阅读说明：本技术 基于少模光纤的混沌bocda温度应变测量装置 (Chaos BOCDA temperature strain measuring device based on few-mode optical fiber ) 是由 张建忠 梁泽锋 刘双双 张晓程 马喆 张明江 于 2021-05-25 设计创作，主要内容包括：本发明属于分布式光纤传感技术领域,公布了一种基于少模光纤的混沌BOCDA温度应变测量方法和装置,利用少模光纤中不同模式组合下光波随温度或应变产生的布里渊频移的线性系数是不同的,可以有效的解决光纤中温度与应变交叉敏感问题。方法包括以下步骤：S1、混沌激光分为探测光信号和泵浦光信号；S2、使探测光信号经光学状态调整、模式选择后入射到少模光纤；使泵浦光信号经光学状态调整、模式选择后从另一端入射到少模光纤,测量少模光纤中产生的斯托克斯光拍频信号,得到布里渊频移量；S3、改变探测光和泵浦光的模式,重复测量得到布里渊频移量；S4、通过测量的布里渊频移量,计算得到传感光纤中的温度和应变信息。(The invention belongs to the technical field of distributed optical fiber sensing, and discloses a chaotic BOCDA (Brillouin optical fiber temperature strain) measurement method and device based on a few-mode optical fiber. The method comprises the following steps: s1, dividing the chaotic laser into a detection light signal and a pumping light signal; s2, making the detecting light signal enter the few-mode optical fiber after optical state adjustment and mode selection; enabling a pump light signal to enter a few-mode fiber from the other end after optical state adjustment and mode selection, and measuring a Stokes light beat frequency signal generated in the few-mode fiber to obtain Brillouin frequency shift quantity; s3, changing the modes of the probe light and the pump light, and repeatedly measuring to obtain Brillouin frequency shift quantity; and S4, calculating the temperature and strain information in the sensing optical fiber according to the measured Brillouin frequency shift quantity.)

1. A chaos BOCDA temperature strain measurement method based on few-mode optical fiber is characterized by comprising the following steps:

s1, dividing the chaotic laser into two paths, wherein one path is used as a detection light signal, and the other path is used as a pumping light signal;

s2, the detection light signal is made to enter the few-mode optical fiber (11) after the optical state adjustment and the mode selection; enabling the pump light signals to enter a few-mode optical fiber (11) from the other end after optical state adjustment and mode selection, measuring Stokes light beat frequency signals generated in the few-mode optical fiber (11), and analyzing and processing the beat frequency signals to obtain Brillouin frequency shift quantity;

s3, changing the modes of the probe light and the pump light, and repeatedly measuring to obtain Brillouin frequency shift quantity;

and S4, calculating the temperature and strain information in the sensing optical fiber through the Brillouin frequency shift amount measured in the steps S2 and S3.

2. The method according to claim 1, wherein in step S2, the optical state adjustment of the probe optical signal includes polarization state adjustment, modulation, delay, amplification, polarization disturbance and isolation;

the optical state of the pump light signal is adjusted, the polarization state is adjusted, amplified, modulated and re-amplified.

3. The method for measuring the chaotic BOCDA temperature strain based on the few-mode fiber according to claim 1, wherein in step S2, the mode selection of the probe light signal and the pump light signal is implemented by a photon lantern, three single-mode fibers are respectively provided at an input end of the photon lantern as input fibers, a pigtail of each single-mode fiber is connected with the few-mode fiber (11), and the three single-mode fibers respectively correspond to: LP01, LP11a, and LP11 b.

4. The method for measuring the temperature strain of the chaotic BOCDA based on the few-mode optical fiber according to claim 1, wherein the formula for obtaining the temperature and strain information in the sensing optical fiber by calculation is as follows:

wherein Δ T and Δ ε are the variation of temperature and strain respectively,andthe Brillouin frequency shift amounts of the combination of LP01-LP01 modes and the combination of LP11a-LP11a modes,andare respectively asThe temperature coefficient and strain coefficient of the brillouin frequency shift in the combination of LP01-LP01 modes,andthe temperature coefficient and the strain coefficient of brillouin frequency shift in the LP11a-LP11a mode combinations, respectively.

