JP2602981B2 - Temperature distribution detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] [Field of Industrial Application] The present invention relates to a temperature distribution detecting device using backscattered light in an optical fiber.

[Conventional technology]

Such an apparatus excites scattered light in an optical fiber by a laser pulse incident on the optical fiber, and includes physical information (for example, temperature) of a place scattered in spectrum, intensity, polarization, etc. By extracting backscattered light propagating toward the oscillation source of the laser pulse as a time-series signal, one of the physical quantities to be measured such as temperature distribution can be reduced.
This is to detect a dimensional distribution. Scattering that occurs when a laser pulse is incident on an optical fiber includes Rayleigh scattering due to density fluctuation, Brillouin scattering due to propagation fluctuation, ie, thermal phonon, and Raman scattering due to molecular vibration and rotation. In particular, temperature distribution measurement using Raman scattering has attracted attention.

By the way, in the above-described temperature distribution detecting device, since the backscattered light is attenuated by the optical fiber when propagating, the intensity of the excitation light pulse (laser pulse) for exciting the scattered light is required to improve the measurement accuracy. There is a need to reliably compensate for the effects of the dependence and its attenuation.

Therefore, in a conventional temperature distribution detecting device using Raman scattered light, anti-Stokes Raman scattered light (hereinafter abbreviated as “anti-Stokes light”) and Stokes Raman scattered light (hereinafter abbreviated as “Stokes light”) are used. By taking the ratio between the scattered light signals, the effect of the intensity of the excitation light pulse and the effect of the attenuation by the optical fiber were compensated.
Here, anti-Stokes Raman scattering means that the energy of one photon shifts to a higher short wavelength side by an excitation light pulse, and Stokes Raman scattering conversely shifts to a lower wavelength longer wavelength side. is there.

Assuming that the shift due to Raman scattering is ν (m ⁻¹ ), the intensity ratio R between the anti-Stokes light and the Stokes light is Becomes Where ν is the Raman shift number, T is the absolute temperature, h
Is the Planck constant, C is the speed of light, λ is the wavelength of the pump light, A is the intensity of the pump light, λa is the wavelength of the anti-Stokes light, λs is the wavelength of the Stokes light, α _λa is the attenuation constant of the optical fiber with respect to the anti-Stokes light, _αλs is the attenuation constant of the optical fiber with respect to the Stokes light.

In the above equation (1), when α _λa α _λs is considered,
Excitation light intensity A and optical fiber attenuation constant α _λa , α
_{[lambda] s} is removed and the following equation is obtained.

Therefore, only the term (KT) based on the temperature is left, and when this calculated value is viewed in time series, it becomes temperature distribution data.

By the way, it has recently been found that the effect of the attenuation of the optical fiber on the anti-Stokes light and the Stokes light does not completely match, and as described above, α
_If it is regarded as _λaαλs , the measurement value will include an error. This is because scattering in the short wavelength band is (1/1 /
λ) ⁴ , and the wavelength shift due to Raman shift between the anti-Stokes light and the Stokes light is, for example, GeO ₂
In this case, it is thought that this is because there is about 100 nm.

[Problems to be solved by the invention]

Therefore, the conventional temperature distribution detecting device using Raman scattered light cannot remove the influence of the attenuation by the optical fiber and cannot perform accurate temperature distribution measurement without being affected by the attenuation by the optical fiber. Was.

The present invention has been made in view of the above circumstances,
It is an object of the present invention to provide a temperature distribution detecting device capable of reliably compensating for the influence of the intensity of the excitation light pulse and the influence of the attenuation of the optical fiber and detecting highly accurate temperature distribution data.

[Constitution of the Invention] [Means for Solving the Problems] In order to solve the above problems, the present invention is to input an optical signal to an input end of a measuring optical fiber, and the optical signal travels in the measuring optical fiber. In the temperature distribution detecting device that takes out the backscattered light generated in the measurement optical fiber as it does and measures the temperature distribution from the time until the backscattered light arrives and the intensity thereof, the measurement optical fiber A temperature distribution detecting portion that is folded back and forth in the same place, and a forward signal consisting of backscattered light generated between the incident end of the measuring optical fiber and the turning point, and a turning point of the measuring optical fiber. Extraction means for extracting a return signal consisting of backscattered light generated up to the end, and combining the forward signal and the return signal extracted by the extraction means to attenuate the signal by the measurement optical fiber. It has a structure provided with the synthesizing means for compensating.

