CN104729750A - Distributed optical fiber temperature sensor based on Brillouin scattering - Google Patents

Abstract

The invention designs a distributed optical fiber temperature sensor based on Brillouin scattering, which includes a narrow line width light source, a coupler, a pulse modulator, an erbium-doped optical fiber amplifier, a sensing fiber 1, a sensing fiber 2 and a detection unit. Among them, the laser light emitted by the narrow-linewidth light source is used as the incident light after pulse modulation and power amplification, and the incident light is split into the sensing fiber 1 and the sensing fiber 2 after being split by a coupler. The back-scattered Brillouin light in the beam is combined at the coupler, and undergoes photoelectric conversion to generate a beat-frequency Brillouin signal. By detecting the frequency shift of the beat-frequency Brillouin signal, we can detect the temperature information of the environment where the sensing fiber is located. On the premise of not introducing a local oscillator optical frequency shifting device, the invention reduces the photoelectric detection bandwidth required in the detection unit through the beat frequency of two sensing optical fibers, and reduces the cost and system complexity.

Description

A Distributed Optical Fiber Temperature Sensor Based on Brillouin Scattering

technical field

The invention relates to the field of optical fiber sensing, in particular to a distributed optical fiber temperature sensor based on Brillouin scattering.

Background technique

A temperature sensor is used to measure the temperature of its environment. The fiber optic temperature sensor uses the sensing fiber as the carrier, and uses the frequency, phase, amplitude and polarization of the optical signal transmitted in the fiber to measure the ambient temperature. Compared with other temperature sensors, the distributed optical fiber temperature sensor has the significant advantage that it can perform distributed measurement of the environment, that is, the temperature distribution along the optical fiber can be measured. Therefore, it can be widely used in the monitoring of large structures, such as pipelines and bridges.

The measurement accuracy, measurement range and spatial resolution of the distributed optical fiber temperature sensor based on Brillouin scattering are superior to other distributed optical fiber sensing technologies, so it has received extensive research and attention. Distributed optical fiber temperature sensor based on Brillouin scattering uses the Brillouin scattered light generated by the interaction of incident light and acoustic wave in the optical fiber to measure temperature. Specifically, by detecting the frequency shift of the Brillouin scattered light relative to the incident light, the temperature of the location where the scattered light occurs can be measured. The above frequency shift is called the Brillouin shift v _B , which can be expressed as

v _B =2nv _A ／λ formula (1)

Among them, n is the effective refractive index of the sensing fiber, v _A and λ are the sound velocity and the wavelength of the incident light in the sensing fiber, respectively. The Brillouin frequency shift is around 11GHz.

When the temperature or stress somewhere in the environment where the sensing fiber is located changes, the Brillouin frequency shift will change linearly accordingly, which can be expressed as

Δv _B (T, ε)=C _T ΔT+C _ε Δε Formula (2)

where C _T and C _c are the response coefficients of the Brillouin frequency shift with respect to temperature and stress changes, respectively.

At present, distributed fiber optic temperature sensors based on Brillouin scattering mainly focus on two aspects:

1) Distributed optical fiber sensor based on Brillouin scattering time domain reflectometry technology;

2) Distributed optical fiber sensor based on time domain analysis technology of Brillouin scattering.

Figure 1 shows the distributed optical fiber sensor structure based on Brillouin scattering time domain reflectometry technology. As shown in Figure 1, the structure includes a light source, a pulse modulator, a coupler, a sensing fiber and a detection unit. The light source may be a narrow linewidth light source. The coupler can be an X-type coupler (that is, a four-port coupler) or a Y-type coupler (that is, a three-port coupler), so as to realize beam splitting and recombination of beams.

In the Brillouin scattering-based time domain reflectometry distributed fiber optic sensor, when the pulse-modulated incident light is transmitted in the sensing fiber, the back Brillouin scattering can be detected at the incident end of the sensing fiber. Since the frequency shift of Brillouin scattering relative to the incident light is affected by temperature, temperature information can be obtained through measurement.

Figure 2 shows the distributed fiber optic sensor based on Brillouin scattering time domain analysis technique. As shown in Figure 3, the structure includes a pump light source, a probe light source, a pulse modulator, a coupler, a sensing fiber and a detection unit. The light source can be a wavelength tunable laser.

In the distributed optical fiber sensor based on Brillouin scattering time domain analysis technology, when the frequency difference between the pump light and the probe light is consistent with the Brillouin frequency shift of a certain area in the sensing fiber, the excited The Brillouin scattering effect causes energy transfer between the two beams. Therefore, by continuously adjusting the frequency of the two light sources and detecting the optical power coupled out from the end of the sensing fiber, the Brillouin frequency shift of the sensing fiber can be determined, thereby obtaining temperature information.