5. The method of claim 4, wherein the temperature coefficient of the combination of LP01-LP01 modes is 1.01690 MHz/degree C, and the strain coefficient is: the temperature coefficient of the 0.05924 MHz/mu epsilon, LP11a-LP11a mode combination is 0.99099 MHz/DEG C, and the strain coefficient is as follows: 0.04872MHz/μ ε.

6. A chaos BOCDA temperature strain measuring device based on few-mode optical fiber is characterized by comprising: the device comprises a chaotic laser (1), a beam splitter (3), a first polarization controller (4), a first electro-optic modulator (5), a programmable optical delay generator (6), a first optical amplifier (7), an optical polarization scrambler (8), a first photon lantern (10), a few-mode optical fiber (11), a second polarization controller (13), a second optical amplifier (14), a second electro-optic modulator (16), a third optical amplifier (17), an optical circulator (18), a tunable filter (19), a photoelectric detector (20), a phase-locked amplifier (22), a data acquisition card (23) and a computer;

the chaotic laser generated by the chaotic laser (1) is divided into two paths by the beam splitter (3), wherein one path is used as a detection light signal, and the other path is used as a pumping light signal;

the detection light signal sequentially passes through a first polarization controller (4), a first electro-optic modulator (5), a programmable light delay generator (6), a first light amplifier (7) and a light polarization scrambler (8) to respectively carry out polarization state adjustment, modulation, delay, amplification and polarization scrambling on the light signal, then enters a first photon lantern (10), and enters a few-mode optical fiber (11) after mode selection is carried out on the first photon lantern (10);

after the pump light signals are subjected to light signal polarization state adjustment, amplification, modulation and re-amplification sequentially through a second polarization controller (13), a second light amplifier (14), a second electro-optic modulator (16) and a third light amplifier (17), the pump light signals are input from a first port of a light circulator (18), output from a second port enters a second photon lantern (12), and are subjected to mode selection through the second photon lantern (12) and then enter a few-mode optical fiber (11) from the other end;

a third port of the optical circulator (18) is connected with the tunable filter (19) and is used for outputting a sensing signal in the few-mode optical fiber (11), and the sensing signal enters the photoelectric detector (20) after being filtered by the tunable filter (19) and is converted into an electric signal; the electric signal is subjected to phase-locked amplification through a phase-locked amplifier (22) in sequence, and is subjected to A/D conversion through a data acquisition card (23) and then enters a computer, and the computer analyzes the acquired data to obtain the temperature and strain information of the position where the detection light and the pumping light signal generate the stimulated Brillouin scattering effect in the sensing optical fiber (11).

7. The chaotic BOCDA temperature strain measuring device based on few-mode optical fiber according to claim 6, wherein the input ends of the first photon lantern (10) and the second photon lantern (12) respectively have three single-mode optical fibers as input optical fibers, and the three single-mode optical fibers respectively correspond to the input optical fibers: LP01, LP11a, and LP11 b.

8. The chaotic BOCDA temperature strain measuring device based on the few-mode optical fiber is characterized by further comprising a signal generator (15) and an arbitrary pulse function generator (21), wherein the output end of the signal generator (15) is respectively connected with the first electro-optical modulator (5) and the lock-in amplifier (22), and the output end of the arbitrary pulse function generator (21) is respectively connected with the second electro-optical modulator (16) and the lock-in amplifier (22).

9. The chaotic BOCDA temperature strain measuring device based on the few-mode optical fiber is characterized by further comprising a first optical isolator (2) and a second optical isolator (9), wherein the first optical isolator (2) is arranged between the chaotic laser (1) and the beam splitter (3), and the second optical isolator (9) is arranged between the optical deflector (8) and the first photon lantern (10).

