CN105136337A - Raman distributed temperature measurement system based on mode multiplexing and temperature measurement method - Google Patents

Abstract

The invention discloses a Raman distributed temperature measurement system based on mode multiplexing. The temperature measurement system includes: a pulse laser; a mode multiplexing demultiplexer, which converts the received pulse laser into m channels of light of equal power, Perform mode multiplexing; the few-mode fiber receives the m-path light output by the mode multiplexing demultiplexer, propagates in the few-mode fiber, and generates backscattered light, and the backscattered light is reversely transmitted into the all The mode multiplexing demultiplexer, the mode multiplexing demultiplexer decomposes the scattered light into m scattered lights and outputs them; m Raman filters receive the m scattered lights and convert the Raman Stokes light and Raman anti-Stokes light are filtered and output respectively; 2m photodetectors receive scattered light for photoelectric conversion and output electrical signals; signal processors process the output electrical signals to obtain temperature information. The above temperature measuring system increases the detection distance of the temperature measuring system and improves the spatial resolution of the temperature measuring system.

Description

A Raman distributed temperature measurement system and temperature measurement method based on mode multiplexing

technical field

The invention relates to the technical field of distributed optical fiber temperature measurement systems, in particular to a Raman distributed temperature measurement system and temperature measurement method based on mode multiplexing.

Background technique

Temperature sensing systems are widely used in infrastructure such as roads, tunnels, bridges, water conservancy projects, high-voltage cables, underground coal mines, and petrochemical fields. The traditional single-point mobile or distributed measurement methods realized by networking of multiple electronic sensors have disadvantages such as difficult installation, difficult maintenance, and susceptibility to electromagnetic interference. Optical fiber-based distributed temperature sensor is an effective means to improve the shortcomings of the above-mentioned sensing system, and the optical fiber has the advantages of low insertion loss, long detection distance, easy laying, etc. It can realize online real-time monitoring and forecasting, and is free from electromagnetic interference. The system is simple and safe.

In the distributed optical fiber sensor, the distributed Raman temperature sensor utilizes the principle of Raman scattering in the optical fiber, and uses the back Raman scattered light in the optical fiber propagation process as the sensing signal, which can realize the detection of each temperature in the entire optical fiber link. The temperature field of the point is monitored.

The sensing fiber used in the existing Raman temperature measurement system is a multi-mode fiber or a single-mode fiber. For the distributed Raman sensing system based on multimode fiber, its advantage is that multimode fiber has a large mode field area and high Raman gain coefficient, and its disadvantage is that the loss of multimode fiber is large, resulting in a limited detection distance. The spatial resolution of sensing is insufficient due to crosstalk introduced by intermodal dispersion. For the distributed Raman sensing system based on single-mode fiber, its advantage is that the loss is small, and its disadvantage is that the mode field area is small, so the input optical power is limited and the detection distance is limited.

Contents of the invention

The present application provides a Raman distributed temperature measurement system and temperature measurement method based on mode multiplexing, which improves the technical problem in the prior art that the detection distance of multi-mode optical fibers and single-mode optical fibers is small.

The application provides a Raman distributed temperature measurement system based on mode multiplexing, and the temperature measurement system includes:

a pulsed laser for emitting pulsed laser light;

A mode multiplexing demultiplexer is connected to the pulsed laser through a single-mode optical fiber, receives the pulsed laser, converts the received pulsed laser into m-channel equal-power light, performs mode multiplexing, and outputs it;

The few-mode fiber is connected to the mode multiplexing and demultiplexer, and receives the m-path light output by the mode multiplexing-demultiplexer, and the m-path light excites the specified m modes in the few-mode fiber, in Propagate in the few-mode optical fiber, and generate backscattered light, the backscattered light is reversely transmitted into the mode multiplexing demultiplexer, and the mode multiplexing demultiplexer decomposes the scattered light into m scattered light light output;

m Raman filters are respectively connected to the mode multiplexing demultiplexer through a single-mode fiber, and the Raman filters receive the m scattered lights and combine the Raman Stokes light and the Raman The anti-Stokes light is filtered separately and output;

2m photoelectric conversion modules are respectively connected to the m Raman filters, and receive m paths of Raman-Stokes scattered light and m paths of Raman anti-Stokes scattered light output by the corresponding Raman filters, Perform photoelectric conversion and output electrical signals;

The signal processor processes the output electrical signals of the 2m photodetectors to obtain temperature information.

