CN101158591A - Detection method suitable for optical fiber distributed temperature and stress sensing device - Google Patents

Abstract

The invention discloses a detection method suitable for an optical fiber distributed temperature and stress sensing device. The optical fiber distributed temperature and stress sensing device mainly includes a light source (1), a frequency discriminator (2), and a thermostat box (3 ) These three modules are all connected by polarization-maintaining optical fiber. The detection method of the present invention is based on optical fiber Raman scattering as the temperature information carrier, Brillouin scattering as the stress information carrier, using Rayleigh scattering to measure the frequency of the relative frequency discriminator of the outgoing laser and performing frequency offset, and using the Fabry-Perot etalon. A direct detection method for frequency discriminative, simultaneous distributed sensing of temperature and stress. It has a simple structure and good stability, which can avoid the instability of the light source output power, light source output frequency, acousto-optic modulation or electro-optic modulation frequency during coherent detection. Insensitive to frequency drift, fluctuations in signal strength.

Description

Detection method suitable for optical fiber distributed temperature and stress sensing device

technical field

The invention relates to a sensing technology, more particularly, to a detection method for a temperature and stress sensing device connected by full optical fiber.

Background technique

Distributed optical fiber sensors can be embedded in the material structure to form a smart material structure (Smart Structure) to realize real-time self-detection and self-diagnosis of the structure itself, which can be applied to: (1) high-rise buildings, intelligent buildings, bridges, highways and other catastrophic Online, dynamic detection, protection and alarm; (2) Online, dynamic detection of aviation and aerospace vehicles and neural network system of robots; (3) Temperature distribution measurement, thermal protection and fault diagnosis of various large and medium-sized transformers and generator sets , underground and overhead high-voltage power cables, thermal power plant piping temperature detection, heating system (warm water, heating) pipes; (4) in the coal mine and petroleum industry for disaster prevention and alarm of coal mines and tunnels; oil depot , oil tanks, dangerous goods warehouses, large warehouses and warehouse fire monitoring and forecasting of large ships; abnormal detection and fault diagnosis of oil pipelines; in addition, it can also be used for online and dynamic detection of chemical raw material production processes, ICU, CCU of hospitals Temperature monitoring and fire monitoring in intensive care units, etc.

At present, distributed optical fiber sensors generally adopt coherent detection methods, which mainly include light sources, detection fibers, couplers, amplifiers, pulse modulators, microwave acousto-optic modulators, coherent detectors, and electrical filters.

Contents of the invention

The object of the present invention is to propose a detection method suitable for optical fiber distributed temperature and stress sensing devices. The optical fiber distributed temperature and stress sensing devices mainly include three components: module. The detection method (A) uses the Raman scattering in the fiber backscattering as the temperature information carrier; (B) uses the Brillouin scattering as the stress information carrier; (C) uses the Rayleigh scattering to measure the relative frequency discriminator of the laser output light Frequency offset; then use the Fabry-Perot etalon for frequency discrimination. Compared with the existing coherent detection means, the detection method of the present invention effectively solves the problem of cross-sensitivity between multiple measured parameters; through the dual-channel Fabry-Perot etalon for differential measurement and frequency offset, it solves the problem of large temperature zero ~400℃ measurement range problem. Due to the absence of frequency scanning, the time resolution can be increased from tens of minutes to tenths of a second.

Description of drawings

Fig. 1 is a structural block diagram of an optical fiber distributed temperature and stress sensing device of the present invention.

Figure 2 is the change curve of the Brillouin scattering spectrum of the detection object with temperature and stress.

Figure 3 is a transmittance curve of a dual-channel Fabry-Perot etalon.

Figure 4 is the transmittance curve of the dual-channel Fabry-Perot etalon near the Brillouin dispersion frequency shift.

Figure 5 is the response curve of Brillouin channel to stress.

Figure 6 is a schematic diagram of the measurement of a large temperature dynamic range.

In the figure: 1. Light source 11. Fiber laser 12. First erbium-doped fiber amplifier 13. Pulse modulator 14. Second erbium-doped fiber amplifier 2. Frequency discriminator 21. French-piano etalon 22. First collimator 23. Second collimator 24. Fourth coupler 25. Fifth coupler 3. Thermostat 31. Polarization controller 32. Reference optical fiber 33. Detection optical fiber 34. Isolator 311. First circulator 312. Second Second circulator 313. Third circulator 321. First fiber Bragg grating 322. Second fiber Bragg grating 323. Third fiber Bragg grating 324. Fourth fiber Bragg grating 331. First coupler 332. Second coupler 333. Third coupling 4. Polarization maintaining fiber 51. First detector 52. Second detector 53. Third detector 54. Fourth detector 55. Fifth detector

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments.