10. The chaotic BOCDA temperature strain measuring device based on the few-mode optical fiber is characterized in that the chaotic laser (1), the first optical isolator (2) and the beam splitter (3) are sequentially connected through a single-mode optical fiber jumper wire;

the beam splitter (3), the first polarization controller (4), the first electro-optic modulator (5), the programmable optical delay generator (6), the first optical amplifier (7), the optical polarization scrambler (8), the second optical isolator (9) and the first photon lantern (10) are sequentially connected through a single-mode optical fiber jumper;

the beam splitter (3), the second polarization controller (13), the second optical amplifier (14), the second electro-optic modulator (16), the third optical amplifier (17), the optical circulator (18) and the second photon lantern (12) are sequentially connected through single-mode optical fiber jumpers;

the optical circulator (18), the tunable filter (19) and the photoelectric detector (20) are connected in sequence through single-mode optical fiber jumpers.

Technical Field

The invention belongs to the field of distributed optical fiber sensing systems, and particularly relates to a chaotic BOCDA temperature strain measuring device and method based on few-mode optical fibers, which can realize simultaneous measurement of temperature and strain of a distributed optical fiber sensing system.

Background

The distributed optical fiber sensing technology has the characteristics of capability of continuously monitoring disturbance information of any point on an optical fiber, long sensing distance, strong anti-electromagnetic interference capability and the like, so that the distributed optical fiber sensing technology has unique advantages for online real-time monitoring of various important infrastructures in severe environments, such as large buildings, oil and gas pipelines, power lines, bridge tunnels, high-speed railways, civil structures and the like, and has wide application prospects in the aspects of safety and health monitoring, early warning and the like of ultra-long lines in traffic, communication, power, transportation and the like.

Currently, distributed sensing technology based on brillouin scattering has been greatly developed because it has great advantages in sensing distance and spatial resolution, etc., and it has the ability to continuously measure temperature or strain along the optical fiber. The brillouin distributed sensing technology is mainly classified into Brillouin Optical Time Domain Reflectometry (BOTDR), Brillouin Optical Time Domain Analysis (BOTDA), Brillouin Optical Coherent Domain Reflectometry (BOCDR), and Brillouin Optical Coherent Domain Analysis (BOCDA). The strain or temperature positioning of the Brillouin optical time domain reflection system and the Brillouin optical time domain analysis system is realized by determining the flight time of a pulse signal, and although long-distance distributed measurement can be realized, the spatial resolution is only a few meters at most due to the service life limitation of an acoustic phonon, so that the contradiction between the sensing distance and the spatial resolution occurs, and the application of the Brillouin time domain technology in specific engineering is limited. In order to improve the spatial resolution, researchers have proposed a brillouin optical coherence domain reflection technique and a brillouin optical coherence domain analysis technique. The former is based on a spontaneous Brillouin scattering process, and the scattered signal is weaker and the signal-to-noise ratio is lower, so that the measurement technology difficulty is higher. The light source is modulated by the sine signal based on the stimulated Brillouin scattering process, a periodic correlation function of a correlation peak can be formed, temperature or strain measurement is finally completed, and high spatial resolution is achieved while a certain sensing distance is guaranteed.

However, BOCDA systems also fail to fundamentally eliminate the conflict between sensing distance and spatial resolution. The chaotic light source is used for solving the problems, namely a distributed optical fiber sensing device and a distributed optical fiber sensing method for chaotic Brillouin optical coherence domain analysis (Chinese patent: ZL201510531253.6) provide that chaotic laser is used as a detection signal for optical fiber sensing, the problem that sensing distance and spatial resolution cannot be considered in a distributed optical fiber sensing system is successfully solved, and optical fiber sensing with long distance and ultrahigh spatial resolution is realized. In addition, because the brillouin frequency shift has the same linear change coefficient with the temperature and the strain, the measurement of the temperature and the strain cannot be simultaneously obtained according to one variable of the brillouin frequency shift, so that the simultaneous measurement of the temperature and the strain information becomes a bottleneck problem in the development of the current chaotic BOCDA system. Therefore, a new device and method are needed to solve the problem of simultaneously measuring temperature and strain in the chaotic BOCDA system.