Further, in order to solve the above problem, the extracting means is further configured to extract the forward signal and the backward signal from each of the anti-Stokes Raman scattered light and the Stokes Raman scattered light, and the combining means, Each of the return signals is converted into a signal at each position as viewed from the outward path, the converted signal is multiplied by the outward signal for each scattered light, and the multiplied value of the anti-Stokes Raman scattered light is subjected to Stokes Raman scattering. The light was multiplied by the multiplied value.

[Action]

According to the present invention, the forward path signal and the return path signal having the same temperature distribution information are respectively extracted, and the forward path signal and the return path signal are combined to compensate for the attenuation by the measurement optical fiber, and the measurement light It is possible to obtain highly accurate temperature distribution data that is not attenuated by the fiber.

Further, the forward path signal and the return path signal are extracted from each of the anti-Stokes Raman scattered light and the Stokes Raman scattered light, and the extracted return path signals are converted into signals at respective positions viewed from the forward path. Then, the converted signal and the outward signal are multiplied for each scattered light,
The effect of the attenuation by the measuring optical fiber is removed, and the product of the anti-Stokes Raman scattered light is divided by the product of the Stokes Raman scattered light to remove the effect of the intensity of the optical signal.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing a configuration of a temperature distribution detecting device according to an embodiment of the present invention. In this device, an excitation light pulse generated from a laser light source 1 composed of a semiconductor laser or the like is supplied to a directional coupler 2. The directional coupler 2 guides the excitation light pulse from the laser light source 1 to the measuring optical fiber 3 and separates the backscattered light from the measuring optical fiber 3. The backscattered light separated by the directional coupler 2 is input to the frequency divider 4. The frequency divider 4 separates and outputs anti-Stokes light and Stokes light from the backscattered light.
The anti-Stokes light separated by the frequency divider 4 is guided to the detector 5, and the Stokes light is guided to the detector 6.
Each of the detectors 5 and 6 includes an avalanche photodiode or the like, and photoelectrically converts each scattered light into an electric signal. Amplifiers 7 and 8 are connected to the detectors 5 and 6, respectively.
The data is input to the arithmetic control unit 11 via the / D converters 9 and 10. The operation control unit 11 has a function of dividing an input signal into a forward path signal and a return path signal, which will be described later, and a combination operation processing function of combining the forward path signal and the return path signal to detect a temperature distribution. Further, the arithmetic and control unit 11 transmits a trigger signal to the pulse generation unit 12 to drive and control the drive unit 13 of the laser light source 1, and
The operation of the A / D converters 9 and 10 is controlled, and the operation is performed so that the calculation result is displayed on the display unit 14.

Here, the optical fiber for measurement 3 is disposed close to the two optical fibers 3a and 3b so as to be at the same temperature condition, and is covered with a light shielding film 15. Moreover,
One end of one of the optical fibers 3a is connected to the directional coupler 2 and serves as an incident end of the excitation light pulse. The opposite end of the optical fiber 3a is connected to the end of the other optical fiber 3b, which is located at the same position as the opposite end, at the connection portion 16. The connection portion 16 is provided with a loss so that a predetermined signal is included in the scattered light generated at this position.

Temperature sensors 17 such as a measuring resistor and a thermocouple are installed at a plurality of positions on the measuring optical fiber 3, and outputs of the temperature sensors 17 are output to a calculation control unit via a signal conversion unit 18.
Sent to 11.

Next, the principle of measuring the temperature distribution according to the present embodiment will be described with reference to FIGS. FIG. 2 is a view schematically showing the measuring optical fiber 3, and the hatched portions in the figure indicate portions where the temperature is higher than the surroundings. The point P corresponds to the connecting portion 16 in FIG. 1, and the two optical fibers 3a and 3b shown in FIG. 1 can be regarded as one fiber obtained by folding the measuring optical fiber 3 at the point P.