Please refer to Fig. 1, the present invention is a detection method suitable for optical fiber distributed temperature and stress sensing devices, based on optical fiber Raman scattering as temperature information carrier, Brillouin scattering as stress information carrier, using Rayleigh It is a direct detection method that uses the Fabry-Perot etalon to perform frequency discrimination by scattering and measuring the frequency of the outgoing laser relative to the frequency discriminator and performs frequency offset, and simultaneously distributes the temperature and stress. It mainly includes three modules: a light source 1, a frequency discriminator 2, and a constant temperature box 3. The connection relationship of the devices in each module is: the fiber laser 11 output pigtail and the first erbium-doped fiber amplifier 12 The input fiber fusion splicing, the first The fiber pigtail of the erbium-doped fiber amplifier 12 is fused with the fiber inlet of the pulse modulator 13, and the fiber pigtail of the pulse modulator 13 is fused with the fiber inlet of the second erbium-doped fiber amplifier 14; The A end of a circulator 311 is fused, the C end of the first circulator 311 is fused with the first fiber grating 321, the B end of the first circulator 311 is fused with the A end of the first coupler 331; the first coupler 331 The B end is fused with the A end of the second circulator 312, the B end of the second circulator 312 is fused with the incoming fiber of the polarization controller 31, and the pigtail of the polarization controller 31 is fused with a section of polarization-maintaining optical fiber 4 used as a detector. The first 20 meters of the polarization-maintaining optical fiber 4 are called the reference fiber 32, and the last 10,000 meters are called the detection fiber 33; the C-end of the first coupler 331 is fused with the C-end of the second coupler 332, and the second coupler 332 The D end is connected to the fiber FC card head of the second detector 52; the C end of the second circulator 312 is welded to the A end of the wavelength division multiplexer 38, and the C end of the wavelength division multiplexer 38 is connected to the fourth fiber grating 324 The input fiber fusion splicing of the fourth fiber grating 324 is connected with the optical fiber FC chuck head of the first detector 51, and the B end of the wavelength division multiplexer 38 is fused with the B end of the third circulator 313; the third ring The C end of the device 313 is fused with the B end of the second coupler 332; the A end of the second coupler 332 is fused with the pigtail of the first collimator 22, and the outgoing light of the first collimator 22 is incident on the French-Pi The A end of (Fabry-Perot) etalon 21, the outgoing light from the B end of Fabry-Perot (Fabry-Perot) etalon 21 is incident in the fourth coupler 24, and the tail fiber of the fourth coupler 24 is connected with the third detector The optical fiber FC card head of device 53 is connected; The A end of the third circulator 313 is fused with the fiber-entry of the third grating 323, the pigtail of the third grating 323 is fused with the fiber-entry of the isolator 34, and the pigtail of the isolator 34 is fused with the fiber of the isolator 34. The fiber-entry fusion of the second grating 322, the pigtail of the second grating 322 is fused with the B end of the third coupler 333, the C end of the third coupler 333 is connected with the fiber FC card head of the fourth detector 54, and the third The A end of the coupler 333 is fused with the pigtail of the second collimator 23, and the outgoing light of the second collimator 23 is incident on the C end of the French-Perot (Fabry-Perot) etalon 21, from the French-Perot (Fabry-Perot) etalon. -Perot) The D-end exit light of the etalon 21 is incident into the fifth coupler 25, and the pigtail of the fifth coupler 25 is connected with the optical fiber FC card head of the fifth detector 55.

In the present invention, the fiber laser adopts a distributed semiconductor laser (model DFB-LD JDSUCQF938) with a working wavelength of 1550nm and a line width of less than 1MHz, and has a fiber coupling output mode.

In the present invention, a laser 11 , a first erbium-doped fiber amplifier 12 , a pulse modulator 13 and a second erbium-doped fiber amplifier 14 constitute a light source 1 . The output light of the light source 1 is pulsed light after two-stage amplification and pulse modulation, and its power is 0.8-1.2W.

In the present invention, the polarization-maintaining fiber 4 adopts PMF-1550-8/125-0.4-L Panda-type polarization-maintaining single-mode fiber, with a numerical aperture NA=0.11 and a core diameter of 8.7 μm.

In the present invention, the central wavelength of the first fiber grating 321, the second fiber grating 322 and the third fiber grating 323 is 1550.92nm, the filtering bandwidth is 0.12nm, and the reflectivity is 98%; the central working wavelength of the fourth fiber grating 324 is 1550nm, and the filtering bandwidth 0.12nm, reflectivity 99.4%.

In the present invention, the isolation of the isolator 34 is >60dB@1550.92±80nm.

In the present invention, the first coupler 331, the second coupler 332 and the third coupler 333 are 1×2 type fiber fusion couplers with a beam splitting ratio of 30/70; the fourth coupler 24 adopts its own Pigtail type fiber optic coupling mirror with a focal length of 21.7mm, an exit beam diameter of 4.8mm, and a divergence angle of 0.42mrad; the fifth coupler 25 adopts a pigtail type fiber optic coupling mirror with a focal length of 16.8mm and an exit beam diameter of 3.38mrad mm, the divergence angle is 0.34mrad.

In the present invention, the first collimator 22 adopts a self-tailored fiber optic collimator, with a focal length of 12.4mm, an outgoing beam diameter of 2.75mm, and a divergence angle of 0.31mrad; the second collimator 23 adopts a self-tailored Fiber optic collimator with a focal length of 15.3mm, a beam diameter of 3.8mm, and a divergence angle of 0.24mrad.

In the present invention, the first detector 51 , the second detector 52 , the third detector 53 , the fourth detector 54 and the fifth detector 55 use InGaAs detector components with high frequency response.

In the present invention, the thermostat box 3 can provide a constant temperature working environment with a working temperature of 55°C and a temperature accuracy of 0.01°C.

In the present invention, the frequency discriminator 2 is made of a French-Pico etalon 21, a first collimator 22, a second collimator 23, a first coupler 24 and a second coupler 25; the first collimator 22 The outgoing light is incident on the A end of the French-Pico etalon 21 (ie, the entrance port of the Rayleigh channel), and exits into the first coupler 24 from the B end (ie, the Rayleigh channel exit port) of the French-Pico etalon 21 , enter into the third detector 53 after being coupled by the first coupler 24; - The D end of the pico etalon 21 (that is, the exit end of the Brillouin channel) exits into the second coupler 25 , and enters the fifth detector 55 after being coupled by the second coupler 25 .