Disclosure of Invention

The invention overcomes the defects of the prior art, and solves the technical problems that: a chaotic BOCDA temperature strain measuring device and method based on few-mode optical fiber are provided.

In order to solve the technical problems, the invention adopts the technical scheme that: a chaos BOCDA temperature strain measurement method based on few-mode optical fiber comprises the following steps:

s1, dividing the chaotic laser into two paths, wherein one path is used as a detection light signal, and the other path is used as a pumping light signal;

s2, making the detecting light signal enter the few-mode optical fiber after optical state adjustment and mode selection; enabling the pump light signal to enter a few-mode fiber from the other end after optical state adjustment and mode selection, measuring a Stokes light beat frequency signal generated in the few-mode fiber, and analyzing and processing the beat frequency signal to obtain Brillouin frequency shift quantity;

s3, changing the modes of the probe light and the pump light, and repeatedly measuring to obtain Brillouin frequency shift quantity;

and S4, calculating the temperature and strain information in the sensing optical fiber through the Brillouin frequency shift amount measured in the steps S2 and S3.

In step S2, the optical state adjustment of the probe optical signal includes polarization state adjustment, modulation, delay, amplification, polarization disturbance and isolation;

the optical state of the pump light signal is adjusted, the polarization state is adjusted, amplified, modulated and re-amplified.

In step S2, the mode selection of the probe light signal and the pump light signal is implemented by a photon lantern, the input end of the photon lantern is provided with three single-mode fibers as input fibers, the tail fiber of the single-mode fiber is connected with a few-mode fiber, and the three single-mode fibers correspond to each other: LP01, LP11a, and LP11 b.

The formula for calculating the temperature and strain information in the sensing optical fiber is as follows:

wherein, Delta T and Delta epsilon are respectively the variation of temperature and strain,andthe Brillouin frequency shift amounts of the combination of LP01-LP01 modes and the combination of LP11a-LP11a modes,andtemperatures of Brillouin frequency shifts in combinations of LP01-LP01 modes, respectivelyThe degree coefficient and the strain coefficient are,andthe temperature coefficient and the strain coefficient of brillouin frequency shift in the LP11a-LP11a mode combinations, respectively.

The temperature coefficient of the combination of LP01-LP01 modes was 1.01690 MHz/deg.C, the strain coefficient was: 0.05924 MHz/. mu.epsilon.and the temperature coefficient of the LP11a-LP11a mode combination is 0.99099 MHz/. degree.C.and the strain coefficient is: 0.04872 MHz/. mu.epsilon.

In addition, the invention also provides a chaos BOCDA temperature strain measuring device based on few-mode optical fiber, which comprises: the device comprises a chaotic laser, a beam splitter, a first polarization controller, a first electro-optic modulator, a programmable optical delay generator, a first optical amplifier, an optical polarization scrambler, a first photon lantern, a few-mode optical fiber, a second polarization controller, a second optical amplifier, a second electro-optic modulator, a third optical amplifier, an optical circulator, a tunable filter, a photoelectric detector, a phase-locked amplifier, a data acquisition card and a computer;

the chaotic laser generated by the chaotic laser is divided into two paths by the beam splitter, wherein one path is used as a detection light signal, and the other path is used as a pumping light signal;

the detection light signal sequentially passes through a first polarization controller, a first electro-optic modulator, a programmable light delay generator, a first light amplifier and a light polarization scrambler to respectively carry out polarization state adjustment, modulation, delay, amplification and polarization scrambling on the light signal, then enters a first photon lantern, and enters a few-mode optical fiber after being subjected to mode selection by the first photon lantern;

after the pump light signal is subjected to light signal polarization state adjustment, amplification, modulation and re-amplification sequentially through a second polarization controller, a second light amplifier, a second electro-optic modulator and a third light amplifier, the pump light signal is input from a first port of the light circulator, output from a second port of the light circulator enters a second photon lantern, and is incident to the few-mode optical fiber from the other end after mode selection is carried out on the pump light signal through the second photon lantern;

the third port of the optical circulator is connected with the tunable filter and used for outputting a sensing signal in the few-mode optical fiber, and the sensing signal is filtered by the tunable filter and enters the photoelectric detector to be converted into an electric signal; the electric signal is subjected to phase-locked amplification by a phase-locked amplifier and A/D conversion by a data acquisition card and then enters a computer, and the computer analyzes the acquired data to obtain the temperature and strain information of the position of the stimulated Brillouin scattering action of the detection light and the pumping light signal in the sensing optical fiber.