Preferably, the temperature measurement system further includes a synchronization source connected to the signal processor and the pulsed laser for synchronous triggering between the pulsed laser and the signal processor.

Preferably, the pulsed laser is an all-fiber pulsed laser, or an integrated semiconductor pulsed laser.

Preferably, the pulsed laser 11 has a wavelength of 1550 Å.

Preferably, the passband ranges of the Raman filter are respectively 1450nm and 1660nm.

The present application also provides a temperature measurement method applied to the temperature measurement system, the method comprising:

Perform temperature calibration on the temperature measurement system of the Raman scattering signal of a mode, in the few-mode optical fiber at the reference temperature T ₀ , the back-to-Raman anti-Stokes measured by the photodetector of the photoelectric conversion module The ratio of scattered power P _as1 (T ₀ ) to Raman Stokes scattered power P _s1 (T ₀ ) is: $\frac{P_{\mathrm{as}1}\left(T_0\right)}{P_{\mathrm{the\; s}1}\left(T_0\right)}=\frac{K_{\mathrm{as}1}}{K_{\mathrm{the\; s}1}}{\left(\frac{v_{\mathrm{as}1}}{v_{\mathrm{the\; s}1}}\right)}^4\exp ({-\mathrm{h\Δv}}_1/{\mathrm{kT}}_0)\exp [-({\mathrm{\α}}_{\mathrm{as}1}-{\mathrm{\α}}_{\mathrm{the\; s}1})L]---\left(1\right),$ Among them, K _as1 and K _s1 are the anti-Stokes scattering cross-section and Stokes scattering cross-section transmitted in the few-mode fiber respectively, and v _as1 and v _s1 are the anti-Stokes scattering cross-section of the light of this mode transmitted in the few-mode fiber The frequency of Stokes light and Stokes light, h is Planck's constant, Δv ₁ is the Raman frequency shift produced by the light of this mode in the few-mode fiber, k is Boltzmann's constant, α _as1 and α _s1 are the loss coefficients of the anti-Stokes light and Stokes light transmitted by the light of this mode in the few-mode fiber, respectively, and L is the length of the fiber;

The ratio of the outputs of two avalanche photodiodes at any temperature T is: $\frac{P_{\mathrm{as}1}\left(T\right)}{P_{\mathrm{the\; s}1}\left(T\right)}=\frac{K_{\mathrm{as}1}}{K_{\mathrm{the\; s}1}}{\left(\frac{v_{\mathrm{as}2}1}{v_{\mathrm{the\; s}1}}\right)}^4\exp ({-\mathrm{h\Δv}}_1/\mathrm{kT})\exp [-({\mathrm{\α}}_{\mathrm{as}1}-{\mathrm{\α}}_{\mathrm{the\; s}1})L]---\left(2\right)$

According to formula 1 and formula 2, the obtained temperature distribution curve is:

The beneficial effects of this application are as follows:

The Raman distributed temperature measurement system based on mode multiplexing provided by this application, due to the small optical fiber transmission loss, and the intermodal dispersion is much smaller than ordinary multimode optical fibers, so it not only increases the detection distance of the temperature measurement system, but also improves The spatial resolution of the temperature measurement system is improved. It solves the technical problem that the existing distributed Raman temperature measurement system of multimode optical fiber has relatively large fiber transmission loss, and due to the influence of intermodal dispersion, the detection distance and spatial resolution of the temperature measurement system are limited.

The Raman distributed temperature measurement system based on mode multiplexing provided by this application, in the process of light transmission in few-mode fibers, the mode field area of some modes is larger than that of single-mode fibers, and can tolerate higher The optical power improves the detection distance, and solves the technical problems of the prior art that the mode field area of the sensing fiber in the distributed Raman temperature measurement system of the single-mode fiber is small, the optical power entering the fiber is limited, and the detection distance is limited. .