The A-end and B-end of the French-Pico etalon 21 form a Rayleigh channel, and the C-end and D-end form a Brillouin channel; a dual-channel etalon is fabricated on the same substrate, so that the cavity length and reflectivity of the dual channels are not equal , can form a frequency discriminator with a fixed frequency interval. Among them, the Brillouin channel is used as a high-resolution frequency discrimination channel, which converts the stress information to be measured into the transmittance change of the Brillouin signal on the French-Pico etalon 21, so as to realize the rapid and direct detection of the stress. The Rayleigh channel is used to measure the frequency of the outgoing laser light of the laser 11 relative to the frequency discriminator 2; when the optical fiber distributed direct simultaneous sensing temperature and stress device of the present invention realizes the detection of a large temperature range of 0-400°C, the Rayleigh channel also It is used to preset the frequency offset Δυ _offset (T) of the emitted laser light of the laser 11 relative to the frequency discriminator 2 .

In the present invention, the diameter of the Fabry-Perot etalon 21 is 50 mm, and the diameter of the single beam incident on the etalon is less than 5 mm, so that about 20 optical fiber sensing optical paths can be installed on the same etalon. It is convenient to realize simultaneous sensing of temperature and stress of a multi-channel optical fiber network (in two-dimensional or three-dimensional space).

The invention detects the temperature by using the Raman scattering spectrum first, and then uses the Brillouin scattering spectrum to detect the stress information under the known temperature condition.

In the present invention, the Rayleigh channel is composed of the second coupler 332, the first collimator 22, the French-Perot (Fabry-Perot) etalon 21 and the fourth coupler 24 in sequence; this channel is used to measure the second The frequency of the laser pulse output by the erbium-doped fiber amplifier 14 relative to the frequency discriminator 2; Long, the frequency offset of the light emitted by the laser 11 relative to the frequency discriminator 2 can be preset.

In the present invention, the Brillouin channel consists of the third circulator 313, the third fiber Bragg grating 323, the isolator 34, the second fiber Bragg grating 322, the third coupler 333, the second collimator 23, the French-Pi (Fabry-Perot) etalon 21, the fifth coupler 25, the fourth detector 54 and the fifth detector 55 constitute; ) changes in transmittance on the etalon 21, so as to realize rapid and direct detection of stress.

In the present invention, the Raman channel is composed of the wavelength division multiplexer 38, the fourth fiber grating 324 and the first detector 51 in sequence; by measuring the Raman scattering of the pulse signal in the optical fiber 4, according to the Raman scattering The variation of intensity with temperature measures the axial distribution of temperature with optical fiber 4 .

The first detector 51 is used to detect the Raman signal light intensity f ₁ of the Raman channel.

The second detector 52 is used to detect the light intensity f ₂ of the Rayleigh signal I _R before entering the frequency discriminator 2 , which is abbreviated as the Rayleigh light intensity f ₂ in the present invention.

The third detector 53 is used to detect the light intensity f ₃ of the Rayleigh signal I _R passing through the Rayleigh channel in the frequency discriminator 2 , which is abbreviated as the transmitted light intensity f ₃ of the Rayleigh signal in the present invention.

The fourth detector 54 is used to detect the light intensity f ₄ of the Brillouin signal I _B before entering the discriminator 2 , which is abbreviated as the Brillouin light intensity f ₄ in the present invention.

The fifth detector 55 is used to detect the light intensity _f5 after the Brillouin signal I _B passes through the Brillouin channel in the discriminator 2, in the present invention, it is abbreviated as the Brillouin signal transmitted light intensity _f5 .

Referring to Fig. 2, in the present invention, the stress detection takes the Brillouin scattering spectrum as the stress detection object. In the figure, the A line refers to the Brillouin scattering spectrum and the B line when the stress is 0 με at a reference temperature of 25°C. Refers to the Brillouin scattering spectrum at a reference temperature of 25°C and a stress of 2000με, line C refers to the Brillouin scattering spectrum at a reference temperature of 400°C and a stress of 0με, line D refers to a reference temperature of 400°C and a stress of 2000με The Brillouin scattering spectrum, as shown in the figure, both temperature and stress will cause the Brillouin scattering spectrum to broaden and move to the high frequency direction. The frequency shift rate of Brillouin scattering spectrum caused by temperature is 1.37MHz/℃, and the broadening rate of Brillouin scattering spectrum caused by temperature is 0.15MHz/℃. ; The stress-induced broadening rate is 0.058MHz/με, and the stress-induced frequency shift rate is 0.077MHz/με.

Referring to Fig. 3, in the present invention, using the measured Rayleigh signal transmittance T _R =f ₃ /f ₂ and the calibrated Rayleigh channel transmittance curve, it is possible to obtain the output light of the laser 11 relative to the frequency discrimination The frequency υ ₀ of device 2. Using the Brillouin signal transmittance T _B =f ₅ /f ₄ and the calibrated Brillouin channel transmittance curve, the stress information ε to be measured in the optical fiber can be obtained. In the figure, line A refers to the specified Rayleigh channel transmittance curve, and line B refers to the transmittance curve of Brillouin channel. The center frequency of line A is 200MHz higher than that of line B, so when setting the frequency offset of the outgoing laser relative to the etalon, the outgoing laser is always on the steep edge of line A.