When an excitation light pulse having a pulse width of 10 to 100 nsec or less is incident on this optical fiber for measurement, scattered light continues to be generated until the excitation light pulse passes through the point P and reaches the end point Q. At this time, when this scattered light is converted into an electric signal, it becomes a signal indicated by a solid line in FIG. This signal is composed of a forward signal Sa ₁ obtained by converting the backscattered light to the turning point P to an electric signal, return obtained by converting the backscattered light from the turning point P to the end point Q into an electric signal the signal Sa ₂ Prefecture. Of the outbound signal Sa ₁ and the inbound signal Sa ₂ ,
Each protruding portion is a portion corresponding to scattered light from a high-temperature region (hatched portion). As shown in Figure 3, in the forward signal Sa ₁ and return signal Sa _2, the coordinates of the temperature distribution are opposite to the traveling time. Contrary to the progress of time, it can be said that the attenuation direction is opposite. Here, in order to simplify the explanation, the return signal Sa ₂
Is turned around the turning point P as shown by the broken line in the figure, and this signal is used as a return path converted signal Sa ₂ ′. It can be seen that when the temperature distributions of the forward pass signal Sa ₁ and the return pass converted signal Sa ₂ ′ are matched, the attenuation direction is reversed.

On the other hand, the Raman scattered light signal from the position 1 (the signal obtained by photoelectrically converting the Raman scattered light) is due to anti-Stokes light, and Sa = A Ka Ta (t, l) e- ^αl · e- ^αal (3) Ss = A Ks Ts (t, l) e− ^α1 · e− ^αsl (4) given by the Stokes light. Here, Ka and Ks are proportional constants determined by the structure and characteristics of the optical fiber, the characteristics of the frequency divider, and the characteristics of the detector. Ta (t, l) and Ts (t, l) are Raman scattered light coefficients, which are functions of the position l and the temperature t. Further, e- ^αl is a term of attenuation applied to the excitation light pulse until the position l, and α is an attenuation constant. e- ^αal and e- ^αsl are terms of attenuation received by the backscattered light from the position l, and ^αa and αs are attenuation constants.

From equation (3), the forward ^path signal Sa ₁ is expressed as Sa ₁ = A Ka Ta (t, l) e− ^α1 · e− ^αal (4), and the return ^path conversion signal Sa ₂ ′ is Sa ₂ ′ = A Ka Ta (t, l) e ^αl・ e ^αal・ e ^-2αL・ e
^−2αaL (5) e- ^2αL e- ^2αaL is a constant determined by the length of the optical fiber ^.
In other ^words , Sa ₂ ′ = A Ka ^* Ta (t, l) e ^αl · e ^αal (6) (4) and (6), taking the product of the forward path signal Sa ₁ and backward conversion signal Sa ₂ ', the signal _{a, a = Sa 1 × Sa 2} = A 2 · Ka · Ka * · Ta ² (t, l) (7), and the effect of optical fiber attenuation is eliminated.

When the same operation is performed on the Stokes light, the following equation is obtained: s = Ss ₁ × Ss ₂ ′ = A ² · Ks · Ks ^* · Ts ² (t, l) (8)

From the equations (7) and (8), taking the ratio of the two signals a and s and calculating the square root thereof, The effect of the intensity A of the excitation light pulse is also compensated. Since the remaining terms Ka, Ks, ^eαaL and ^eαsL are all determined by the structure, characteristics, length, frequency divider and detector characteristics of the optical fiber, they should be treated as constants once the measurement system is determined. Can be.

The above-described synthesis operation processing of the return path signal and the forward path signal is executed by the operation control unit 11.

 Hereinafter, the operation of the present embodiment will be described.

The trigger signal is transmitted from the arithmetic control unit 11 to the pulse generation unit 12, whereby the driving unit 13 is driven by the pulse signal output from the pulse generation unit 12. As a result, the laser light source 1 emits about 10 ³ to 10 ⁵ times at a cycle of 1 K to 10 KHz,
The excitation light pulse generated each time is incident on the measuring optical fiber 3 via the directional coupler 2. The backscattered light by all the excitation light pulses is extracted again through the directional coupler 2 and input to the frequency divider 4. Here, the light is separated into anti-Stokes light and Stokes light, and
At 6 it is converted to an electrical signal. The signals respectively amplified by the amplifiers 7 and 8 are subjected to high-speed A / D conversion by the A / D converters 9 and 10 which are operated by sampling control signals having an appropriate delay provided from the arithmetic control unit 11. After being converted into a digital signal, it is input to the arithmetic control unit 11 and temporarily stored.