In addition, the present application introduces a mode multiplexer and demultiplexer, and a single sensing fiber (few-mode fiber) is used to realize simultaneous measurement of multiple channels, so that the measurement time is greatly reduced.

Description of drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the accompanying drawings that need to be used in the description of the embodiments. Obviously, the accompanying drawings in the following description are only of the present invention. some examples.

FIG. 1 is a schematic structural diagram of a Raman distributed temperature measurement system based on mode multiplexing according to a preferred embodiment of the present application.

Detailed ways

Embodiments of the present application provide a Raman distributed temperature measurement system and temperature measurement method based on mode multiplexing, which solves the technical problem of the short detection distance of multi-mode optical fibers and single-mode optical fibers in the prior art.

The technical solution in the embodiment of the present application is to solve the above-mentioned technical problems, and the general idea is as follows:

The application provides a Raman distributed temperature measurement system based on mode multiplexing, and the temperature measurement system includes:

a pulsed laser for emitting pulsed laser light;

A mode multiplexing demultiplexer is connected to the pulsed laser through a single-mode optical fiber, receives the pulsed laser, converts the received pulsed laser into m-channel equal-power light, performs mode multiplexing, and outputs it;

The few-mode fiber is connected to the mode multiplexing and demultiplexer, and receives the m-path light output by the mode multiplexing-demultiplexer, and the m-path light excites the specified m modes in the few-mode fiber, in Propagate in the few-mode optical fiber, and generate backscattered light, the backscattered light is reversely transmitted into the mode multiplexing demultiplexer, and the mode multiplexing demultiplexer decomposes the scattered light into m scattered light light output;

m Raman filters are respectively connected to the mode multiplexing demultiplexer through a single-mode fiber, and the Raman filters receive the m scattered lights and combine the Raman Stokes light and the Raman The anti-Stokes light is filtered separately and output;

2m photoelectric conversion modules are respectively connected to the m Raman filters, and receive m paths of Raman-Stokes scattered light and m paths of Raman anti-Stokes scattered light output by the corresponding Raman filters, Perform photoelectric conversion and output electrical signals;

The signal processor processes the output electrical signals of the 2m photodetectors to obtain temperature information.

The Raman distributed temperature measurement system based on mode multiplexing provided by this application, due to the small optical fiber transmission loss, and the intermodal dispersion is much smaller than ordinary multimode optical fibers, so it not only increases the detection distance of the temperature measurement system, but also improves The spatial resolution of the temperature measurement system is improved. It solves the technical problem that the existing distributed Raman temperature measurement system of multimode optical fiber has relatively large fiber transmission loss, and the detection distance and spatial resolution of the temperature measurement system are limited due to the influence of intermodal dispersion.

The Raman distributed temperature measurement system based on mode multiplexing provided by this application, in the process of light transmission in few-mode fibers, the mode field area of some modes is larger than that of single-mode fibers, and can tolerate higher The optical power improves the detection distance, and solves the technical problems of the prior art that the mode field area of the sensing fiber in the distributed Raman temperature measurement system of the single-mode fiber is small, the optical power entering the fiber is limited, and the detection distance is limited. .

In addition, the present application introduces a mode multiplexer and demultiplexer, and a single sensing fiber (few-mode fiber) is used to realize simultaneous measurement of multiple channels, so that the measurement time is greatly reduced.

In order to better understand the above-mentioned technical solution, the above-mentioned technical solution will be described in detail below in conjunction with the accompanying drawings and specific implementation methods.

The present application provides a Raman distributed temperature measurement system based on mode multiplexing, as shown in FIG. 1 , which is a schematic structural diagram of a Raman distributed temperature measurement system based on mode multiplexing in a preferred embodiment of the present application. The temperature measurement system includes a pulsed laser 11 , a mode multiplexer/demultiplexer 12 , a few-mode fiber 13 , m Raman filters 17 , a 2m photoelectric conversion module 14 , a signal processor 15 and a synchronization source 16 .

The pulsed laser 11 is used to emit pulsed laser light. The pulsed laser 11 can be an all-fiber pulsed laser, or an integrated semiconductor pulsed laser. In this embodiment, the wavelength of the pulsed laser 11 is 1550.

