A method and device for synchronously measuring distributed strain and FBG strain dual mechanisms

Abstract

A method and device for synchronously measuring a distributed strain and FBG strain dual mechanism. The method comprises winding an optical cable into a fiber winding structure, and fusing FBG broadband light, continuous detection light and pulsed pump light in the same fiber winding structure, so that three kinds of measurement lights work simultaneously in the same optical fiber, and two kinds of strain measurements do not interfere with each other. The device realizes a method for synchronously measuring a distributed strain and FBG strain dual mechanism through multiple laser sources, couplers, photoelectric modulators, circulators and photoelectric detectors. The present invention performs two measurement mechanisms simultaneously in the same optical fiber, improves the accuracy of the sensor, greatly reduces the length of optical fiber wiring, and is particularly suitable for characterizing the internal strain state of a three-dimensional precision winding structure.

Description

Technical Field

[0001] The present invention belongs to the technical field of distributed strain measurement, and in particular relates to a method and a device for synchronously measuring a distributed strain and an FBG strain dual mechanism. Background Art

[0002] Autonomous underwater vehicles (AUVs) have been widely used in marine scientific research, marine resource exploration, and marine safety assurance. The key to achieving underwater vehicle control and real-time transmission of high-definition video information is to establish rapid communication between the vehicle and the ground. Unlike electromagnetic communication in the air, electromagnetic waves will decay rapidly in seawater, and the transmission depth is only a few meters. Compared with electromagnetic communication, optical fiber communication has outstanding advantages such as anti-electromagnetic interference, cross-media transmission, strong environmental adaptability, and large information transmission capacity.

[0003] According to certain process criteria, the optical fiber is wound into a cable package with a complex geometric structure. During the navigation of the spacecraft, the optical fiber is passively released from this precise structure layer by layer. The reliability of the successful release of the optical fiber is closely related to the initial strain state in the cable package structure. The breakage of the optical fiber will lead to the failure of the deep-sea exploration mission. Obtaining reliable strain in the entire cable package structure is crucial to the safe release assessment of the optical fiber winding structure.

[0004] Fiber-optic distributed strain measurement uses the optical fiber itself as the sensing unit and measures the strain distribution along the length of the optical fiber based on Rayleigh scattering, Brillouin scattering or Raman scattering. Among them, the distributed strain measurement method based on stimulated Brillouin scattering (BOTDA) has the characteristics of high spatial resolution, large number of sensing points and low cost in long-distance measurement, and has been applied in many fields. However, due to the random scattering characteristics of Brillouin scattering, gain spectrum fitting errors and random noise, the strain measured by Brillouin has low reliability, and sometimes the measurement error reaches 15%. For precise optical fiber structures such as optical cable sheaths and fiber optic gyroscopes, low distributed strain confidence can easily cause safety assessment errors, increasing mission costs and cycles.

[0005] Fiber Bragg grating (FBG) point strain measurement has the characteristics of high sensitivity and high accuracy, but the capacity of the sensor in the same optical fiber is limited by the bandwidth of the reflection spectrum. In addition, it is mainly used to measure the strain of the surface at discrete locations and cannot be used to measure the internal strain of a three-dimensional fiber-wound structure.

[0006] Integrating BOTDA distributed strain and FBG strain in the same optical path and realizing distributed strain and FBG strain measurement on a long-distance optical fiber will provide a basis for the safe release assessment of fiber-wound structures. However, when the BOTDA measurement system and the FBG measurement system are directly connected to the optical fiber, the two measurement signals interfere with each other, and the grating recorded in the optical fiber will excite strong signal noise in the Brillouin spectrum, which significantly increases the distributed error of the BOTDA measurement. How to integrate the two measurement mechanisms in the same system and realize distributed strain and multi-point FBG strain measurement on the same optical fiber at the same time is still a recognized problem. Summary of the invention