In the detection units of the above two structures, there are currently two detection methods, namely direct detection method and coherent detection method. Direct detection filters the scattered light at the detection end to extract useful Brillouin scattered light. Preferably, the filter device may be a fiber grating, a Fabry-Perot interferometer or a Mach-Zehnder interferometer. The coherent detection method introduces local oscillator light into the structure, reducing the detection bandwidth to the Brillouin frequency shift level, that is, 11GHz. There is also a method to use the incident light as local oscillator light after frequency shifting. Frequency shifting methods include acousto-optic modulators, electro-optic modulators, Brillouin fiber lasers, etc. The detection bandwidth of these methods can be reduced to several hundred MHz or so.

However, the detection units mentioned above all have disadvantages such as complex structure and high cost, which increases the difficulty of application.

Contents of the invention

The problem to be solved by the present invention is to propose a low-cost Brillouin scattering distribution with reduced detection bandwidth for the problems of complex structure and high cost of the detection unit in the existing distributed optical fiber temperature sensor based on Brillouin scattering. fiber optic temperature sensor.

In order to achieve the above object, the present invention provides a Brillouin scattering distributed optical fiber temperature sensor, which includes a narrow linewidth light source, a pulse modulator, an erbium-doped fiber amplifier, a coupler, a sensing fiber 1 and a sensing fiber laid together Optical fiber 2, and a detection unit.

The laser light emitted by the narrow-linewidth light source is pulse-modulated and amplified by an erbium-doped fiber amplifier, and then divided into two paths of detection light by a coupler. The first path of detection light enters the sensing fiber 1 , and the second path of detection light enters the sensing fiber 2 . Both sensing fibers generate backscattered Brillouin light containing temperature information. The two backscattered lights are combined at the coupler and coupled to the detection unit, so that we can extract the beat-frequency Brillouin signal containing temperature information on the sensing fiber link.

Different sensing fibers have different Brillouin frequency shifts due to the doping in the fiber core and the difference in fiber core diameter. The Brillouin backscattered light of multiple sensing fibers is combined and passed through the detection unit to generate a beat-frequency Brillouin signal with multi-peak characteristics in the frequency domain. The beat frequency Brillouin signal frequency shift of sensing fiber 1 and sensing fiber 2 is:

$\mathrm{\δ}{v}_B^{12}=(v_{B1,\mathrm{ref}}-v_{B2,\mathrm{ref}})+(C_T^1-C_T^2)(T-T_{\mathrm{ref}})+(C_{\mathrm{\ϵ}}^1-C_{\mathrm{\ϵ}}^2)(\mathrm{\ϵ}-{\mathrm{\ϵ}}_{\mathrm{ref}})$ Formula (3)

Because different sensing fibers have different temperature and stress coefficients, when the external temperature and stress change, the frequency shift of the beat-frequency Brillouin signal also changes linearly. The variable value is:

$\mathrm{\Δ\δ}{v}_B^{\mathrm{ij}}=(C_T^i-C_T^j)\mathrm{\ΔT}+(C_{\mathrm{\ϵ}}^i-C_{\mathrm{\ϵ}}^j)\mathrm{\Δ\ϵ}$ Formula (4)

Therefore, temperature information can be obtained by detecting the frequency shift of the beat frequency Brillouin signal.

Preferably, the coupler is a 3dB coupler with a splitting ratio of 50:50. The output of the erbium-doped fiber amplifier is coupled to port 1 of the coupler; the sensing fiber 1 and sensing fiber 2 are respectively coupled to ports 3 and 4 of the coupler; the detection unit is coupled to port 2 of the coupler.

Preferably, the detection unit in this structure includes a photoelectric converter and an electric spectrum analyzer. The photoelectric converter converts the optical signal into an electrical signal, thereby obtaining a beat-frequency Brillouin signal. An electrical spectrum analyzer can detect beat-frequency Brillouin signals.

Preferably, the pulse modulator in this structure may be an acousto-optic modulator or an electro-optic modulator. The pulse modulator modulates the incident light into a rectangular pulse signal with a certain interval. The repetition frequency of the modulated pulse signal determines the sensing range, and the pulse width of the pulse signal determines the sensing resolution.

Preferably, the photoelectric converter in the detection unit only needs a relatively low bandwidth to complete the measurement, because the bandwidth of the beat-frequency Brillouin signal is about 1 GHz.

Compared with the prior art, the present invention has the advantage that two-way sensing optical fiber Brillouin scattered light is used for beating frequency, so that high-frequency detection is converted into low-frequency detection, so that there is no need for a high-cost high-bandwidth photoelectric converter, and no The frequency modulation of the local oscillator light needs to be performed in advance to increase the complexity of the structure.

To the accomplishment of the above and related ends, one or more aspects of the invention comprise the features hereinafter described in detail and particularly pointed out in the claims. The following description and accompanying drawings detail certain exemplary aspects of the invention. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Furthermore, the invention is intended to include all such aspects and their equivalents.

Description of drawings

By referring to the following description combined with the accompanying drawings and the contents of the claims, and with a more comprehensive understanding of the present invention, other objectives and results of the present invention will be more clear and easy to understand. In the attached picture:

Figure 1 is a schematic diagram of the distributed optical fiber sensor structure based on Brillouin scattering time domain reflectometry technology;

Fig. 2 is the schematic diagram of the distributed optical fiber sensor structure based on Brillouin scattering time domain analysis technique;

Fig. 3 is the overall structural representation of the present invention;

Fig. 4 is the spectrogram of the measured beat frequency Brillouin signal;

Figure 5 shows the frequency shift and linear fitting results of the beat frequency Brillouin signal measured at different set temperatures.