See Figure 4. In the figure, line A refers to the specified Rayleigh channel transmittance curve, line B refers to the transmittance curve of Brillouin channel, and line C refers to the reference temperature of 25°C and stress of 0με The Brillouin scattering spectrum and line D refer to the Brillouin scattering spectrum when the reference temperature is 25°C and the stress is 2000με. It can be seen from the figure that when the temperature is known, the stress will cause the Brillouin scattering spectrum to It moves in the direction of high frequency and continuously broadens. But when the Raman signal has been used to measure the temperature, the transmittance function Res(ε) of the Brillouin scattering signal on the Brillouin channel changes monotonously, as shown in Figure 5. The stress information can be retrieved by measuring the transmittance T _B =f ₅ /f ₄ of the Brillouin scattering signal on the Brillouin channel.

Referring to Fig. 5, in the Brillouin Optical Time Domain Reflectometry (BOTDR), it is considered that a "panda-type" polarization-maintaining fiber is used as the sensing element to achieve higher measurement sensitivity. The response function Res(ε) in the stress detection method proposed by the present invention is related to the Brillouin posterior scattering spectrum characteristic of the selected optical fiber, and the present invention proposes to use a "panda type" polarization-maintaining optical fiber as the sensing element. Since the Brillouin backscattering spectrum of the "Panda-type" polarization-maintaining fiber becomes wider with the increase of stress, the slope of the response function Res(ε) becomes larger, and the detection sensitivity of the system increases. In the figure, when the axial stress of the fiber changes within 2000με, the response function of the corresponding "bow-tie" polarization-maintaining fiber varies from 26% to 45%; while the response function of the corresponding "panda-type" polarization-maintaining fiber varies from 12-48%. Therefore, the detection sensitivity of the "panda type" polarization maintaining fiber is higher.

As shown in Figure 6, compared to the characteristics of the coherent detection method that requires stepwise frequency scanning, in order to maintain the measurement accuracy of the sensor, the scanning step size must be as small as 5 MHz; in order to maintain the measurement dynamic range, the scanning range must be as large as 1 GHz. Therefore, the coherent detection takes about 8-20 minutes for a single measurement. In actual work, many applications require the measurement of transient temperature and stress information. In order to solve the above long-time-consuming contradiction, the present invention proposes to convert the stress information ε into the transmission of the Brillouin signal on the discriminator 2 The rate value Res(ε), thus realizing direct detection, no frequency scanning is required, and the time-consuming is short, a single measurement only needs 0.1-10s (the specific single measurement time depends on the measurement accuracy requirements). Compared with the coherent detection method, the present invention can avoid many potential noise sources during the coherent detection (during the coherent detection, the output power fluctuation of the light source, the output frequency drift of the light source, the frequency instability of the acousto-optic modulator and the electro-optic modulator will be directly introduced measurement error); the temperature and stress detection method provided by the invention avoids the use of acousto-optic modulators and electro-optic modulators, and is insensitive to the frequency drift of the light source and the intensity fluctuation of the light source itself. In the figure, line A refers to the specified Rayleigh channel transmittance curve, line B refers to the transmittance curve of Brillouin channel, and line E refers to the Brillouin channel transmittance curve of the optical fiber at the reference temperature of 25°C without stress. The Brillouin scattering spectrum and the F line refer to the Brillouin scattering spectrum of the fiber at the reference temperature of 250°C without stress, and the G line refers to the Brillouin scattering spectrum of the fiber at the reference temperature of 400°C without stress. It can be seen that to realize the measurement range of 2000με, as long as the frequency offset of the outgoing laser relative to the frequency discriminator 2 is set, it can be ensured that the Brillouin scattering spectrum is always on the A curve when detecting at high temperature (less than or equal to 400°C) On the steep edge, in order to achieve the purpose of high resolution (1MHz).

In actual detection, the narrower the transmittance curve of the frequency discriminator, the higher the measurement sensitivity, but the smaller the measurement dynamic range. In order to solve this contradiction, the present invention proposes a frequency pre-bias method, which solves the problem of high-precision stress measurement in a large temperature range (0-400°C), and the stress measurement range is 0-2000uε. In the present invention, a low-precision Rayleigh channel and a high-precision Brillouin channel are set. Among them, the high-precision Brillouin channel is used to measure the stress information ε and maintain high measurement sensitivity; the low-precision Rayleigh channel is used to preset the frequency offset Δυ _offset of the outgoing laser relative to the frequency discriminator. The frequency offset Δυ _offset at different temperatures is realized by changing the cavity length of the French-Pico etalon 21 . The frequency pre-bias method can ensure that the Brillouin backscattering spectrum of the detection fiber 4 is always on the steep edge of the transmittance curve of the Brillouin channel of the French-Pico etalon 21 (as shown in Fig. line), to achieve the purpose of high-precision detection.

According to the specific characteristics of the detection target spectrum (the Brillouin scattering spectrum is sensitive to both temperature and stress) in the present invention, the structure of the frequency discriminator 2 is designed. One of the features of the present invention is that the Rayleigh channel in the discriminator is set, and the transmittance function h(υ) of the outgoing laser light on the Rayleigh channel in the frequency discriminator and the actual measured outgoing laser light in the discriminator The transmittance value T _R =f ₃ /f ₂ on the Rayleigh channel in the frequency discriminator can measure the frequency offset υ ₀ of the outgoing laser relative to the Rayleigh channel of the frequency discriminator 2 (the reference value is 234MHz).

