Multipoint strain sensing with temperature compensation based on bragg grating and optical fmcw interferometer

Abstract

The application provides a multipoint strain sensor with temperature compensation based on a Bragg grating and an optical FMCW interferometer. The multipoint strain sensor with temperature compensation is characterized in that it is composed of a sweep laser 1, an optical coupler 2, an auxiliary interference device 3, a main interference device 4, a multiplexing device 5, a data acquisition card 6 and a data processing computer 7. The application can be used for real-time monitoring and measurement of multipoint quasi-distributed strain after temperature compensation, and can be widely used in the fields of aviation, ship engineering, bridge measurement and structural health monitoring.

Description

(I)TECHNICAL FIELD

[0001] The present application relates to a multi-point strain sensor with temperature compensation based on Bragg grating and optical FMCW interferometer, which can be used for real-time monitoring and measurement of quasi-distributed strain, and belongs to the field of optical fiber sensing technology. (II)BACKGROUND

[0002] Optical fiber strain sensor has excellent performance such as anti-electromagnetic and radiation, miniaturization, high temperature resistance and corrosion resistance, and can stand out among many sensing technologies, and has great development prospect. Strain measurement plays an important role in the field of aviation, ship engineering, bridge measurement and other structural health monitoring. In recent years, FBG measurement technology has developed rapidly. Among various optical fiber strain sensors, FBG measurement has high sensitivity and fast response time, and its strong multiplexing capability and low cost make it popular in engineering field. However, in actual engineering, the center wavelength of the grating is easily affected by the external environment temperature when FBG is used to measure strain. In order to improve the accuracy of strain measurement, the method of cascading FBG is used to separate temperature and strain, so as to realize strain measurement with temperature compensation. However, FBG can only measure at specified positions, and as the number of FBG multiplexing increases, the spectral range of the light source is required to be wide, and the specific position of strain occurrence cannot be clearly distinguished.

[0003] The application of FMCW interference technology in radar field to optics can solve the above problems. Optical FMCW interference technology, also known as optical frequency domain reflection technology (OFDR), acquires position information on optical fiber link by detecting backscattering Rayleigh scattering, which makes up for the shortcomings of FBG and ensures the measurement accuracy and reliability. In recent years, many researchers have studied strain by combining optical fiber grating and OFDR technology.

[0004] The document "Chan P K C, Jin W, Lau K T, et al. Multi-point strain measurement of composite-bonded concrete materials with a RF-band FMCW multiplexed FBG sensor array [J]. Sensors and Actuators A: physical, 2000, 87(1-2): 19-25" proposes a strain measurement sensor system based on fiber Bragg grating (FBG) and FMCW technology multiplexing, and uses this system to measure the strain force between the surface of the composite steel and the concrete beam. The system uses an erbium-doped fiber broadband light source with a peak wavelength of about 1558 nm, and through the use of a VCO-driven integrated optical modulator, the light intensity is modulated by a triangular swept carrier wave of 58 to 99 MHz at a repetition frequency of 5 kHz. Although the sensor has a temperature dependence of about 10 pm / °C, no temperature compensation is performed, and the system is only a three-sensor system, with fewer strain measurement points.

[0005] The document "Lau K, Chan C, Zhou L, et al. Strain monitoring in composite-strengthened concrete structures using optical fiber sensors [J]. Composites part B: engineering, 2001, 32(1): 33-45." again experiments on the system, expands the FBG to N, but still experiments in a constant temperature environment, and the sensor still does not have temperature compensation function.

[0006] The document "Zhu M, Murayama H, Wada D, et al. Dependence of measurement accuracy on the birefringence of PANDA fiber Bragg gratings in distributed simultaneous strain and temperature sensing [J]. Optics Express, 2017, 25(4): 4000-4017." studies the relationship between the measurement accuracy of a distributed strain and temperature sensor using polarization maintaining fiber Bragg gratings (PANDA_FBGs) and birefringence, verifies the feasibility of combining PANDA_FBGs with OFDR to realize distributed measurement, and improves the measurement accuracy of strain and temperature by increasing birefringence. However, the polarization maintaining fiber Bragg grating in this document is expensive to manufacture.