Detailed ways

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident, however, that these embodiments may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing one or more embodiments.

Various embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

Referring to Fig. 3, the Brillouin scattering distributed optical fiber temperature sensor based on the present invention includes a light source, a pulse modulator, an erbium-doped fiber amplifier, a coupler, a sensing fiber 1 and a sensing fiber 2 laid together , photoelectric converter and electrical spectrum analyzer. Preferably, the light source may be a laser with a narrow linewidth, the center wavelength of which is 1550 nm, and the linewidth is on the order of MHz. The output of the narrow-linewidth light source is modulated by the pulse modulator and amplified by the erbium-doped fiber amplifier as the incident light, and then coupled with port 1 of the coupler. Preferably, a coupler with a splitting ratio of 50:50 is used, that is, a 3dB coupler.

After the incident light is split by the coupler, it is respectively used as the probe light and coupled with the sensing fiber 1 and the sensing fiber 2 through the 3 port and 4 port of the coupler. The two sensing fibers laid together will generate backscattered Brillouin light containing temperature information. The two backscattered lights return to the coupler to combine and convert them into beat-frequency Brillouin electrical signals by the detection unit, so that we can extract the temperature information on the sensing fiber link.

At the detection end of the electric spectrum analyzer, the beat frequency Brillouin signal is roughly around 1GHz. Compared with the 11GHz of the traditional solution, it has been greatly optimized, thereby reducing the cost of detection.

Figure 4 shows the spectrum of the beat-frequency Brillouin signal measured at 20°C, 50°C and 80°C and the results of Lorentzian fitting after 50 averages of the amplitude and the original data (fitting degrees, respectively is R ² =0.948, 0.977, 0.975). When the external temperature rises, the power of the beat-frequency Brillouin signal increases; at the same time, the full width at half maximum of its spectrum decreases. This phenomenon is consistent with the characteristics of the Brillouin signal gain spectrum of the traditional structure. We still use the center frequency of Lorentz fitting as the frequency shift of the beat frequency Brillouin signal at the current set temperature. It can be seen from the figure that the frequency shift of the beat frequency Brillouin signal is about 1.1GHz, and the variation range within the set temperature variation range is 25MHz, so the detection bandwidth is lower than that of the traditional structure.

Fig. 5 shows the results of the frequency shift of the beat frequency Brillouin signal measured at different set temperatures, and the straight line represents the result of linear fitting using the least square method. The slope of the linear fit is the temperature coefficient of the beat-frequency Brillouin shift. The linear relationship between the beat frequency Brillouin frequency shift and the set temperature is

δv _B (MHz)=0.34166T+1116.5 Formula (5)

In the formula, T is the temperature set for the environment where the sensing fiber is located. In the traditional structure, the temperature coefficient of the Brillouin frequency shift is about 1MHz/℃. The smaller the temperature coefficient of the beat frequency Brillouin signal frequency shift may increase the temperature measurement error, and the multiple average measurement method can be used for this Errors have been corrected.

Therefore, the present invention utilizes the beat-frequency Brillouin signal generated by the Brillouin scattered light of the two sensing optical fibers laid together, and can use a photoelectric detection device with a lower bandwidth, thereby reducing the cost of the structure.

While the foregoing disclosure shows exemplary embodiments of the invention, it should be noted that various changes and modifications can be made without departing from the scope of the invention as defined in the claims. Depending on the structure of the inventive embodiments described herein, constituent elements of the claims may be replaced with any functionally equivalent elements. Therefore, the protection scope of the present invention should be determined by the contents of the appended claims.

Claims (4)

1. A distributed optical fiber temperature sensor based on Brillouin scattering, comprising a narrow linewidth laser, a pulse modulator, an erbium-doped fiber amplifier, a coupler, a sensing fiber 1, a sensing fiber 2, and a detection unit. Among them, after pulse modulation and power amplification, the laser light emitted by the narrow linewidth laser is split by the coupler and enters the sensing fiber 1 and sensing fiber 2. The backscattered light in the two sensing fibers is coupled The beams are combined at the detector and converted into an electrical signal by the photoelectric converter in the detection unit, that is, the beat frequency Brillouin signal. 2. The distributed optical fiber temperature sensor based on Brillouin scattering according to claim 1, wherein the two sensing fibers have different Brillouin frequency shifts. 3. The distributed optical fiber temperature sensor based on Brillouin scattering according to claim 1, wherein the detection unit comprises a photoelectric converter and an electric spectrum analyzer. 4. The distributed optical fiber temperature sensor based on Brillouin scattering according to claim 3, wherein the photoelectric converter has a bandwidth of 1.5 GHz, and the electric spectrum analyzer is used to display the beat frequency Brillouin signal spectrum.