In general design, an independent circular channel structure is adopted; in actual use, the change of the cavity length of each single-channel etalon will cause the drift of the central frequency of the etalon, so the frequency interval between multiple etalons will drift randomly, which will lead to Serious measurement error. The invention discloses a technology of the same substrate of etalons with multiple uses. One of the characteristics of the frequency discriminator in the present invention is that a dual-channel Fabry-Perot etalon is used as a core device, which is composed of two circular reflectors arranged in parallel. When the lens is coated, the reflectivity of the left and right semicircular channels is different, forming a Brillouin channel and a Rayleigh channel. Coating on the inner semicircle of the front reflector of the Brillouin channel forms a step with a height of 27.9nm, so that the cavity length of the Brillouin channel is slightly smaller than that of the Rayleigh channel, so that the peak transmittance of the Brillouin channel corresponds to The frequency is 200 MHz higher than the frequency corresponding to the peak transmittance of the Rayleigh channel (as shown in Figure 3). In use, by adopting the dual-channel structure disclosed in the present invention, the cavity length drift will not affect the frequency interval of 200 MHz. Therefore, the measured frequency offset ν ₀ of the outgoing laser relative to the Rayleigh channel in the discriminator 2 can be used to calculate the frequency offset ν ₁ of the outgoing laser relative to the Brillouin channel of the discriminator 2 (the reference value is 34 MHz).

In Brillouin optical time-domain reflectometry (BOTDR), the cross-sensitivity of Brillouin backscattering spectra to temperature and stress cannot be solved at present. The invention proposes a simultaneous detection combination mode of Raman spectrum and Brillouin spectrum, which solves the problem of temperature and stress cross-sensitivity. The change value ΔI _R (L) of the Raman backscattering intensity at the corresponding optical fiber length L measured in the present invention relative to the Rayleigh backscattering intensity (that is, the first detector 51 output power f ₁ and the second detector 52 Output power f ₂ ratio change value). According to the temperature response coefficient C _R ^T and temperature response characteristics of optical fiber 4 $(\mathrm{\Δ}{T}_R\left(L\right)=\mathrm{\Δ}{I}_R\left(L\right)C_R^T)$ The temperature change at the length L on the optical fiber 4 relative to a reference value can be measured.

In the present invention, the axial stress suffered by the optical fiber 4 is converted into the transmittance information of the Brillouin backscattering at the detected temperature on the Brillouin channel in the discriminator 2, by measuring the transmittance of the Brillouin signal The transition rate T _B =f ₅ /f ₄ and the response curve of the calibrated Brillouin channel to stress (see Figure 5) can measure the stress information ε.

In the present invention, a self-calibration assembly is designed, that is, the first 20 m of the optical fiber 4 is used as the reference optical fiber 32, and the reference optical fiber 32 is placed in the thermostat 3; the rear 10 km of the optical fiber 4 is used as the detection optical fiber 33, and the detection optical fiber 33 is placed at a constant temperature Box 3 outside. During actual measurement, the preset temperature of the incubator 3 is 25±0.1°C, and the temperature of the optical fiber measured by comparing the preset temperature with the reference optical fiber 32 (under normal working conditions, the temperature of the optical fiber should be equal to the preset temperature), so The measurement results of the device of the present invention can be calibrated in real time using the fiber temperature.

The present invention combines four fiber gratings (first fiber grating 321, second fiber grating 322, third fiber grating 323 and fourth fiber grating 324), three circulators (first circulator 311, second circulator 312 and The third circulator 313), three fiber couplers (the first fiber coupler 331, the second fiber coupler 332 and the third fiber coupler 333), the wavelength division multiplexer 38 and the isolator 34 are placed in the incubator 3, the influence of ambient temperature on the stability of the device of the present invention is effectively eliminated.

The principle of the detection method applicable to the optical fiber distributed temperature and stress sensing device in the present invention is described in detail as follows:

1. Determination of temperature by Raman scattering spectrum

The Raman scattering power is only sensitive to temperature and has no response to stress, and its sensitivity is nearly three times greater than that of the Brillouin scattering spectrum. Therefore, in direct detection, the power ratio of Raman backscattering relative to Rayleigh backscattering varies with temperature. Changes to detect temperature. Note that the temperature change of the detection optical fiber 33 relative to the reference temperature 25°C at the optical fiber length L is $\mathrm{\Δ}{T}_R\left(L\right)=\mathrm{\Δ}{I}_R\left(L\right)C_R^T\mathrm{\Δ}{T}_R\left(L\right),$ In the formula, ΔI _R (L) is the change in power ratio of Raman backscattering relative to Rayleigh backscattering at the fiber length L (the change in the ratio of the output power of the first detector 51 to the output power of the second detector 52), C _R ^T is the temperature response coefficient, which is determined by the detection optical fiber 33 and can be measured when the instrument is calibrated.

2. Measurement of stress

In coherent detection, any two of the three physical quantities of Brillouin scattering power, spectral width, and frequency shift must be detected simultaneously through frequency scanning, and then the temperature and stress information can be retrieved respectively. Because the Brillouin backscattering power is interfered by many factors, the research suggests that the highest accuracy of BOTDR technology can be achieved by simultaneously detecting the frequency shift and spectral width change of the Brillouin scattering spectrum by using a "panda-type" polarization-maintaining fiber.

The invention converts the change of the Brillouin scattering spectrum characteristics (frequency shift, spectral width, power) caused by temperature and stress into the monotonous change of the transmittance of the Brillouin signal on the high-resolution discriminator, thereby realizing the temperature and pressure sensing at the same time.