The mode multiplexing and demultiplexing device 12 is connected to the pulsed laser through a single-mode fiber, and the mode multiplexing and demultiplexing device 12 includes an interface A, an interface B and m interfaces C, wherein the interface A is connected to a single-mode optical fiber; the interface B is connected to the few-mode optical fiber 13, and the interface C is connected to the single-mode optical fiber.

The working principle of the mode multiplexing and demultiplexing device 12 is as follows, the mode multiplexing and demultiplexing device 12 receives the input of the pulsed laser of the pulsed laser 11 from the interface A, and then converts the received pulsed laser into m-channel The light of equal power performs mode multiplexing, enters the few-mode fiber 13 through the interface B, and simultaneously excites the specified m modes in the few-mode fiber 13, propagates in the few-mode fiber 13, and generates backscattered light, The backscattered light is reversely transmitted into the mode multiplexing and demultiplexing device 12, and the mode multiplexing and demultiplexing device 12 can decompose the scattered light returned from the few-mode fiber 13 into m according to different modes. channels, respectively entering the m Raman filters 17 through the m interface C. In this embodiment, the few-mode fiber 13 supports n modes, where n is greater than or equal to m.

The few-mode fiber 13 is used as a sensing fiber, which can support more than two spatial modes of the few-mode fiber, and the pulsed laser only propagates in the fundamental mode LP01 state in the few-mode fiber. During the propagation of the pulsed laser in the state of the base film LP01 in the few-mode optical fiber 13, backscattering is continuously generated, and the backscattered light returns to the mode multiplexing and demultiplexing device 12, and passes through the mode multiplexing and demultiplexing The interface C of the device 12 is input to the Raman filter 17. When the fundamental mode of the few-mode optical fiber 13 is excited, the mode field area of the fundamental mode is larger than that of ordinary single-mode optical fibers, and the intermodal dispersion is much smaller than that of ordinary multi-mode optical fibers.

The m Raman filters 17 are respectively connected to the m interfaces C of the mode multiplexer/demultiplexer 12 through single-mode optical fibers. The m paths of scattered light enter the m Raman filters 17 respectively. The Raman filter 17 can respectively filter the Raman-Stokes light and the Raman anti-Stokes light and output them to the photoelectric conversion module 14 . The Raman Stokes light frequency is 10-13THz lower than the signal light frequency, and the Raman anti-Stokes light frequency is 10-13THz higher than the signal light frequency. The passband ranges of the Raman filter 17 are respectively 1450nm band range and 1660nm band range.

The 2m photoelectric conversion modules 14 are respectively connected to the m Raman filters 17, and are used to perform m-path Raman-Stokes light and m-path Raman anti-Stokes light output by the corresponding Raman filter Photoelectric conversion to obtain 2m output electrical signals.

The signal processor 15 is connected to the 2m photoelectric conversion modules 14 for processing the 2m output electrical signals to obtain temperature distribution information in the few-mode optical fiber 13 .

The synchronization source 16 is connected to the signal processor 15 and the pulsed laser 11 for synchronous triggering between the pulsed laser 11 and the signal processor 15 .

The specific working process of the temperature measurement system is as follows: the pulsed laser 11 emits a pulsed laser, and the mode multiplexing demultiplexer 12 receives the input of the pulsed laser of the pulsed laser 11 from the interface A, and then sends the received pulsed laser Transform into m channels of equal power light, perform mode multiplexing, enter the few-mode fiber 13 through the interface B, and simultaneously stimulate the specified m modes in the few-mode fiber 13, propagate in the few-mode fiber 13, and generate back To the scattered light, the backscattered light is reversely transmitted into the mode multiplexing demultiplexer 12, and the mode multiplexing demultiplexer 12 can return the scattered light from the few-mode fiber 13 according to different modes The light is decomposed into m paths, and enters the m Raman filters 17 respectively through the m interface C. The Raman Stokes light and the Raman anti-Stokes light are filtered out by the Raman filter 17, and are respectively output to the corresponding electrical conversion module 14 for photoelectric conversion, and the signal processor 15 pairs of photodetectors The output signal of 14 is processed to obtain temperature information.