[0007] The document "Bremer K, Alwis L S M, Weigand F, et al. Evaluating the performance of functionalized carbon structures with integrated optical fiber sensors under practical conditions [J]. Sensors, 2018, 18(11): 3923." proposes a two-FBG sensing system based on OFDR, and successfully monitors the spatial strain distribution of a concrete block through a three-point bending experiment, but this document does not realize temperature compensation function.

[0008] Document CN104296965 discloses an OFDR experimental system, which uses a frequency-sweeping laser source, a polarization controller, an electro-optic modulator, an interference system, a lead sulfide detector, a balanced receiver, and a signal processing system to measure the stress distribution of key components such as bridges with mm-level high spatial resolution, but this document does not realize temperature compensation function.

[0009] Document CN112880715A discloses an optical fiber sensing system based on OFDR technology, which solves the problem of insertion loss of Rayleigh scattering signals from large core diameter to small core diameter in the traditional OFDR sensing system by changing the core diameter size of the transmission fiber and the connection relationship of the mode matcher, but this document does not realize temperature compensation function.

[0010] Document CN111678456A discloses an OFDR device and a measuring method for simultaneously measuring temperature and strain, which compensates for the double-parameter frequency shift amount with the temperature frequency shift amount and combines the frequency shift coefficient to obtain temperature change and strain respectively, but the method in the document has a short sensing distance.

[0011] Document CN113218320A discloses an OFDR large strain measurement method based on distance domain compensation, which can improve the correlation of test signals and reference signals, thereby greatly improving the strain measurement range of the system and realizing large strain measurement of the OFDR system, but the method in the document does not realize temperature compensation.

[0012] Document CN113358240A discloses a temperature and pressure sensor for large-area flexible intelligent skin based on DUS FBG, which includes a temperature-sensitive flexible material and a sensing array encapsulated by the temperature-sensitive flexible material; the sensing array is a two-root DUS and FBG staggered U-shaped array with different center wavelengths. It realizes a high-sensitivity, large-area wearable flexible intelligent skin temperature and pressure sensor, but the method in the document has a high price.

[0013] In summary, in order to realize real-time measurement of temperature and strain double parameters and improve the sensitivity of the sensing system, how to design a multi-point strain sensing system, while realizing high spatial resolution multi-point strain measurement with temperature compensation, making the system have high temperature and strain sensitivity is the key to the problem, and how to choose a suitable process scheme to reduce the cost is also a problem to be solved. (III) SUMMARY

[0014] The purpose of the present application is to provide a novel multi-point strain sensing system based on cascaded Bragg gratings and optical FMCW interference technology with temperature compensation, high spatial resolution and low cost.

[0015] The purpose of the present application is achieved as follows:

[0016] To achieve the above object, the application is implemented by the following technical solutions: a multi-point strain sensing system based on a cascaded Bragg grating and an optical FMCW interference technology, comprising a sweep frequency laser 1, an optical coupler 2, an auxiliary interference device 3, a main interference device 4, a multiplexing device 5, a data acquisition card 6 and a data processing computer 7, two output ends of the sweep frequency laser are connected with the optical coupler 2 and the data processing computer 7 respectively, two ports of the optical coupler 2 are connected with the auxiliary interference device 3 and the main interference device 4 respectively, the main interference device 4 is connected with the multiplexing device 5 through a three-port fiber ring, the output ends of the auxiliary interference device 3 and the main interference device are connected with the input end of the data acquisition card 6, and the output end of the data acquisition card 6 is connected with the input end of the data processing computer 7. In the application, the sweep frequency laser 1 has a sweep frequency range of 1516-1564 nm, a Δf of 6.25 THz, a sweep frequency rate of 40.01 nm / s (5 THz / s) and an output power of 8 mW, and the laser has a line width of 1.5 MHz.