The input end of first photon lantern and second photon lantern has three single mode fiber as input fiber respectively, and three single mode fiber correspond respectively: LP, LPa and LPb.

The chaotic BOCDA temperature strain measuring device based on the few-mode optical fiber further comprises a signal generator and an arbitrary pulse function generator, wherein the output end of the signal generator is respectively connected with the first electro-optical modulator and the phase-locked amplifier, and the output end of the arbitrary pulse function generator is respectively connected with the second electro-optical modulator and the phase-locked amplifier.

The chaotic BOCDA temperature strain measuring device based on the few-mode optical fiber further comprises a first optical isolator and a second optical isolator, wherein the first optical isolator is arranged between the chaotic laser and the beam splitter, and the second optical isolator is arranged between the optical deflector and the first photon lantern.

According to the chaotic BOCDA temperature strain measuring device based on the few-mode optical fiber, a chaotic laser, a first optical isolator and a beam splitter are sequentially connected through a single-mode optical fiber jumper;

the beam splitter, the first polarization controller, the first electro-optic modulator, the programmable optical delay generator, the first optical amplifier, the optical polarization scrambler, the second optical isolator and the first photon lantern are sequentially connected through a single-mode optical fiber jumper;

the beam splitter, the second polarization controller, the second optical amplifier, the second electro-optic modulator, the third optical amplifier, the optical circulator and the second photon lantern are sequentially connected through single-mode optical fiber jumpers;

the optical circulator (), the tunable filter (19) and the photoelectric detector (20) are connected in sequence through a single-mode optical fiber jumper.

Compared with the prior art, the invention has the following beneficial effects:

1. the invention provides a chaotic BOCDA temperature strain measuring device and method based on few-mode optical fibers, which realize simultaneous measurement of temperature and strain and solve the problem of cross sensitivity of the sensing system application.

2. Compared with the prior art that the LEAF optical fiber is adopted as the sensing optical fiber, the invention utilizes the limited orthogonal mode in the few-mode optical fiber as the independent channel to carry out information transmission, realizes the simultaneous measurement of the temperature and the strain according to the difference of the coefficient of linear relation between the Brillouin frequency shift and the temperature and the strain in different modes, can effectively solve the problem of cross sensitivity of the optical fiber temperature and the strain, has higher precision, simpler experimental device, larger measurement range and lower cost, and is easier to popularize in practical application.

Drawings

Fig. 1 is a schematic structural diagram of a device for simultaneously measuring a chaos BOCDA temperature and a strain based on a few-mode optical fiber according to an embodiment of the present invention;

fig. 2 shows brillouin gain spectrograms of four kinds of few-mode optical fibers at two ends of incident light in different modes in combination, LP01-LP01, LP01-LP01, LP11a-LP11a and LP11a-LP11b, which are experimentally measured in the embodiment of the present invention, and fig. 2 shows that full widths at half maximum in the combination of the four kinds of modes are 53.78MHz, 57.54MHz, 57.59MHz and 57.55MHz, respectively.