The Raman distributed temperature measurement system based on mode multiplexing provided by this application, due to the small optical fiber transmission loss, and the intermodal dispersion is much smaller than ordinary multimode optical fibers, so it not only increases the detection distance of the temperature measurement system, but also improves The spatial resolution of the temperature measurement system is improved. It solves the technical problem that the existing distributed Raman temperature measurement system of multimode optical fiber has relatively large fiber transmission loss, and the detection distance and spatial resolution of the temperature measurement system are limited due to the influence of intermodal dispersion. Both the light source and the light receiving device used in this application are single-mode devices.

The Raman distributed temperature measurement system based on mode multiplexing provided by this application, in the process of light transmission in few-mode fibers, the mode field area of some modes is larger than that of single-mode fibers, and can tolerate higher The optical power improves the detection distance, and solves the technical problems of the prior art that the mode field area of the sensing fiber in the distributed Raman temperature measurement system of the single-mode fiber is small, the optical power entering the fiber is limited, and the detection distance is limited. .

In addition, the present application introduces a mode multiplexer and demultiplexer, and a single sensing fiber (few-mode fiber) is used to realize simultaneous measurement of multiple channels, so that the measurement time is greatly reduced.

The present application also provides a temperature measurement method, which is applied to the above-mentioned Raman distributed temperature measurement system based on mode multiplexing. According to the principle of Raman temperature measurement using two-way demodulation of Raman Stokes scattering and Raman anti-Stokes scattering, the following steps of signal processing can be obtained:

First, for a Raman scattering signal of a certain mode, the Raman temperature measurement system is temperature calibrated. In the entire few-mode optical fiber at the reference temperature T ₀ , the back-facing Raman measured by the photodetector of the photoelectric conversion module 14 The ratio of anti-Stokes scattering power P _as1 (T ₀ ) to Raman Stokes scattering power P _s1 (T ₀ ) is:

$\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}}{<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 4</span>}}\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span>{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; h\&Delta;v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; kT</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Among them, K _as1 and K _s1 are the anti-Stokes scattering cross-section and Stokes scattering cross-section transmitted in the few-mode fiber respectively, and v _as1 and v _s1 are the anti-Stokes scattering cross-section of the light of this mode transmitted in the few-mode fiber The frequency of Stokes light and Stokes light, h is Planck's constant, Δv ₁ is the Raman frequency shift produced by the light of this mode in the few-mode fiber, k is Boltzmann's constant, α _as1 and α _s1 are the loss coefficients of the anti-Stokes light and Stokes light transmitted in the few-mode fiber by the light of this mode, respectively, and L is the length of the fiber.

Then the ratio of the output of the two avalanche photodiodes at any temperature T is obtained as:

$\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}}{<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 4</span>}}\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span>{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; h\&Delta;v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; kT</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

From the above two formulas (1) and (2), we can get:

$\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; /</span>\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; h\&Delta;v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; kT</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; h\&Delta;v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; kT</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

The temperature distribution curve can be obtained as:

$\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; k</span>}}{{\mathrm{<span\; class="notranslate">\; h\&Delta;v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; ln</span>}\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; /</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

Define the ratio R(T) of Raman anti-Stokes light and Stokes light power detected by two photodetectors as:

R ₁ (T) = P _as1 (T)/P _s1 (T) (5)

Through formula (1) reference temperature T ₀ , formula (4) and formula (5), by measuring the ratio R ₁ (T) of anti-Stokes light and Stokes light power, the output from port C can be obtained The temperature distribution in an optical fiber link measured by the mode of light.

Similarly, by processing the Raman scattering signals in different modes, the temperature distribution in another group of optical fiber links can be obtained at the same time. After obtaining the distribution of m groups of optical fiber links at the same time, the influence of noise in the links can be eliminated by means of weighted average, and the measurement time can be greatly shortened.

While preferred embodiments of the invention have been described, additional changes and modifications to these embodiments can be made by those skilled in the art once the basic inventive concept is appreciated. Therefore, it is intended that the appended claims be construed to cover the preferred embodiment as well as all changes and modifications which fall within the scope of the invention.

Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention also intends to include these modifications and variations.