[0007] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a method and device for synchronous measurement of distributed strain and FBG strain dual mechanisms. By combining the advantages of a large number of distributed measurement points and the high accuracy of FBG measurement, the two measurement mechanisms are integrated in the same optical path to achieve the purpose of simultaneous measurement of distributed strain and point strain, and comprehensively evaluate the internal health status of the structure. In this method, the two measurement mechanisms are carried out simultaneously in the same optical fiber, which improves the accuracy of sensor strain measurement and greatly reduces the optical fiber wiring length. It is particularly suitable for the internal strain state characterization of three-dimensional precision winding structures.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is:

[0009] A method for synchronously measuring a distributed strain and an FBG strain dual mechanism comprises the following steps:

[0010] Step 1: Inscribe a series of discrete fiber Bragg gratings FBG2 in the optical fiber, weave a sheath outside the optical fiber to form an optical cable with FBG2, and wind the optical cable into an optical fiber winding structure 3 according to the design structure;

[0011] Step 2: Input the FBG broadband light 5 and the continuous detection light 4 in the same direction into one end of the fiber winding structure 3 obtained in step 1 through the fiber coupler, and input the pulsed pump light 1 from the other end of the fiber winding structure 3 obtained in step 1. The detection light 4 and the pump light 1 excite Brillouin frequency shift in the optical fiber. The detection light passes through each grating losslessly in turn and demodulates the distributed Brillouin strain. No Brillouin frequency shift is excited at the position of FBG2. At the same time, the FBG broadband light 5 reflects back the light of the central wavelength after reaching each grating. At the input end of the FBG broadband light 5, the multi-point FBG strain is demodulated according to the power peak, so that the three measurement lights can work simultaneously in the same optical fiber, and the two strain measurements do not interfere with each other.

[0012] The condition for not exciting the Brillouin frequency shift at the position of FBG2 in step 2 is that the modulation depth of the grating refractive index is less than 0.2% of the strain refractive index coefficient of the optical fiber.

[0013] The theoretical analysis of the detection light passing through each grating without loss and not exciting the Brillouin frequency shift at the grating position is as follows:

[0014] In single-mode optical fiber, there is only backward stimulated Brillouin scattering, and the Brillouin frequency shift v B It can be expressed as:

[0015] v B =2nV a / λ p (1)

[0016] Where n is the effective refractive index of the optical fiber, λ p represents the wavelength of the pump light, V a Express the speed of sound waves in optical fiber:

[0017]

[0018] Where k is Poisson's ratio, E is Young's modulus, and ρ is the fiber density.

[0019] Brillouin scattering is a nonlinear scattering caused by acoustic phonons in optical fibers. When the temperature and stress in the optical fiber change, its effective refractive index and acoustic wave velocity will also change, causing the change of the Brillouin frequency shift of the optical fiber. If the fiber refractive index n, Young's modulus E, Poisson's ratio k and density ρ are recorded as n(ε,T), E(ε,T), k(ε,T) and ρ(ε,T) as a function of temperature T and strain ε, respectively, when the temperature remains unchanged, the reference temperature is T0, and the relationship between stress and Brillouin frequency shift is:

[0020]

[0021] SiO2, a component of optical fiber, has a small tensile stress. By performing Taylor series expansion on equation (3) and ignoring high-order terms, we can obtain:

[0022] v B (ε,T0)≈v B (0,T0)[1+ε(Δn ε +Δk ε +ΔE ε +Δρ ε )](4)

[0023] If p is 1550nm, at room temperature 20℃, Δn ε =-0.22, Δk ε =1.49, ΔE ε =2.88,Δρ ε =0.33.

[0024] The refractive index modulation period (grating period) of the fiber Bragg grating is relatively short, about 0.5 to 1 μm, and the refractive index modulation distribution is sinusoidally uniform:

[0025]

[0026] Where n1(z) is the refractive index of the grating core, Δn is the maximum value of the refractive index change, i.e., the refractive index modulation depth, n eff The effective refractive index of the fiber core, Λ is the grating period, and L is the grating length.

[0027] After the point sensor fiber Bragg grating is introduced into the optical fiber to be tested, it mainly causes the refractive index of the fiber core to change. At the fiber Bragg grating, the Brillouin frequency shift caused by strain can be expressed as:

[0028]

[0029] According to equation (6), whether the detection light passing through the grating will stimulate the Brillouin shift is determined by the grating modulation depth Δn relative to the strain refractive index coefficient Δn. ε The probe light will not trigger the Brillouin frequency shift after passing through the grating, and the condition that the probe light will pass through the grating losslessly in sequence is that the grating refractive index modulation depth is less than 0.2% of the optical fiber strain refractive index coefficient.

[0030] A device for synchronously measuring a distributed strain and a FBG strain dual mechanism, comprising a first laser source 6, the first laser source 6 is connected to a first line port of a first coupler 8 through an optical fiber, the two line ports of the first coupler 8 are respectively connected to a first photoelectric modulator EOM1 and a third photoelectric modulator EOM3 through optical fibers, the first photoelectric modulator EOM1 is connected to a first port of a first optical circulator C1 through an optical fiber, the second port of the first optical circulator C1 is connected to a second laser source 7 through an optical fiber, the third port of the first optical circulator C1 is connected to a second photoelectric modulator EOM2 through an optical fiber, the second photoelectric modulator EOM2 is connected to a first port of a second optical circulator C2 through an optical fiber, the second port of the second optical circulator C2 is connected to an optical fiber winding structure 3 through an optical fiber, the third port of the second optical circulator C2 is connected to a first photodetector PD1 through an optical fiber, and the first photodetector PD1 is used to detect a Brillouin frequency shift signal;

[0031] The first electro-optical modulator EOM1 and the second electro-optical modulator EOM2 are simultaneously connected to a waveform generator AWG for generating pulses;

[0032] The third optoelectronic modulator EOM3 and the third laser source 11 are connected to the two-line port of the second coupler 9 through the optical fiber, the one-line port of the second coupler 9 is connected to the first port of the third optical circulator C3 through the optical fiber, the second port of the third optical circulator C3 is connected to the optical fiber winding structure 3 through the optical fiber, and the third port of the third optical circulator C3 is connected to the second photodetector PD2 through the optical fiber, and the second photodetector PD2 is used to detect the FBG signal;

[0033] The third electro-optical modulator EOM3 is also connected to the arbitrary waveform generator 10 for generating the detection light.

[0034] The frequency range of the detection light generated by the laser emitted by the first laser source 6 through the third electro-optical modulator EOM3 is 10.5 GHz to 11.1 GHz.

[0035] The laser light emitted by the first laser source 6 passes through the first electro-optical modulator EOM1 and the second electro-optical modulator EOM2 to generate a pump light pulse with a width ranging from 1 to 20 ns.

[0036] The working wavelength range of the FBG broadband light emitted by the third laser source 11 is 1460-1620 nm.

[0037] The first laser source 6 is an ultra-narrow-band laser with an operating wavelength of 1550 nm or 1310 nm.

[0038] The power of the detection light in the two paths divided by the first laser source is not less than 5% and not more than 25%.

[0039] Compared with the prior art, the beneficial effects of the present invention are:

[0040] 1. The present invention proposes a method for synchronously measuring distributed strain and FBG point strain in the same optical fiber. The two measurement mechanisms are integrated into one optical fiber, and the measurement accuracy of the two strains does not affect each other. Both Brillouin distributed strain and FBG point strain can be obtained, avoiding the fixed error caused by the separate wiring of the two sensors in the traditional method and greatly reducing the optical fiber wiring length.

[0041] 2. The present invention proposes a method for accurately characterizing the internal strain state of a three-dimensional precision optical fiber winding structure. Under the traditional method, the two measurement mechanisms work independently, and the FBG can only be pasted on the surface of the structure, and the true strain state inside the structure cannot be accurately obtained. However, the method of the present invention can simultaneously obtain the two strains in the structure, providing reliable data for the safety assessment of the structure.

[0042] 3. The present invention applies multiple strains at both ends of an optical fiber, obtains the Brillouin strain and the FBG strain at the same time, evaluates the Brillouin strain error distribution, and avoids the clamping error and position approximate equivalent error caused by the traditional method of applying strain through two optical fibers.

[0043] In summary, the present invention improves the accuracy of the sensor, reduces the length of optical fiber wiring, reduces the wiring workload and cost, and can achieve accurate characterization of the internal strain of the three-dimensional structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a schematic diagram of the principle of the fusion of the distributed strain and FBG strain measurement mechanism of the present invention;

[0045] Figure 2 An optical path diagram of the fusion of the distributed strain and FBG strain measurement mechanisms of the present invention;

[0046] In the figure, 1. pump light; 2. fiber Bragg grating; 3. fiber winding structure; 4. detection light; 5. FBG broadband light; 6. first laser source; 7. second laser source; 8. first coupler; 9. second coupler; 10. arbitrary waveform generator; 11. third laser source; EOM1. first photoelectric modulator; EOM2. second photoelectric modulator; EOM3. third photoelectric modulator; C1. first optical circulator; C2. second optical circulator; C3. third optical circulator; PD1. first photodetector; PD2. second photodetector; AWG. waveform generator;

[0047] Figure 3 A distributed strain diagram measured by the dual-mechanism fusion device of Example 1 of the present invention in the initial state;

[0048] Figure 4 This is a distributed strain diagram measured by the dual-mechanism fusion device of Example 1 of the present invention at 77.6 με;

[0049] Figure 5 This is a distributed strain diagram measured by the dual-mechanism fusion device of Example 1 of the present invention at 533.3 με;

[0050] Figure 6 The demodulated FBG spectrum diagram of Example 1 of the present invention when the FBG broadband light source and the continuous detection light are input in the same direction;

[0051] Figure 7 The demodulated FBG spectrum diagram of Example 1 of the present invention when the FBG broadband light source and the continuous detection light are input in reverse;

[0052] Figure 8 Brillouin strain plot when the grating has a high refractive index modulation depth;

[0053] Fig. 9This is a Brillouin strain error distribution diagram of Example 1 of the present invention;

[0054] Fig.10 Graphs of the Brillouin strain and FBG strain simultaneously demodulated in the optical fiber line package of Example 2 of the present invention. DETAILED DESCRIPTION

[0055] The present invention will be described in detail below in conjunction with the accompanying drawings.

[0056] Embodiment 1:

[0057] This example uses an example of an optical fiber with a length of 1 km containing 6 FBGs to illustrate the measurement method and device for the fusion of the distributed strain and point strain dual mechanisms, but is not limited to this example.

[0058] like Figure 1 As shown, a method for synchronously measuring a distributed strain and a FBG strain dual mechanism comprises the following steps:

[0059] Step 1: Inscribe a series of discrete fiber Bragg gratings FBG2 in the optical fiber, weave a sheath outside the optical fiber to form an optical cable with FBG2, and wind the optical cable into an optical fiber winding structure 3 according to the design structure;

[0060] Step 2: Input the FBG broadband light 5 and the continuous detection light 4 in the same direction into one end of the fiber winding structure 3 obtained in step 1 through the fiber coupler, and input the pulsed pump light 1 from the other end of the fiber winding structure 3 obtained in step 1. The detection light 4 and the pump light 1 excite Brillouin frequency shift in the optical fiber. The detection light passes through each grating losslessly in turn and demodulates the distributed Brillouin strain. No Brillouin frequency shift is excited at the position of FBG2. At the same time, the FBG broadband light 5 reflects back the light of the central wavelength after reaching each grating. At the input end of the FBG broadband light 5, the multi-point FBG strain is demodulated according to the power peak, so that the three measurement lights can work simultaneously in the same optical fiber, and the two strain measurements do not interfere with each other.

[0061] The condition for not exciting the Brillouin frequency shift at the position of FBG2 in step 2 is that the modulation depth of the grating refractive index is less than 0.2% of the strain refractive index coefficient of the optical fiber.

[0062] The theoretical analysis of the detection light passing through each grating without loss and not exciting the Brillouin frequency shift at the grating position is as follows:

[0063] In single-mode optical fiber, there is only backward stimulated Brillouin scattering, and the Brillouin frequency shift v B It can be expressed as:

[0064] v B =2nV a / λ p (1)

[0065] Where n is the effective refractive index of the optical fiber, λ p represents the wavelength of the pump light, V a Express the speed of sound waves in optical fiber:

[0066]

[0067] Where k is Poisson's ratio, E is Young's modulus, and ρ is the fiber density.

[0068] Brillouin scattering is a nonlinear scattering caused by acoustic phonons in optical fibers. When the temperature and stress in the optical fiber change, its effective refractive index and acoustic wave velocity will also change, causing the change of the Brillouin frequency shift of the optical fiber. If the fiber refractive index n, Young's modulus E, Poisson's ratio k and density ρ are recorded as n(ε,T), E(ε,T), k(ε,T) and ρ(ε,T) as a function of temperature T and strain ε, respectively, when the temperature remains unchanged, the reference temperature is T0, and the relationship between stress and Brillouin frequency shift is:

[0069]

[0070] SiO2, a component of optical fiber, has a small tensile stress. By performing Taylor series expansion on equation (3) and ignoring high-order terms, we can obtain:

[0071] v B (ε,T0)≈v B (0,T0)[1+ε(Δn ε +Δk ε +ΔE ε +Δρ ε )](4)

[0072] If p is 1550nm, at room temperature 20℃, Δn ε =-0.22, Δk ε =1.49, ΔE ε =2.88,Δρ ε =0.33.

[0073] The refractive index modulation period (grating period) of the fiber Bragg grating is relatively short, about 0.5 to 1 μm, and the refractive index modulation distribution is sinusoidally uniform:

[0074]

[0075] Where n1(z) is the refractive index of the grating core, Δn is the maximum value of the refractive index change, i.e., the refractive index modulation depth, n eff The effective refractive index of the fiber core, Λ is the grating period, and L is the grating length.

[0076] After the point sensor fiber Bragg grating is introduced into the optical fiber to be tested, it mainly causes the refractive index of the fiber core to change. At the fiber Bragg grating, the Brillouin frequency shift caused by strain can be expressed as:

[0077]

[0078] According to equation (6), whether the detection light passing through the grating will stimulate the Brillouin shift is determined by the grating modulation depth Δn relative to the strain refractive index coefficient Δn. ε The probe light will not trigger the Brillouin frequency shift after passing through the grating, and the condition that the probe light will pass through the grating losslessly in sequence is that the grating refractive index modulation depth is less than 0.2% of the optical fiber strain refractive index coefficient.

[0079] A device for synchronously measuring a distributed strain and a FBG strain dual mechanism, comprising a first laser source 6, the first laser source 6 is connected to a first line port of a first coupler 8 through an optical fiber, the two line ports of the first coupler 8 are respectively connected to a first photoelectric modulator EOM1 and a third photoelectric modulator EOM3 through optical fibers, the first photoelectric modulator EOM1 is connected to a first port of a first optical circulator C1 through an optical fiber, the second port of the first optical circulator C1 is connected to a second laser source 7 through an optical fiber, the third port of the first optical circulator C1 is connected to a second photoelectric modulator EOM2 through an optical fiber, the second photoelectric modulator EOM2 is connected to a first port of a second optical circulator C2 through an optical fiber, the second port of the second optical circulator C2 is connected to an optical fiber winding structure 3 through an optical fiber, the third port of the second optical circulator C2 is connected to a first photodetector PD1 through an optical fiber, and the first photodetector PD1 is used to detect a Brillouin frequency shift signal;

[0080] The first electro-optical modulator EOM1 and the second electro-optical modulator EOM2 are simultaneously connected to a waveform generator AWG for generating pulses;

[0081] The third optoelectronic modulator EOM3 and the third laser source 11 are connected to the two-line port of the second coupler 9 through the optical fiber, the one-line port of the second coupler 9 is connected to the first port of the third optical circulator C3 through the optical fiber, the second port of the third optical circulator C3 is connected to the optical fiber winding structure 3 through the optical fiber, and the third port of the third optical circulator C3 is connected to the second photodetector PD2 through the optical fiber, and the second photodetector PD2 is used to detect the FBG signal;

[0082] The third electro-optical modulator EOM3 is also connected to the arbitrary waveform generator 10 for generating the detection light.

[0083] The power of the detection light in the two paths divided by the first laser source is not less than 5% and not more than 25%.

[0084] As a verification of the fusion of the two strain measurement mechanisms, the optical path of the measurement system for the fusion of stimulated Brillouin distributed strain and FBG strain is as follows: Figure 2 The first laser source 6 is an ultra-narrow-band laser with an operating wavelength of 1550 nm, and is divided into two paths with a power ratio of 10:90, 10% of the laser is used as detection light for BOTDA measurement, and 90% of the laser is used to generate pulse light. The first optoelectronic modulator EOM1, the second optoelectronic modulator EOM2 and the second laser source 7 cooperate to adjust 90% of the laser light in the upper branch to generate 5ns pulse pump light, which is input into the optical fiber winding structure 3 from the second port of the second optical circulator C2; 10% of the laser light in the lower branch generates continuous detection light with a frequency of 10.5-11.1GHz through the third optoelectronic modulator EOM3, which is injected into the other end of the optical fiber to be tested from the second port of the third optical circulator C3 together with the FBG broadband light source (third laser source 11), and the working wavelength of the FBG broadband light source is 1460-1620nm; the Brillouin signal is measured from the third port of the second optical circulator C2 through the first photodetector PD1, and the FBG signal is measured from the third port of the third optical circulator C3 through the second optical circulator C2; the pump light of the upper branch and the detection light / FBG broadband light of the lower branch pass through the optical fiber winding structure 3 containing FBG in opposite directions. The first photodetector PD1 receives the Brillouin signal carried by the detection light from the third port of the second optical circulator C2, and demodulates the distributed Brillouin strain by the frequency sweep method; the second photodetector PD2 receives the central wavelength information reflected from each FBG from the third port of the third optical circulator C3.

[0085] A 1km long single-mode silica fiber (Yangfei Fiber, Bl1011-A) was used to record six gratings at a length of 516-517m using the mask method, with an interval of 10cm. The fiber was wound around the receiving tray from both sides. The grating refractive index modulation depth was 1×10 -4 The strain refractive index coefficient of the optical fiber is -0.22, and the modulation depth of the grating refractive index is less than 0.02 of the strain refractive index coefficient of the optical fiber. Figure 2 In the device shown, one end of the optical fiber is connected to the pulse pump light, and the other end is connected to the continuous detection light and FBG broadband light. The optical fiber segment of 516-517m is fixed to the micro-strain application platform, and the relaxation state is used as the zero strain reference. Different strains are applied to the optical fiber segment from low to high. The distributed strain measured by the dual-mechanism fusion system of the present invention in the initial state is as follows Figure 3 As shown in the figure, the part marked between the two dots belongs to the unfolded optical fiber, the optical fibers on both sides are in a tangled state, and the part marked with the strip belongs to the optical fiber segment of 516-517m. In the relaxed state, the strain of this optical fiber segment is close to 0, and the strain of the tangled optical fiber is close to 200με. The distributed strain measured by the dual-mechanism fusion system of the present invention at 77.6με and 533.3με is shown in the figure. Figure 4 and Figure 5 As shown, from Figure 3-5 It can be seen that only the optical fiber strain in the stress application section is deformed, and the strain of the other optical fibers does not change, and no signal noise is generated, indicating that the fiber Bragg grating in the method of the present invention will not affect the BOTDA strain measurement.

[0086] When the FBG broadband light 5 and the continuous detection light 4 are input in the same direction, the demodulated FBG spectrum of the present invention is as follows: Figure 6 As shown in the figure, the peak of the central wavelength of the spectrum is prominent, and the FBG strain corresponding to each grating can be demodulated. This shows that in the present invention, the two strain measurement mechanisms work in the same optical fiber and can achieve non-interference. However, when the FBG broadband light 5 and the continuous detection light 4 are input in the same direction, the demodulated FBG spectrum is as follows: Figure 7 As shown, it is impossible to demodulate to the FBG strain, and the two measurement mechanisms cannot be combined.

[0087] When the grating refractive index modulation depth is equal to 0.3% of the fiber strain refractive index coefficient, when the two measurement mechanisms work simultaneously in the fiber, a large amount of signal noise is excited in the generated distributed Brillouin strain, such as Figure 8 , distributed strain measurement cannot be achieved.

[0088] Based on the present invention, one optical fiber simultaneously obtains the Brillouin strain and the FBG strain, evaluates the Brillouin strain error distribution, and avoids the clamping error caused by the traditional strain application through two optical fibers. The average value of the 6 FBG strains is taken as the accurate strain, and the accuracy of the Brillouin strain is evaluated by the relative percentage error RPE:

[0089]

[0090] Among them, ε B represents the Brillouin strain, ε F represents the FBG strain. The relationship between Brillouin strain and relative percentage error is as follows: Fig. 9 As shown, it can be seen that the Brillouin distributed strain measurement error is highly nonlinear and changes with strain.

[0091] Embodiment 2:

[0092] The difference from Example 1 is that Example 2 selects a 2.5 km optical fiber-wound underwater AUV navigation communication line package as an example to illustrate the method of characterizing the internal strain state of a three-dimensional precision optical fiber winding structure. 35 FBGs are evenly distributed in the optical fiber, and after being wound into a line package, the present invention is used to simultaneously measure the distributed strain and the FBG strain. The obtained strain results are as follows: Fig.10 As shown in the figure, although the Brillouin strain has low accuracy, it can reflect the strain distribution within the structure, and the FBG strain can accurately reflect the strain levels at multiple locations within the structure.

[0093] Embodiment 3:

[0094] The difference from Example 1 is that Example 3 selects the application scenario of ocean crust surface strain monitoring, lays the optical cable with FBG on the surface of the seabed crust, and obtains the slow changes of the crust surface through accurate strain changes, providing reliable data for earth science research.

[0095] Embodiment 4:

[0096] The difference from Example 1 and Example 3 is that the embodiment is used for monitoring the health status of large space structures. By adopting the present invention, only one or a small number of optical fibers need to be laid in the entire structure to monitor the surface strain of the structure. There is no need to lay strain sensors at multiple points respectively, which greatly reduces the length of optical fibers that need to be laid and reduces the risk of optical fibers to the safety of spatial structure deployment.

[0097] Therefore, by adopting the distributed strain and FBG strain dual-mechanism synchronous measurement method and device proposed in the present invention, distributed Brillouin strain and FBG strain synchronous measurement can be realized in the same optical fiber. There is no coupling between the two strain measurement results, and no new signal noise is introduced, which provides reliable data for the accurate evaluation of the internal strain state of the three-dimensional precision winding structure. In addition, the present invention integrates the two strain measurements into one optical fiber, overcoming the problems of long fiber laying length and high cost caused by traditional independent wiring, avoiding the optical fiber fixed error existing in the method of equivalently characterizing the strain at the position by wiring at a similar position, and adopting the method of the present invention to accurately characterize the error distribution characteristics of the optical fiber Brillouin strain measurement for the first time, providing new knowledge for the accurate measurement of BOTDA.

Claims

1. A method for synchronous measurement of distributed strain and FBG strain dual mechanism, characterized in that: The following steps are involved: Step 1: Inscribe a series of discrete fiber Bragg gratings (FBGs) (2) in an optical fiber, weave a sheath outside the optical fiber to form an optical cable with FBGs (2), and wind the optical cable into an optical fiber winding structure (3) according to the designed structure; Step 2: The FBG broadband light (5) and the continuous detection light (4) are inputted in the same direction into one end of the fiber winding structure (3) obtained in step 1 through a fiber coupler, and the pulsed pump light (1) is inputted from the other end of the fiber winding structure (3) obtained in step 1. The detection light (4) and the pump light (1) excite Brillouin frequency shift in the optical fiber. The detection light is demodulated to obtain distributed Brillouin strain after passing through each grating in turn without loss, and no Brillouin frequency shift is excited at the position of FBG (2). At the same time, the FBG broadband light (5) is reflected back as light of the central wavelength after reaching each grating, and multi-point FBG strain is demodulated at the input end of the FBG broadband light (5) according to the power peak value, so that the three measurement lights can work simultaneously in the same optical fiber, and the two strain measurements do not interfere with each other. The condition for not exciting the Brillouin frequency shift at the position of FBG (2) in step 2 is that the modulation depth of the grating refractive index is less than 0.2% of the strain refractive index coefficient of the optical fiber.

2. A dual-mechanism synchronous measurement device based on the method for dual-mechanism synchronous measurement of distributed strain and FBG strain according to claim 1, characterized in that: The invention comprises a first laser source (6), wherein the first laser source (6) is connected to a first line port of a first coupler (8) through an optical fiber, the two line ports of the first coupler (8) are respectively connected to a first photoelectric modulator (EOM1) and a third photoelectric modulator (EOM3) through optical fibers, the first photoelectric modulator (EOM1) is connected to a first port of a first optical circulator (C1) through an optical fiber, the second port of the first optical circulator (C1) is connected to a second laser source (7) through an optical fiber, the third port of the first optical circulator (C1) is connected to a second photoelectric modulator (EOM2) through an optical fiber, the second photoelectric modulator (EOM2) is connected to a first port of a second optical circulator (C2) through an optical fiber, the second port of the second optical circulator (C2) is connected to an optical fiber winding structure (3) through an optical fiber, the third port of the second optical circulator (C2) is connected to a first photoelectric detector (PD1) through an optical fiber, and the first photoelectric detector (PD1) is used to detect a Brillouin frequency shift signal; The first electro-optical modulator (EOM1) and the second electro-optical modulator (EOM2) are simultaneously connected to a waveform generator (AWG) for generating pulses; The third optoelectronic modulator (EOM3) and the third laser source (11) are connected to two-line ports of the second coupler (9) through optical fibers, one line port of the second coupler (9) is connected to a first port of the third optical circulator (C3) through optical fibers, the second port of the third optical circulator (C3) is connected to the optical fiber winding structure (3) through optical fibers, the third port of the third optical circulator (C3) is connected to a second photodetector (PD2) through optical fibers, and the second photodetector (PD2) is used to detect FBG signals; The third electro-optical modulator (EOM3) is also connected to the arbitrary waveform generator (10) for generating the detection light; The frequency range of the detection light generated by the laser emitted by the first laser source (6) through the third electro-optical modulator (EOM3) is 10.5 GHz to 11.1 GHz; The laser light emitted by the first laser source (6) passes through the first electro-optical modulator (EOM1) and the second electro-optical modulator (EOM2) to generate a pump light pulse with a width range of 1-20 ns; The FBG broadband light emitted by the third laser source (11) has an operating wavelength range of 1460-1620 nm, and the first laser source (6) is an ultra-narrowband laser with an operating wavelength of 1550 nm or 1310 nm; The power of the detection light in the two paths divided by the first laser source (6) is not less than 5% and not more than 25%.

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