The arithmetic control unit 11 executes the above-described synthesis arithmetic processing for each input signal corresponding to each excitation light pulse. Therefore, first, it divides the input signal into the forward signal Sa ₁ and return signal Sa _2. At this time, in order to detect the turning point P, since an appropriate loss is given to the connecting portion 16 which becomes the turning point P,
The position of the loss is read from the input signal. Or,
The turning point P may be detected as a half distance from the Fresnel reflection at the end of the optical fiber. Further, temperature information at a plurality of points on the measuring optical fiber 3 is sent from the temperature sensor 17 to the arithmetic and control unit 11, and furthermore, a constant determined by a measuring system such as the measuring optical fiber 3, the frequency divider 4, the detectors 7, 8 and the like. Is entered. Based on the temperature information from the temperature sensor 17, the arithmetic control unit 11
The constants in equation (9) such as a, Ks, and L are corrected.

Next, the above-described synthesis operation is performed for each input signal, and the operation results for all the input signals are averaged.
This combining operation is performed during the time when the next excitation light pulse is generated or every time a predetermined period elapses.

As described above, the temperature distribution data from which the influence of the attenuation of the measuring optical fiber 3 and the influence of the intensity of the excitation light pulse are removed is obtained. The temperature distribution data obtained in this manner is displayed on the display 14.

As described above, according to the present embodiment, one ends of the two optical fibers 3a and 3b placed in the same temperature distribution are connected by the connecting portion 16, and the backscattered light from the measuring optical fiber 3 is used. The forward signal Sa ₁ and the return signal Sa ₂ are separated, and the forward signal Sa
₁ and the return signal Sa ₂ are used to execute the above-described combining operation, so that it is possible to obtain temperature distribution data from which the influence of the attenuation of the measuring optical fiber 3 and the influence of the intensity of the excitation light pulse have been removed. The measurement accuracy can be greatly improved, and an accurate temperature distribution can be detected.

In addition, since the temperature distribution data obtained from about 10 ³ to 10 ⁵ pulses corresponding to the excitation light pulse are averaged and the average value is obtained, noise can be reduced sufficiently and data reliability can be improved. Can be done.

Further, the temperature is measured at a plurality of locations on the measuring optical fiber 3, Is obtained and corrected, so that a more accurate measurement value can be obtained.

[Effects of the Invention] As described above in detail, according to the present invention, a temperature distribution detecting device capable of reliably compensating for the influence of the intensity of the excitation light pulse and the influence of the attenuation of the optical fiber and detecting highly accurate temperature distribution data. Can be provided.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a temperature distribution detecting device according to an embodiment of the present invention, FIG. 2 is a diagram schematically showing the shape of a measuring optical fiber, and FIG. 3 is a waveform of a signal obtained by photoelectrically converting backscattered light. FIG. DESCRIPTION OF SYMBOLS 1 ... Laser light source, 2 ... Directional coupler, 3 ... Measurement optical fiber, 4 ... Divider, 5,6 ... Detector, 11 ...
... Calculation control unit.

Claims (2)

(57) [Claims] An optical signal is incident on an incident end of an optical fiber for measurement, and backscattered light generated in the optical fiber for measurement as the optical signal travels through the optical fiber for measurement is taken out. In a temperature distribution detecting device that measures a temperature distribution from a time until the backscattered light arrives and an intensity thereof, a temperature distribution detecting unit in which the measuring optical fiber is folded back and forth in the same place, and the measuring light. Extraction means for extracting a forward signal consisting of backscattered light generated from the incident end of the fiber to the return point and a return signal consisting of backscattered light generated from the return point to the end of the measuring optical fiber. A temperature distribution detecting device comprising: a synthesizing means for synthesizing a forward signal and a return signal extracted by the extracting means and compensating for attenuation by the optical fiber for measurement. . 2. The extraction means extracts the outbound signal and the inbound signal from each of the anti-Stokes Raman scattered light and the Stokes Raman scattered light, respectively, and the combining means extracts each of the outbound path signals as viewed from the outbound path. Converting the signal into a position signal, multiplying the converted signal by the outgoing signal for each scattered light, and dividing the multiplied value of the anti-Stokes Raman scattered light by the multiplied value of the Stokes Raman scattered light. The temperature distribution detecting device according to claim 1, wherein