In the figure 1, 1-chaotic laser, 2-first optical isolator, 3-beam splitter, 4-first polarization controller, 5-first electro-optical modulator, 6-programmable optical delay generator, 7-first optical amplifier, 8-optical scrambler, 9-second optical isolator, 10-first photon lantern, 11-sensing optical fiber, 12-second photon lantern, 13-second polarization controller, 14-second optical amplifier, 15-signal generator, 16-second electro-optical modulator, 17-third optical amplifier, 18-optical circulator, 19-tunable optical filter, 20-photoelectric detector, 21-arbitrary pulse function generator, 22-phase-locked amplifier and 23-data acquisition card.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below, and it is obvious that the described embodiments are some embodiments of the present invention, but not all embodiments; all other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Example one

As shown in fig. 1, an embodiment of the present invention provides a chaotic BOCDA temperature strain measurement device based on a few-mode optical fiber, including: the device comprises a chaotic laser 1, a first optical isolator 2, a beam splitter 3, a first polarization controller 4, a first electro-optic modulator 5, a programmable optical delay generator 6, a first optical amplifier 7, an optical polarization scrambler 8, a second optical isolator 9, a first photon lantern 10, a few-mode optical fiber 11, a second polarization controller 13, a second optical amplifier 14, a second electro-optic modulator 16, a third optical amplifier 17, an optical circulator 18, a tunable filter 19, a photoelectric detector 20, a phase-locked amplifier 22, a data acquisition card 23 and a computer.

After passing through the first optical isolator 2, the chaotic laser with low coherence generated by the chaotic laser 1 is divided into two paths by the beam splitter 3, wherein one path is used as a probe light signal, and the other path is used as a pumping light signal.

The detection light signal sequentially passes through a first polarization controller 4, a first electro-optic modulator 5, a programmable light delay generator 6, a first light amplifier 7, a light polarization scrambler 8 and a second light isolator 9 to respectively perform polarization state adjustment, modulation, delay, amplification, polarization scrambling and isolation of the light signal, enters a first photon lantern 10, and enters a few-mode optical fiber 11 after mode selection is performed on the first photon lantern 10.

After the optical signal polarization state adjustment, amplification, modulation and re-amplification are sequentially performed on the pump optical signal through the second polarization controller 13, the second optical amplifier 14, the second electro-optical modulator 16 and the third optical amplifier 17, the pump optical signal is input from the first port of the optical circulator 18, output from the second port enters the second photon lantern 12, and enters the few-mode optical fiber 11 from the other end after mode selection is performed on the pump optical signal through the second photon lantern 12.

The detection light signal and the pump light signal generate stimulated brillouin scattering in the sensing optical fiber 11 and generate stokes light, the stokes light and the pump light signal generate interference beat frequency effect, and the generated beat frequency signal is the sensing signal. After being output from the few-mode optical fiber 11, the sensing signal returns to the optical circulator 18 through the second photon lantern 12, is output from a third port of the optical circulator 18, is filtered by the tunable filter 19, and enters the photoelectric detector 20 to be converted into an electric signal; the electrical signal is phase-locked and amplified by the phase-locked amplifier 22, the data acquisition card 23 performs A/D conversion and then enters the computer, and the computer analyzes the acquired data to obtain the temperature and strain information of the position where the detection light and the pump light signal generate the stimulated Brillouin scattering effect in the sensing fiber 11.

Specifically, the chaotic BOCDA temperature strain measurement device based on the few-mode optical fiber further includes a signal generator 15 and an arbitrary pulse function generator 21, an output end of the signal generator 15 is connected to the first electro-optical modulator 5 and the lock-in amplifier 22, respectively, and an output end of the arbitrary pulse function generator 21 is connected to the second electro-optical modulator 16 and the lock-in amplifier 22, respectively. The signal generator 15 is configured to drive the first electro-optical modulator 5 to modulate the probe optical signal, and the arbitrary pulse function generator 21 is configured to drive the second electro-optical modulator 16 to pulse-modulate the probe optical signal. The modulated signals from the signal generator 15 and the arbitrary pulse function generator 21 are also sent to a lock-in amplifier, and the detected electric signals are subjected to lock-in amplification processing.