C_υ ^ε是探测光纤33的布里渊频移的应力响应系数，且 $C_{\mathrm{\&upsi;}}^{\mathrm{\&epsiv;}}=0.077\mathrm{MHz}/\mathrm{\&mu;\&epsiv;}.$ Because the transmittance of the Brillouin signal is finally detected, the absolute strength of the signal only affects the signal-to-noise ratio of the measurement. The frequency shift of the Brillouin scattering spectrum is a function of the temperature T and the axial stress ε that the detection fiber 33 bears, denoted as υ _B (T, ε), and ${\mathrm{\&upsi;}}_B(T,\mathrm{\&epsiv;})={\mathrm{\&upsi;}}_B({\mathrm{<figure-callout\; id="0"\; label="T"\; filenames="S2007101758670E00021.png,S2007101758670E00031.png"\; state="\{\{state\}\}">T</figure-callout>}}_0,{\mathrm{\&epsiv;}}_0)+C_{\mathrm{\&upsi;}}^T\&times;(T-T_0)+C_{\mathrm{\&upsi;}}^{\mathrm{\&epsiv;}}\&times;\mathrm{\&epsiv;},$ In the formula, υ _B (T ₀ , ε ₀ ) = 11.2 GHz is the frequency shift of the Brillouin scattering spectrum measured at the reference temperature T ₀ = 25°C and the stress-free state ε ₀ = 0, and C _υ ^T is the detection fiber 33 The temperature response coefficient of the Brillouin frequency shift of , and

C _υ ^ε is the stress response coefficient of the Brillouin frequency shift of the detection fiber 33, and $C_{\mathrm{\&upsi;}}^{\mathrm{\&epsiv;}}=0.077\mathrm{MHz}/\mathrm{\&mu;\&epsiv;}.$

The Brillouin scattering spectral width is a function of the temperature T and the axial stress ε that the detection fiber 33 bears, which is denoted as Δυ _B (T, ε), and $\mathrm{\&Delta;}{\mathrm{\&upsi;}}_B(T,\mathrm{\&epsiv;})=\mathrm{\&Delta;}{\mathrm{\&upsi;}}_B({\mathrm{<figure-callout\; id="0"\; label="T"\; filenames="S2007101758670E00021.png,S2007101758670E00031.png"\; state="\{\{state\}\}">T</figure-callout>}}_0,{\mathrm{\&epsiv;}}_0)+C_b^T\&times;(T-T_0)+C_b^{\mathrm{\&epsiv;}}\&times;\mathrm{\&epsiv;},$ In the formula, Δυ _B (T ₀ , ε ₀ )=71MHz is the spectral width of the Brillouin scattering spectrum measured at the reference temperature T ₀ =25°C and the stress-free state ε ₀ =0, and C _b ^T is the distribution of the detection fiber 33 The temperature response coefficient of the Rieouin scattering spectrum width, and C _b ^ε is the stress response coefficient of the Brillouin scattering spectrum width of the detection fiber 33, and $C_b^{\mathrm{\&epsiv;}}=0.058\mathrm{MHz}/\mathrm{\&mu;\&epsiv;}.$

In the present invention, due to the use of differential measurement technology, the frequency shift and spectrum broadening of the Brillouin scattering spectrum caused by the stress of temperature are converted into the Brillouin scattering signal on the frequency discriminator 2, by measuring the transmittance to achieve Retrieve stress information. The inversion process has nothing to do with the intensity of the Brillouin scattering signal, so the peak value of the Brillouin scattering spectrum is normalized. The normalized Brillouin backscattering spectral function is denoted as S _B (υ, T, ε), and S _B (υ, T, ε) = Δυ _B (T, ε) ² /{4[υ- υ _B (T, ε)] ² +Δυ _B (T, ε) ² }, where υ is the incident light frequency. The normalized Brillouin scattering spectrum curves under different temperature and stress conditions are shown in Fig. 2 .

In the present invention, the Fabry-Perot etalon in the frequency discriminator 2 is a core device, and its Brillouin channel is used to detect stress information; its Rayleigh channel is used to detect the frequency of the outgoing laser relative to the frequency discriminator 2. During dynamic range measurement (wide temperature range 0-400°C, stress range 0-2000με), by measuring the transmittance of the outgoing laser frequency on the Rayleigh channel and adjusting the cavity length of the etalon 21, the outgoing laser can be preset Frequency offset relative to discriminator 2. The spectral function of the Fabry-Perot etalon denote h(υ), and $h\left(\mathrm{\&upsi;}\right)=\frac{2}{{\mathrm{\&theta;}}_{\max }^2}{\&Integral;}_0^{{\mathrm{\&theta;}}_{\max }}{(1-\frac{a}{R})}^2\sin \mathrm{\&theta;}/\{1+4f_{\mathrm{E.}}^2{\sin }^2[\mathrm{\&pi;\&upsi;}\cos \mathrm{\&theta;}/{\mathrm{\&upsi;}}_{\mathrm{FSR}}]/{\mathrm{\&pi;}}^2\}\mathrm{d\&theta;},$ In the formula, θ is the beam incidence angle of the Fabry-Perot etalon (during the test, the beam is normal incident θ = 0, the incident beam is not completely collimated, and the maximum divergence angle is θ _max = 0.31mrad); a≈0.2% is the etalon The absorption loss of the reflective surface; R is the reflectivity of the etalon; _FE is the effective fineness; υ is the frequency of the incident light; υ _FSR is the free spectrum _spacing ;

