High-precision temperature demodulation method oriented toward distributed fiber Raman sensor

Abstract

A temperature demodulation method oriented toward a distributed fiber Raman temperature sensing system, the method comprising the following steps: step 1 of constructing a high-precision temperature detection device oriented towards a distributed fiber Raman sensing system; step 2 of performing signal processing with respect to Stokes light and anti-Stokes light at a calibration stage; step 3 of performing signal processing with respect to Stokes light and the anti-Stokes light at a measurement stage; and step 4 of obtaining a high-precision temperature demodulation technique oriented toward the distributed fiber Raman sensor. The method is used to effectively resolve the issue of low temperature measuring accuracy caused by Rayleigh crosstalk in existing distributed fiber Raman temperature measurement systems, and temperature measurement accuracy thereof is expected to fall within ±0.1° C. The method is applicable to distributed fiber Raman temperature measurement systems.

Claims

1. A high-precision temperature demodulation method oriented to a distributed fiber Raman sensor, characterized in comprising the following steps: Step 1: constructing a high-precision temperature detection device oriented to a distributed fiber Raman sensing system comprising a pulsed laser, an output end of the pulsed laser being connected to an input end of a Raman wavelength division multiplexer; two output ends of the WDM being respectively connected to input ends of a first Avalanche Photodiode (APD) and a second APD; an output end of the first APD being connected to an input end of a first Low-Noise Amplifier (LNA); an output end of the second APD being connected to an input end of a second LNA; output ends of the first LNA and the second LNA being connected to input ends of a data acquisition card; an output end of the data acquisition card being connected to an input end of a computer; a common end of the WDM being connected to an input end of a sensing fiber to be tested; Step 2: processing Stokes light and anti-Stokes light signals in a calibration stage transmitting laser pulses emitted by the pulsed laser to the sensing fiber to be tested; conducting spontaneous Raman scattering when the laser pulses propagate in a multi-mode sensing fiber, such that Stokes light and anti-Stokes light are generated at each position of the multi-mode sensing fiber; wherein backward Stokes light and anti-Stokes light generated in the fiber firstly respectively arrive at the first APD, the first LNA, and the second APD, the second LNA via the WDM, and are performed a photoelectric conversion and amplification, finally, enter the high-speed acquisition card and computer to acquire data to obtain positional and light intensity information of the Stokes light and anti-Stokes light along the fiber; before temperature measurement, performing a calibration processing for all of the sensing fiber to be tested at a constant temperature, and the calibration process being carried out twice in total in a calibration stage; in a first calibration stage, obtaining, by the data acquisition card, a backscattering light intensity curve of the anti-Stokes light and the Stokes light, whose light intensity ratio is expressed as: $\begin{array}{cc}\frac{{\varphi }_{a0}}{{\varphi }_{s0}}=\frac{K_a{V}_a^4}{K_s{V}_s^4}\exp \left(-\frac{h\Delta v}{{\mathrm{kT}}_{c0}}\right)\exp \left[\left({\alpha }_s-{\alpha }_a\right)l\right]+P_0{K}_r{V}_0^4\exp \left(-2{\alpha }_{o}l\right)& \left(1\right)\end{array}$ where Ks, Ka and Kr are coefficients related to a scattering end cross section of the fiber; Vs, Va and Vo are frequencies of the Stokes light, anti-Stokes light and incident light; h and k are Planck Constant and Boltzmann Constant respectively; 4v is a Raman frequency offset of the fiber, which is 13.2 THz; as, aa, ao are attenuation coefficients of the Stokes light, anti-Stokes light and Rayleigh scattering light per unit length of the fiber, respectively; Teo represents a temperature value of the sensing fiber to be tested in the first calibration stage; / represents a distance between the position and a front end of the multi-mode sensing fiber; and Po is intensity of incident light; in a second calibration stage, obtaining, by the data acquisition card, the backscattering light intensity curve of the anti-Stokes light and the Stokes light, whose light intensity ratio is expressed as: $\begin{array}{cc}\frac{{\varphi }_{a1}}{{\varphi }_{s1}}=\frac{K_a{V}_a^4}{K_s{V}_s^4}\exp \left(-\frac{h\Delta v}{k{T}_{c1}}\right)\exp \left[\left({\alpha }_s-{\alpha }_a\right)l\right]+P_0{K}_r{V}_0^4\exp \left(-2{\alpha }_{a}l\right),& \left(2\right)\end{array}$ where Tai represents the temperature value of the sensing fiber to be tested in the second calibration stage; Step 3: processing the Stokes light and anti-Stokes light signals in a measurement stage in the measurement stage, obtaining, by the data acquisition card, the backscattering light intensity curve of the anti-Stokes light and the Stokes light, whose light intensity ratio is expressed as: $\begin{array}{cc}\frac{{\varphi }_a}{{\varphi }_s}=\frac{K_a{V}_a^4}{K_s{V}_s^4}\exp \left(-\frac{h\Delta v}{kT}\right)\exp \left[\left({\alpha }_s-{\alpha }_a\right)l\right]+P_0{K}_r{V}_0^4\exp \left(-2{\alpha }_{o}l\right).& \left(3\right)\end{array}$ where T represents the temperature value at a position 1 of the sensing fiber to be tested in the measurement stage; Step 4: a high-precision temperature demodulation method oriented to a distributed fiber Raman sensing system subtracting formula (1) from formula (3) to obtain: $\begin{array}{cc}\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varphi }_{a0}}{{\varphi }_{\mathrm{s0}}}=\frac{K_a{V}_a^4}{K_s{V}_s^4}\exp \left[\left({\alpha }_s-{\alpha }_a\right)l\right]\left[\exp \left(-\frac{h\Delta \nu }{\mathrm{kT}}\right)-\exp \left(-\frac{h\Delta v}{k{T}_{c0}}\right)\right]& \left(4\right)\end{array}$ subtracting formula (2) from formula (3) to obtain:
ϕ.sub.a/ϕ.sub.s−ϕ.sub.a1/ϕ.sub.s1=K.sub.aV.sub.a.sup.4/K.sub.sV.sub.s.sup.4exp[(α.sub.s−α.sub.a)l]exp(−hΔv/kT)−exp(−hΔv/kT.sub.c1)  (5) making a ratio of formula (4) to formula (5) to obtain: $\begin{array}{cc}\frac{\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varphi }_{a0}}{{\varphi }_{s0}}}{\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varphi }_{a1}}{{\varphi }_{s1}}}=\frac{\exp \left(-\frac{h\Delta v}{\mathrm{kT}}\right)-\exp \left(-\frac{h\Delta v}{{\mathrm{kT}}_{c0}}\right)}{\exp \left(-\frac{h\Delta v}{\mathrm{kT}}\right)-\exp \left(-\frac{h\Delta v}{{\mathrm{kT}}_{c1}}\right)};& \left(6\right)\end{array}$ resolving formula (6) to obtain: $\begin{array}{cc}T={.Math.\ln \left\{\frac{\left(\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varphi }_{a0}}{{\varphi }_{s0}}/\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varphi }_{a1}}{{\varphi }_{s1}}\right)\exp \left(-\frac{h\Delta v}{k{T}_{c1}}\right)-\exp \left(-\frac{h\Delta v}{k{T}_{c0}}\right)}{\left(\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varphi }_{a0}}{{\varphi }_{s0}}/\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varphi }_{a1}}{{\varphi }_{s1}}\right)-1}\right\}\left(-\frac{k}{h\Delta v}\right).Math.}^{-1}.& \left(7\right)\end{array}$

2. The high-precision temperature demodulation method oriented to a distributed fiber Raman sensor of claim 1, characterized in that: the pulsed laser has a wavelength of 1550 nm, a pulse width of 10 ns, and a repetition frequency of 8 KHz; an operating wavelength of the WDM is 1550 nm/1450 nm/1663 nm; the APD has a bandwidth of 100 MHz and a spectral response range of 900-1700 nm; a bandwidth of the LNA is 100 MHz; the data acquisition card has four channels, a sampling rate of 100M/s and a bandwidth of 100 MHz; and the sensing fiber to be tested is an ordinary multi-mode fiber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic diagram of a high-precision detection device oriented to a distributed fiber Raman sensing system of the present disclosure.

(2) 1—pulse laser; 2—WDM (wavelength division multiplexer 1550 nm/1450 nm/1650 nm); 3—to-test sensing fiber; 4—first APD; 5—second APD; 6—first LNA; 7—second LNA; 8—high-data acquisition card; 9—computer.

DETAILED DESCRIPTION

(3) Embodiments of the present disclosure will be described in detail below with reference to the drawings.

(4) A high-precision temperature demodulation method oriented to a distributed fiber Raman sensing system is implemented by a following device. The device includes a 1550 nm high-power pulsed laser, a WDM, two APDs, two LNAs, a sensing fiber to be tested (ordinary multi-mode fiber), a data acquisition card, and a computer.

(5) The method is divided into following four steps.

(6) Step 1: Constructing a High-Precision Temperature Detection Device Oriented to a Distributed Fiber Raman Sensing System.

(7) As shown in FIG. 1, a fiber Raman thermodetector includes a pulsed laser, a WDM, two APDs, two LNAs, a data acquisition card, and a computer; wherein an output end of the pulsed laser 1 is connected to an input end of the WDM 2. An input end of the sensing fiber to be tested 3 is connected to a common end of the WDM 2. Two output ends of the WDM 2 are respectively connected to input ends of a first APD 4 and a second APD 5; an output end of the first APD 4 is connected to an input end of a first LNA 6; an output end of the second APD 5 is connected to an input end of a second LNA 7; output ends of the first LNA 6 and the second LNA 7 are connected to input ends of the data acquisition card 8; and an output end of the data acquisition card 8 is connected to an input end of the computer 9.

(8) In specific implementation, the pulsed laser has a wavelength of 1550 nm, a pulse width of 10 ns, and a repetition frequency of 8 KHz. The operating wavelength of the WDM is 1550 nm/1450 nm/1663 nm. The APD has a bandwidth of 100 MHz, and a spectral response range of 900-1700 nm. The bandwidth of the LNA is 100 MHz. The data acquisition card has four channels, a sampling rate of 100M/s and a bandwidth of 100 MHz. The sensing fiber to be tested is an ordinary multi-mode fiber.

(9) Step 2: Processing Stokes Light and Anti-Stokes Light Signals in a Calibration Stage.

(10) Starting the fiber Raman thermodetector, and transmitting laser pulses emitted by the high-power pulsed laser to the sensing fiber to be tested; conducting spontaneous Raman scattering when the laser pulses propagate in the sensing fiber to be tested, thereby enabling Stokes light and anti-Stokes light to be generated at each position of the multi-mode sensing fiber.

(11) The Stokes light is incident on the data acquisition card successively via the WDM, the first APD, and the first LNA, and the data acquisition card performs an analog-digital conversion on the Stokes light and obtains a light intensity curve of the Stokes light.

(12) The anti-Stokes light is incident on the data acquisition card successively via the WDM, the second APD, and the second LNA, and the data acquisition card performs an analog-digital conversion on the anti-Stokes light and obtains a light intensity curve of the anti-Stokes light.

(13) Before temperature measurement, it is necessary to perform a calibration processing for all of the sensing fiber at a constant temperature, and the calibration processing is carried out twice in total during the calibration stage. In a first calibration stage, the data acquisition card obtains a backscattering light intensity curve of the anti-Stokes light and Stokes light, and the light intensity ratio between the anti-Stokes light and Stokes light is expressed as:

(14) $\begin{array}{cc}\frac{{\varphi }_{a0}}{{\varphi }_{s0}}=\frac{K_a{V}_a^4}{K_s{V}_s^4}\exp \left(-\frac{h\Delta v}{k{T}_{c0}}\right)\exp \left[\left({\alpha }_s-{\alpha }_a\right)l\right]+P_0{K}_r{V}_0^4\exp \left(-2{\alpha }_{o}l\right),& \left(8\right)\end{array}$

(15) where K.sub.s, K.sub.a and K.sub.r are coefficients related to a scattering end cross section of the fiber; V.sub.s, V.sub.a and V.sub.0 are frequencies of Stokes light, anti-Stokes light and incident light; h and k are Planck Constant and Boltzmann Constant respectively; Δv is a Raman frequency offset of the fiber, which is 13.2 THz; α.sub.s, α.sub.a, α.sub.0 are attenuation coefficients of the Stokes light, anti-Stokes light and Rayleigh scattering light per unit length of the fiber, respectively; T.sub.c0 represents a temperature value of the sensing fiber to be tested in the first calibration stage; l represents a distance between the position and a front end of the multi-mode sensing fiber; and P.sub.0 is intensity of incident light.

(16) In a second calibration stage, the data acquisition card obtains the backscattering light intensity curve of the anti-Stokes light and Stokes light, and the light intensity ratio between the anti-Stokes light and Stokes light is expressed as:

(17) $\begin{array}{cc}\frac{{\varphi }_{a1}}{{\varphi }_{s1}}=\frac{K_a{V}_a^4}{K_s{V}_s^4}\exp \left(-\frac{h\Delta v}{k{T}_{c1}}\right)\exp \left[\left({\alpha }_s-{\alpha }_a\right)l\right]+P_0{K}_r{V}_0^4\exp \left(-2{\alpha }_{o}l\right),& \left(9\right)\end{array}$

(18) where T.sub.c1 represents the temperature value of the sensing fiber to be tested in the second calibration stage.

(19) Step 3: Processing the Stokes Light and Anti-Stokes Light Signals in a Measurement Stage

(20) In the measurement stage, the data acquisition card obtains the backscattering light intensity curve of the anti-Stokes light and Stokes light, and the light intensity ratio between the anti-Stokes light and Stokes light may be expressed as:

(21) 0$\begin{array}{cc}\frac{{\varphi }_a}{{\varphi }_s}=\frac{K_a{V}_a^4}{K_s{V}_s^4}\exp \left(-\frac{h\Delta v}{kT}\right)\exp \left[\left({\alpha }_s-{\alpha }_a\right)l\right]+P_0{K}_r{V}_0^4\exp \left(-2{\alpha }_{o}l\right),& \left(10\right)\end{array}$

(22) where T represents the temperature value at a position l of the sensing fiber to be tested in the measurement stage.

(23) Step 4: A High-Precision Temperature Demodulation Method Oriented to a Distributed Fiber Raman Sensing System

(24) A following formula can be obtained by subtracting formula (8) from formula (10):

(25) $\begin{array}{cc}\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varphi }_{a0}}{{\varphi }_{s0}}=\frac{K_a{V}_a^4}{K_s{V}_s^4}\exp \left[\left({\alpha }_s-{\alpha }_a\right)l\right]\left[\exp \left(-\frac{h\Delta v}{kT}\right)-\exp \left(-\frac{h\Delta v}{k{T}_{c0}}\right)\right].& \left(11\right)\end{array}$

(26) A following formula can be obtained by subtracting formula (9) from formula (10):

(27) $\begin{array}{cc}\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varphi }_{a1}}{{\varphi }_{s1}}=\frac{K_a{V}_a^4}{K_s{V}_s^4}\exp \left[\left({\alpha }_s-{\alpha }_a\right)l\right]\left[\exp \left(-\frac{h\Delta v}{kT}\right)-\exp \left(-\frac{h\Delta v}{k{T}_{c1}}\right)\right].& \left(12\right)\end{array}$

(28) A following formula can be obtained by making a ratio of formula (12) to formula (11):

(29) $\begin{array}{cc}\frac{\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varphi }_{a0}}{{\varphi }_{s0}}}{\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varphi }_{a1}}{{\varphi }_{s1}}}=\frac{\exp \left(-\frac{h\Delta v}{kT}\right)-\exp \left(-\frac{h\Delta v}{k{T}_{c0}}\right)}{\exp \left(-\frac{h\Delta v}{kT}\right)-\exp \left(-\frac{h\Delta v}{k{T}_{c1}}\right)}.& \left(13\right)\end{array}$

(30) After formula (13) is resolved, a follow formula may be obtained:

(31) $\begin{array}{cc}T={.Math.\ln \left\{\frac{\left(\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varphi }_{a0}}{{\varphi }_{s0}}/\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varnothing }_{a1}}{{\varphi }_{s1}}\right)\exp \left(-\frac{h\Delta v}{k{T}_{c1}}\right)-\exp \left(-\frac{h\Delta v}{k{T}_{c0}}\right)}{\left(\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varphi }_{a0}}{{\varphi }_{s0}}/\frac{{\varphi }_a}{{\varphi }_s}-\frac{{\varphi }_{a1}}{{\varphi }_{s1}}\right)-1}\right\}\left(-\frac{k}{h\Delta v}\right).Math.}^{-1}.& \left(14\right)\end{array}$

(32) Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present disclosure rather than limiting, and although detailed descriptions have been made with reference to the embodiments of the present disclosure, it should be understood for those skilled of the art that modifications or equivalent replacements to the technical solutions of the present disclosure do not depart from the spirit and scope of the technical solutions of the present disclosure, and should be covered by the scope of the claims of the present disclosure.