Specifically, in this embodiment, three single-mode fibers are respectively provided at the input ends of the first photon lantern 10 and the second photon lantern 12 as input fibers, and input light enters different input fibers and then exits into the few-mode sensing fiber 11 through a single-mode pigtail. In this embodiment, the core structure of the few-mode fiber is step-type, and the inner cladding is doped with fluorine (F) with a certain concentration, which has a fixed radius of the core and the cladding and a refractive index, so that the propagation mode of the few-mode fiber is determined. Due to the existence of degenerate modes, in fact, three spatial modes exist for a two-mode step fiber: LP01, LP11a, and LP11 b. Therefore, in the present embodiment, the first photon lantern 10 and the second photon lantern 12 having three single mode fibers at the input end are connected to the few-mode fiber, and the three single mode fibers respectively correspond to: the three spatial modes LP01, LP11a and LP11b can convert the fundamental mode in the single-mode fiber into corresponding different modes in the few-mode fiber, thereby realizing mode selection. That is, by coupling the probe light and the pump light into different input fibers of the photon lanterns at two ends of the sensing fiber, the mode selection of the probe light and the pump light can be performed, so that different combinations based on the three spatial modes, namely, four mode combinations of LP01-LP01, LP11a-LP11a, LP01-LP11 and LP11a-LP11b, are obtained. The LP01-LP01 modes represent that the probe light is switched into the LP01 mode, and the pump light is switched into the LP01 mode; the LP11a-LP11a modes represent that the probe light is switched into the LP11a mode, and the pump light is switched into the LP11a mode; the LP01-LP11 modes represent that probe light is switched into the LP01 mode, and pump light is switched into the LP11a or LP11b mode; the LP11a-LP11b modes indicate that the probe light is switched into the LP11a mode and the pump light is switched into the LP11b mode.

Specifically, in this embodiment, the chaotic laser 1, the first optical isolator 2, and the beam splitter 3 are connected in sequence by a single-mode optical fiber jumper; the beam splitter 3, the first polarization controller 4, the first electro-optic modulator 5, the programmable optical delay generator 6, the first optical amplifier 7, the optical polarization scrambler 8, the second optical isolator 9 and the first photon lantern 10 are sequentially connected through a single-mode optical fiber jumper; the beam splitter 3, the second polarization controller 13, the second optical amplifier 14, the second electro-optical modulator 16, the third optical amplifier 17, the optical circulator 18 and the second photon lantern 12 are sequentially connected through a single-mode optical fiber jumper; the optical circulator 18, the tunable filter 19 and the photoelectric detector 20 are connected in sequence through a single-mode optical fiber jumper.

In the specific implementation, the central wavelength of the chaotic laser source is 1550nm, the beam splitter 3 adopts a 1x2 optical fiber coupler, the coupling ratio is 90:10 (detection light path: pumping light path), the first electro-optical modulator 5 and the second electro-optical modulator 16 adopt AZ-DK5-20-FFU-SFU-LV-SRF1W type intensity modulators, the programmable optical delay generator 6 adopts an ODG-101 high-precision programmable optical delay line, the first optical amplifier 7, the second optical amplifier 14 and the third optical amplifier 17 adopt erbium-doped fiber amplifiers, the optical polarization scrambler 8 adopts a PCD-104 type polarization scrambler, the sensing fiber 11 adopts a few-mode fiber, the length of which is 10km, the signal generator 15 adopts an EXG-N5173B type signal source, the tunable filter 19 is an XTM-50 bandwidth wavelength tunable filter, and the photodetector 20 is a PM100D detector.

The embodiment provides a chaotic BOCDA temperature strain measuring device based on a few-mode optical fiber, which can measure the frequency shift quantity of a Brillouin gain spectrum by changing the mode combination of incident light at two ends of the few-mode optical fiber so as to realize simultaneous detection of temperature and strain.

In this embodiment, mode selection is performed on probe light and pump light through two photon lanterns, and brillouin frequency shift measurement is performed through two different modes (LP01-LP01 mode combination and LP11a-LP11a mode combination), that is, temperature and strain information in a few-mode fiber can be demodulated simultaneously.

The Brillouin frequency shift versus temperature and strain for the combination of LP01-LP01 modes and the combination of LP11a-LP11a modes are:

from equations (1) and (2), the amount of change in temperature and strain is:

wherein, Delta T and Delta epsilon are respectively the variation of temperature and strain,andthe brillouin frequency shifts for the LP01-LP01 mode combination and the LP11a-LP11a mode combination respectively,andthe temperature coefficient and the strain coefficient of the brillouin frequency shift in the combination of LP01-LP01 modes,andthe temperature coefficient and the strain coefficient of brillouin frequency shift in the LP11a-LP11a mode combinations, respectively.

Through experimental measurement, the temperature and strain coefficients of the combination of LP01-LP01 mode and the combination of LP11a-LP11a mode are shown in Table 1, and the Brillouin gain spectra of the combination of LP01-LP01, LP01-LP11, LP11a-LP11a and LP11a-LP11b in four different modes are shown in FIG. 2.

TABLE 1 experimentally measured temperature and strain coefficients for the combination of LP01-LP01 and LP11a-LP11a modes obtained

From table 1, values of Δ T and Δ ∈ can be calculated by substituting brillouin frequency shifts into formulas (3) and (4) measured by combining LP01 to LP01 modes and LP11a to LP11a modes. According to the simulation results, the Δ T and Δ ε were calculated to be 24.8 ℃ and 1003.5 μ ε, respectively, in the obtained Brillouin frequency shifts 84.7MHz and 73.5MHz equations. The temperature error and the strain error are respectively 0.2 ℃ and 3.5 mu epsilon, which are very close to the actual set temperature and strain changes.

Example two

The embodiment of the invention provides a chaotic BOCDA temperature strain measurement method based on a few-mode optical fiber, which comprises the following steps:

s1, dividing the chaotic laser into two paths, wherein one path is used as a detection light signal, and the other path is used as a pumping light signal;

s2, making the detecting light signal enter the few-mode optical fiber 11 after the optical state adjustment and the mode selection; enabling the pump light signal to enter the few-mode fiber 11 from the other end after optical state adjustment and mode selection, measuring a Stokes light beat frequency signal generated in the few-mode fiber 11, and analyzing and processing the beat frequency signal to obtain Brillouin frequency shift quantity;

s3, changing the modes of the probe light and the pump light, and repeatedly measuring to obtain Brillouin frequency shift quantity;

and S4, calculating the temperature and strain information in the sensing optical fiber through the Brillouin frequency shift amount measured in the steps S2 and S3.

Specifically, in step S2, the optical state adjustment of the probe optical signal includes polarization state adjustment, modulation, delay, amplification, polarization disturbance and isolation; the optical state of the pump light signal is adjusted, the polarization state is adjusted, amplified, modulated and re-amplified. The polarization state adjustment, modulation, delay, amplification, polarization disturbance and isolation of the light beam are respectively realized by a polarization controller, a modulator, an optical delay generator, an optical amplifier, an optical polarization winder and an optical isolator.

Specifically, in step S2, the mode selection of the probe light signal and the pump light signal is implemented by a photon lantern, three single-mode fibers are respectively provided at an input end of the photon lantern as input fibers, a pigtail of the single-mode fiber is connected with the few-mode fiber 11, and the three single-mode fibers respectively correspond to: LP01, LP11a, and LP11 b.

In summary, the invention provides a chaotic BOCDA temperature strain measurement method and device based on a few-mode fiber, which can realize ultrahigh spatial resolution based on low-coherence chaotic laser, use a sensing fiber as the few-mode fiber, use a limited orthogonal mode in the few-mode fiber as an independent channel for information transmission, realize simultaneous measurement of temperature and strain according to different linear relation coefficients of brillouin frequency shift and temperature and strain in different modes, can effectively solve the problem of cross sensitivity of fiber temperature and strain, and have the advantages of higher precision, simpler experimental device, larger measurement range, lower cost and easier popularization in practical application.

Finally, it should be noted that: the above embodiments are only used to illustrate the technical solution of the present invention, and not to limit the same; while the invention has been described in detail and with reference to the foregoing embodiments, it will be understood by those skilled in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.