The designed dual-channel Fabry-Perot etalon curve is shown in Figure 3. Fabry-Perot etalon cavity length l = 27mm (which determines the free spectral spacing υ _FSR = c/2nl, c is the speed of light in vacuum, n is the refractive index of the strong inner medium of the etalon). The Fabry-Perot etalon consists of two front and rear mirrors, the right semicircle of the front mirror is 28nm higher than the left semicircle, and the aperture is 50mm. This paper refers to the left channel as the Brillouin channel (line B), and the right channel as the Rayleigh channel (line A). The reflectivity R of the left and right channels is 59.6% and 95.4% respectively, which determine the reflection fineness $(f_R=\frac{\mathrm{\&pi;}{R}^{-1/2}}{1-R}).$ According to the current manufacturing process level, the defect fineness (F _D ) of the dual channel is 200, so that the effective fineness $(f_{\mathrm{E.}}={(f_R^{-2}+f_{\mathrm{D.}}^{-2})}^{-1/2})$ 6 and 63 respectively.

In the present invention, after the temperature T _D is measured by the Rayleigh signal, the stress response function under the measured temperature T _D is defined as Res _TD (ε), and ${\mathrm{Res}}_{T_{\mathrm{D.}}}\left(\mathrm{\&epsiv;}\right){\&Integral;}_{-\&infin;}^{\&infin;}{S}_B(\mathrm{\&upsi;},T_{\mathrm{D.}},\mathrm{\&epsiv;})h_h\left(\mathrm{\&upsi;}\right)\mathrm{d\&upsi;}/{\&Integral;}_{-\&infin;}^{\&infin;}{S}_B(\mathrm{\&upsi;},T_{\mathrm{D.}},\mathrm{\&epsiv;})\mathrm{d\&upsi;},$ In the formula, h _H (υ) is the spectrum function of the Brillouin channel (shown as line B in Figure 3), and dυ represents the integral of the Brillouin backscattering frequency υ. The local display of the transmittance curve of the dual-channel Fabry-Perot etalon at the frequency center of the Brillouin scattering spectrum is shown in Figure 4.

Figure 5 shows the stress response curves Res _TD (ε) of the “Panda-type” polarization-maintaining fiber and the “bow-tie” polarization-maintaining fiber at 25°C. Measure the transmittance T _B = f ₅ /f ₄ of the Brillouin scattering signal on the Brillouin channel (line A), and solve the nonlinear equation $T_B={\mathrm{Res}}_{T_{\mathrm{D.}}}\left(\mathrm{\&epsiv;}\right)$ Then the stress distribution can be measured.

3. Realization of large temperature dynamic range

In direct detection technology, high resolution and large dynamic range are a prominent contradiction. When the resolution of the frequency discrimination device is higher, the slope of the transmittance curve is larger, but the half maximum width is smaller, and the corresponding measurement range is smaller. . The present invention discloses a solution. As shown in Figure 6, the temperature range of 400 °C corresponds to a change in the frequency shift of the Brillouin scattering spectrum close to 900 MHz; the design stress measurement range is 2000 με, and the corresponding change in the frequency shift of the Brillouin scattering spectrum is 46 MHz. Therefore, another reference channel (Rayleigh channel) is formed by coating on the same substrate that forms the high-resolution frequency discrimination channel. The FWHM of the Rayleigh channel is 1 GHz. Because the two channels are fabricated on the same substrate, there is no relative frequency error between the two channels. In order to make the Brillouin scattering spectra at different temperatures on the steep edge of the frequency discrimination channel (line A), the frequency interval of the reference channel (line B) relative to the outgoing laser can be adjusted. Because the cavity lengths of the two channels are the same, and increasing the cavity length can make the center frequency of the etalon shift down, the relationship between the frequency shift Δυ _offset and the cavity length increment Δl is $\frac{\mathrm{\&Delta;}{\mathrm{\&upsi;}}_{\mathrm{offset}}}{\mathrm{\&Delta;l}}=-\frac{\mathrm{\&upsi;}}{l},$ In the formula, the sign "-" indicates that when the frequency of the etalon 21 is required to move up relative to the emitted laser light, the cavity length needs to be shortened.

In the present invention, when detecting the stress distribution of a certain section of the optical fiber 33, the temperature T _D can be firstly measured by the Raman scattering spectrum; then by the frequency shift of the Brillouin scattering spectrum ${\mathrm{\&upsi;}}_B(T,\mathrm{\&epsiv;})={\mathrm{\&upsi;}}_B({\mathrm{<figure-callout\; id="0"\; label="T"\; filenames="S2007101758670E00021.png,S2007101758670E00031.png"\; state="\{\{state\}\}">T</figure-callout>}}_0,{\mathrm{\&epsiv;}}_0)+C_{\mathrm{\&upsi;}}^T\&times;(T-T_0)+C_{\mathrm{\&upsi;}}^{\mathrm{\&epsiv;}}\&times;\mathrm{\&epsiv;}$ We can know the Brillouin frequency shift υ _B (T _D , 0) caused by the stress-free state at this temperature, and thus we can know the frequency upshift value of the etalon relative to the outgoing laser Δυ _offset (T) = υ _B (T _D , 0)-υ _B (T ₀ , 0).