Claims (6)

1. a kind of Raman distributed temperature measurement system based on mode multiplexing, it is characterized in that, described temperature measurement system comprises: a pulsed laser for emitting pulsed laser light; A mode multiplexing demultiplexer is connected to the pulsed laser through a single-mode optical fiber, receives the pulsed laser, converts the received pulsed laser into m-channel equal-power light, performs mode multiplexing, and outputs it; The few-mode fiber is connected to the mode multiplexing and demultiplexer, and receives the m-path light output by the mode multiplexing-demultiplexer, and the m-path light excites the specified m modes in the few-mode fiber, in Propagate in the few-mode optical fiber, and generate backscattered light, the backscattered light is reversely transmitted into the mode multiplexing demultiplexer, and the mode multiplexing demultiplexer decomposes the scattered light into m scattered light light output; m Raman filters are respectively connected to the mode multiplexing demultiplexer through a single-mode fiber, and the Raman filters receive the m scattered lights and combine the Raman Stokes light and the Raman The anti-Stokes light is filtered separately and output; 2m photoelectric conversion modules are respectively connected to the m Raman filters, and receive m paths of Raman-Stokes scattered light and m paths of Raman anti-Stokes scattered light output by the corresponding Raman filters, Perform photoelectric conversion and output electrical signals; The signal processor processes the output electrical signals of the 2m photodetectors to obtain temperature information. 2. The temperature measurement system according to claim 1, wherein the temperature measurement system also includes a synchronization source connecting the signal processor and the pulsed laser for synchronization between the pulsed laser and the signal processor trigger. 3. The temperature measuring system according to claim 1 or 2, characterized in that, the pulsed laser is an all-fiber pulsed laser, or an integrated semiconductor pulsed laser. 4. The temperature measuring system according to claim 3, characterized in that, the wavelength of the pulsed laser 11 is 1550. 5. The temperature measurement system according to claim 1, characterized in that, the passband ranges of the Raman filter are respectively 1450nm band range and 1660nm band range. 6. A temperature measurement method, applied to the temperature measurement system according to any one of claims 1-5, characterized in that the method comprises: Perform temperature calibration on the temperature measurement system of the Raman scattering signal of a mode, in the few-mode optical fiber at the reference temperature T ₀ , the back-to-Raman anti-Stokes measured by the photodetector of the photoelectric conversion module The ratio of scattered power P _as1 (T ₀ ) to Raman Stokes scattered power P _s1 (T ₀ ) is: $\frac{P_{\mathrm{as}1}\left(T_0\right)}{P_{\mathrm{the\; s}1}\left(T_0\right)}=\frac{K_{\mathrm{as}1}}{K_{\mathrm{the\; s}1}}{\left(\frac{v_{\mathrm{as}1}}{v_{\mathrm{the\; s}1}}\right)}^4\exp (-\mathrm{h\&Delta;}{v}_1/k{T}_0)\exp [-({\mathrm{\&alpha;}}_{\mathrm{as}1}-{\mathrm{\&alpha;}}_{\mathrm{the\; s}1})L]---\left(1\right),$ Among them, K _as1 and K _s1 are the anti-Stokes scattering cross-section and Stokes scattering cross-section transmitted in the few-mode fiber respectively, and v _as1 and v _s1 are the anti-Stokes scattering cross-section of the light of this mode transmitted in the few-mode fiber The frequency of Stokes light and Stokes light, h is Planck's constant, Δv ₁ is the Raman frequency shift produced by the light of this mode in the few-mode fiber, k is Boltzmann's constant, α _as1 and α _s1 are the loss coefficients of the anti-Stokes light and Stokes light transmitted by the light of this mode in the few-mode fiber, respectively, and L is the length of the fiber; The ratio of the outputs of two avalanche photodiodes at any temperature T is: $\frac{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span>}{{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}}{<span\; class="notranslate">\; (</span>\frac{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 4</span>}}\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; (</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; h\&Delta;</span>}{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; k</span>}\mathrm{<span\; class="notranslate">\; T</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; exp</span>}<span\; class="notranslate">\; [</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; as</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; L</span>}<span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$ According to formula 1 and formula 2, the obtained temperature distribution curve is: