium forming the optical rod, you can measure the temperature from the resonance wavelength λ group.

On the other hand, the intensity mI of the laser beam incident on the optical rod 18 is determined using the diameter d and length l of the optical rod, the reflectance R of the end face, the refractive index K of the outer peripheral film, and the morphological number M of the laser beam. ing. While this laser light continues as a sine wave, the coherence distance Δl can be large. Here, the transmittance T of the etalon is determined by the following equation.

T=mo/mI=1/1+Asin ² (δ/2)...(1) However, A=4R/(1-R) ² Also, the reflectance r of the etalon is determined by the following equation.

r=mr/mI=Asin ² (δ/2)/1+Asin ² (δ/2)…
...(2) That is, the reflectance r becomes maximum when δ=(2m+1)π. Note that m is an integer and mr is the reflected light intensity.

r max = 4R/(1+R) ² ... (3) Therefore, the resonance condition is δ = 2mπ, and the finesse F is F = (λ/2)/Δ.

FIG. 4 shows this state, and is a diagram showing the relationship between rod length and transmitted light.

If the length of the optical rod is l, the refractive index is n, and the wavelength of the transmitted laser beam is λ, then when N is the adjustment, the interference peak is expressed as 2nl=Nλ. That is, when the length of the optical rod changes, a peak appears every λ/2.

Thereby, it can be determined whether the temperature change is increasing or decreasing from the incidental oscillation state of the laser beam described above.

As is well known, when a laser beam is oscillated, the generated laser beam does not have a purely single wavelength, but consists of multiple forms of light with different wavelengths and planes of polarization (incidental oscillation). Here, two lights with slightly different wavelengths are generated, and if the wavelength of the main light with a large amplitude is λ ₁ and the wavelength of the sub-light with a small amplitude is λ ₂ , the characteristics will be as shown in Figure 4, and the temperature will change. When the rod is rising and extending, it changes in the direction of arrow A, so first the secondary mountain B appears, and then the main mountain P appears, and on the other hand,
When the temperature is decreasing, it changes in the direction of arrow B, so first the main peak P appears, and then the secondary peak B appears. Note that if there is not much difference in amplitude between the main light and the sub-light generated by the incidental oscillation, the sub-light is used by passing it through a polarizing filter to arbitrarily reduce the amplitude of the sub-light.

Therefore, from this measurement result, the amount of temperature change and whether it is rising or falling can be identified.

That is, as shown in FIG. 4, as the thickness of the optical rod 18 changes with temperature, the intensity of the laser beam transmitted through the optical rod 18 increases (or decreases). Once the maximum value (or minimum value) is reached, it will then fall (rise). For this reason, when measuring temperature with this laser light, for example, even if the intensity of the output light is rising, it is either because the temperature is falling and the intensity of the output light is rising, or because the temperature is falling and the output light is increasing. I don't know if the strength is increasing. However, as mentioned above, if a secondary peak b with a smaller peak value is provided adjacent to the main peak P, the temperature will not change because the secondary peak is on the left side of the main peak in this example in Fig. 4. When the intensity of the output light changes, depending on which peak or secondary peak appears first, it can be determined whether the temperature is decreasing or increasing. . Of course, the amount of change in temperature can also be measured with high precision depending on the intensity of the transmitted light.

FIG. 5 is a block diagram of an embodiment in which a thermometer using an optical rod having such an operating principle as a temperature sensor is applied to high-voltage heavy electrical equipment and power cables. In the figure, 31 is a light source that emits white light, 32 is a laser diode, 33, 34, 35
36 is a focusing lens, 36 is a beam splitter, and 37 is an optical combiner that integrates these parts. The output light of this optical combiner 37 is guided to a temperature sensor section 39 by an optical fiber 38. In this temperature sensor section 39, the light passes through a condenser lens 40 that condenses the light from the optical fiber 38 and enters an optical rod 43 via a polarizer 41, a wavelength plate, and the like. Both end faces of the optical rod 43 are coated with dielectric multilayer films 43a and 43b having a high refractive index, and the outer peripheral surface is coated with a dielectric multilayer film 43c having a low refractive index.

Therefore, the white light incident on the optical rod 43 is subjected to multiple reflections between the films 43a and 43b, causing optical resonance as described above. The light emitted from the film 43b of the optical rod 43 is focused by a focusing lens 44 and input into an optical fiber 45. The temperature sensor section 39 consisting of these parts is integrated with the ends of the optical fibers 38 and 45, and is made of a material that is resistant to petroleum-based insulating oil or insulating gas (SF ₆ ) (for example, the product name Afwan).
COP) and sealed. This temperature sensor section 39 can be housed in a size and shape smaller than that of a general-purpose thermometer.

The light passing through the optical fiber 45 enters the optical combiner 37 and a photoelectric processing section 46 located at hand. After passing through a focusing lens 47, this feedback light is separated by a beam splitter 48.
1, is converted into an electrical signal, and is sent to a microprocessor 52 as an identification circuit for identifying the main mountain and the sub-mountain.

On the other hand, white light with only a specific wavelength strengthened passes through a filter 53 and is separated by a spectrometer 54, and then converted into an electrical signal by a photodiode array IC board 55, and the electrical signal corresponding to the wavelength is sent to a microprocessor. 52. 56 is a storage device that exchanges data with the microprocessor 52. Peak value wavelengths λa, λb, λc that are integral multiples of the half wavelength that resonate in proportion to the length of the optical rod 43
The transmitted light intensity P shown in FIG. 4 is decoded by the microprocessor 52. This output is displayed in analog on display 57 and digitally displayed on display 58.

In this embodiment, the optical fibers for the outward and round trips are provided separately, but a common optical fiber may be used for both the outward and return routes.

Furthermore, although the optical rod used in the example had an end face diameter of 900 μm, it is possible to perform multiple reflections between the end faces if the diameter is 600 μm or more. Also,
Temperature measurements were possible in the range of 40-130°C. Note that the optical rod can have various shapes such as a cylindrical shape with rounded ends or a prismatic shape with square end faces. In addition, the laser beam used was a neon helium gas laser with a wavelength of 6300 Å, but
Any material within the range of 8500 Å can be used.

In addition, to detect light of a specific wavelength, a prism-shaped spectrometer was used to change the refraction angle and make the light enter a photodiode array. It can also be detected.

[Effects of the Invention] As described above, according to the present invention, since temperature is measured by using a laser beam and transmitting the laser beam through an optical rod, stable and highly accurate measurement of minute temperature changes is possible. Moreover, by skillfully utilizing the incidental oscillation phenomenon of the laser emitter, there is an excellent effect in that the direction of change in minute temperature changes can be reliably determined.

Furthermore, since an optical rod is used, the distance between both end faces, that is, the length of the portion through which light passes, is longer than that of a thin etalon plate. The absolute amount of expansion and contraction for the same temperature change is greater when the length of the light-transmitting part is longer, so a larger amount of rod length change can be obtained in response to temperature change, which has the effect of improving detection sensitivity. .

Also, since it uses a cylindrical light rod,
The optical rod, including the optical fiber that inputs and outputs light to and from the optical rod, has no bulges and can be made elongated and compact, making it easy to install and embed in the measurement location.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the operation of the etalon plate, Fig. 2 is a configuration diagram of an embodiment of a thermometer implementing the temperature change measuring method according to the present invention, Fig. 3 is a diagram of observed spectrum waveforms, and Fig. 4 is a diagram illustrating the observed spectrum. FIG. 5 is a graph showing the intensity of transmitted light versus rod length, and is a diagram of another embodiment. 10... White light source, 11, 17, 19... Focusing lens, 12, 21... Beam splitter, 1
3... Laser emitter, 14... Polarizer, 15...
Wave plate, 16, 20... Optical fiber, 18... Optical rod, 18a, 18b... Dielectric multilayer film, 18
c...Membrane, 22, 25...Optical filter, 23...
Spectrometer, 24...Photodiode array IC board, 2
6...Photoelectric converter, 27...Microprocessor, 28...Storage device, 29, 30...Display device.

Claims (1)

[Scope of Claims] 1. An optical rod that is placed in the temperature measurement area and has both end faces of a cylindrical shape polished to be parallel and flat, on one end face of which oscillates in a plurality of states including main oscillation and incidental oscillation. Laser light emitted from a laser emitter is made to enter through an optical fiber, transmitted light emitted from the other end face of the optical rod is converted into an electrical signal by a photoelectric converter, and the converted electrical signal and the optical rod are converted into electrical signals. The amount of temperature change in the temperature-measuring part is measured from the change characteristics of the transmitted light intensity in which a main peak based on the main oscillation of the laser beam and a secondary peak based on the incidental oscillation occur periodically in response to a change in length. Also, a method for measuring temperature change, characterized in that the direction of temperature change in the temperature-measuring portion is detected from the order in which the main peak and secondary peak of the transmitted light intensity occur.