In the present invention, the relationship between the frequency shift Δυ _offset and the cavity length increment Δl can be obtained $\frac{\mathrm{\&Delta;}{\mathrm{\&upsi;}}_{\mathrm{offset}}}{\mathrm{\&Delta;l}}=-\frac{\mathrm{\&upsi;}}{l},$ And the frequency up-shift value of the etalon relative to the outgoing laser light Δυ _offset (T)=υ _B (T _D , 0)-υ _B (T ₀ , 0) shows the cavity length increment corresponding to the temperature value. By modulating the voltage value of the piezoelectric ceramic driver in the Fabry-Perot etalon, the cavity length can be accurate to 0.1nm, corresponding to a frequency error of 0.71MHz (corresponding to a stress measurement error of about 31με). However, in actual measurement, the frequency of the outgoing laser relative to the center of the etalon can be known by measuring the transmittance value of the outgoing light on the reference channel, so as to eliminate this error.

The present invention is suitable for the characteristics of the detection method of the optical fiber distributed temperature and stress sensing device:

1. Compared with the existing optical fiber distributed sensing coherent detection system, the device of the present invention has simple structure and good stability, and can avoid many potential noise sources during coherent detection (during coherent detection, light source output power, light source output frequency, The frequency instability of acousto-optic modulation or electro-optic modulation will directly introduce measurement errors); using differential direct detection technology for frequency discrimination, it is not sensitive to the frequency drift of the light source and the fluctuation of signal strength.

2. No need for frequency scanning, high time resolution (10Hz), suitable for transient environment detection.

3. Large measurement dynamic range (400°C temperature range, stress measurement range 2000με).

4. The later data processing is simple and does not require a lot of complicated calculations.

5. The diameter of the Fabry-Perot etalon is 50mm, and the diameter of the beam incident on the etalon by a single path is less than 5mm, so that about 20 optical fiber sensing optical paths can be installed on the same etalon. It is convenient to realize the simultaneous sensing of temperature and stress in multi-channel optical fiber network.

Claims (1)

1. detection method that is applicable to optical fiber distributed temperature and stress sensing device, it is characterized in that: the temperature measuring in described optical fiber distributed temperature and the stress sensing device is to be in the temperature variation of 25 ℃ of fiber lengths L place relative reference temperature with detection optical fiber (33) $\mathrm{\&Delta;}{T}_R\left(L\right)=\mathrm{\&Delta;}{I}_R\left(L\right)C_R^T\mathrm{\&Delta;}{T}_R\left(L\right),$ In the formula, Δ I _R(L) be the backward scattered power ratio variation of the relative Rayleigh of Raman back scattering at fiber lengths L place, C _R ^TIt is the temperature-responsive coefficient;

Stress measurement in described optical fiber distributed temperature and the stress sensing device is:

(A) frequency displacement of Brillouin spectrum is with the function of detection optical fiber (33) bearing temperature T of institute and axial stress ε, is designated as υ _B(T, ε), and ${\mathrm{\&upsi;}}_B(T,\mathrm{\&epsiv;})={\mathrm{\&upsi;}}_B(T_0,{\mathrm{\&epsiv;}}_0)+C_{\mathrm{\&upsi;}}^T\&times;(T-T_0)+C_{\mathrm{\&upsi;}}^{\mathrm{\&epsiv;}}\&times;\mathrm{\&epsiv;},$ In the formula, υ _B(T ₀, ε ₀)=11.2GHz is reference temperature T ₀=25 ℃ and unstress state ε ₀=0 records the frequency shift amount of Brillouin spectrum, C _υ ^TBe the temperature-responsive coefficient of the Brillouin shift of detection optical fiber (33), and

C _υ ^εBe the stress response coefficient of the Brillouin shift of detection optical fiber (33), and $C_{\mathrm{\&upsi;}}^{\mathrm{\&epsiv;}}=0.077\mathrm{MHz}/\mathrm{\&mu;\&epsiv;};$ (B) the Brillouin scattering spectrum width is the function of detection optical fiber (33) bearing temperature T of institute and axial stress ε, is designated as Δ υ _R(T, ε), and $\mathrm{\&Delta;}{\mathrm{\&upsi;}}_B(T,\mathrm{\&epsiv;})=\mathrm{\&Delta;}{\mathrm{\&upsi;}}_B(T_0,{\mathrm{\&epsiv;}}_0)+C_b^T\&times;(T-T_0)+C_b^{\mathrm{\&epsiv;}}\&times;\mathrm{\&epsiv;},$ In the formula, Δ υ _B(T ₀, ε ₀)=71MHz is reference temperature T ₀=25 ℃ and unstress state ε ₀=0 records the spectrum width of Brillouin spectrum, C _b ^TBe the temperature-responsive coefficient of the Brillouin spectrum spectrum width of detection optical fiber 33, and

C _b ^εBe the stress response coefficient of the Brillouin spectrum spectrum width of detection optical fiber 33, and $C_b^{\mathrm{\&epsiv;}}=0.058\mathrm{MHz}/\mathrm{\&mu;\&epsiv;};$

(C) adopt the difference measurement technology, the frequency displacement of the Brillouin spectrum that the stress of temperature is caused and spectrum widening change brillouin scattering signal on frequency discriminator (2), reach the inverting stress information by measuring this transmitance; The intensity of this refutation process and brillouin scattering signal is irrelevant, so with the peak value normalization of Brillouin spectrum; Brillouin's back scattering spectral function after the normalization is designated as S _B(υ, T, ε), and S _B(υ, T, ε)=Δ υ _B(T, ε) ²/ { 4[υ-υ _B(T, ε)] ²+ Δ υ _B(T, ε) ², in the formula, υ is the incident light frequency.

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