[0017] The auxiliary interference device 3 comprises optical couplers 31-32, a delay optical fiber 33 and a balanced photodetector 34, and the signal transmitted from the optical coupler 2 into the auxiliary interference device 3 is sequentially transmitted into the data acquisition card 6 after passing through the optical couplers 31, the delay optical fiber 33, the optical coupler 32 and the balanced photodetector 34.

[0018] The main interference device 4 comprises optical couplers 41-42, a polarization controller 43, a three-port fiber ring 44 and a balanced photodetector 45, and the signal transmitted from the optical coupler 2 into the main interference device 4 is transmitted into the optical coupler 41, is divided into two paths and is connected with the polarization controller 43 and the three-port fiber ring 44 respectively, and the polarization controller 43 and the three-port fiber ring 44 are combined into one path through the optical coupler 42 and are connected with the balanced photodetector 45.

[0019] The multiplexing device 5 comprises optical couplers 51-1, 51-2, …, 51-M, a total of M, FBG sensors 52-1, 52-2, …, 52-N, a total of N, N is an even number and M=N-1, FBG is in a series relationship in each group of two, and each group is in a parallel relationship.

[0020] The application also discloses a measurement method of the multi-point strain sensing system based on the cascaded Bragg grating and the optical FMCW interference technology.

[0021] (1) Theoretical analysis: the optical signal output by the laser is divided into a reference arm with a polarization controller and a test arm with a three-port ring through the optical coupler, the input end of the multiplexing device is connected with the three-port ring, and the mode coupling between the two counter-propagating beams occurs at the FBG resonance wavelength:

[0022] λ Bragg= 2n eff Λ (1)

[0023] wherein n eff is the effective refractive index of the core, Λ is the spatial period of the FBG, λ Bragg is the Bragg wavelength. The change of the Bragg wavelength is only proportional to the effective refractive index and the period. When the external environment of the grating (such as temperature and strain) changes, the effective refractive index and the period of the grating will change, thereby causing the resonance wavelength of the grating to shift. The corresponding wavelength shift can be expressed as:

[0024]

[0025] wherein Δε represents the axial strain experienced by the optical fiber, ΔT represents the temperature change experienced by the optical fiber, ρ e is the elasto-optic coefficient of the optical fiber, α f and ξ f respectively represent the thermo-optic coefficient and the thermal expansion coefficient of the optical fiber.

[0026] (2) Spatial resolution study: the relationship between the positioning accuracy on the fiber link and other parameters is:

[0027]

[0028] wherein δ a represents the spatial resolution, C represents the speed of light in vacuum, n eff represents the effective refractive index of the core, F s represents the sweep frequency width.

[0029] Preferably, in the present application, the effective refractive index of the core is 1.46, and the present application uses 1000-point moving average filtering to obtain a spatial resolution of 17.12 mm.

[0030] (3) Study on the multiplexing number of Bragg gratings:

[0031]

[0032] wherein N represents the multiplexing number of Bragg gratings, L represents the maximum sensing distance, δ a represents the spatial resolution.

[0033] Preferably, in the present application, the sensing distance is 100 m.

[0034] Preferably, in the present application, the FBG multiplexing number of the multiplexing device can be expanded, the maximum sensing distance is 100 m, the spatial resolution is 17.12 mm, the multiplexing number of Bragg gratings is 5840, and the maximum number of multiplexing groups is 2920.

[0035] (4) Two-parameter characteristic research: in the same sensor channel, two measuring units are influenced by temperature change, and their changing trends are basically consistent. With the increase of temperature, the center wavelength will have red shift. Keeping temperature unchanged, increasing strain, the strain sensing unit FBG52-2, FBG52-4……FBG52-x (x≤5840 and is even) have red shift of center wavelength with the increase of strain, but the temperature sensing unit FBG52-1, FBG52-3……FBG52-y (y≤5840 and is odd) is not sensitive to strain, and the center wavelength is unchanged.

[0036] Preferably, in the present application, the temperature change ΔT of the multiplexing sensor is demodulated as i (i=1, 2, 3……) and strain change ΔS i (i=1, 2, 3……) of the sensing matrix are written as:

[0037]

[0038] Where S ε_ε,i is the sensitivity of the strain sensing unit in the i-th sensor to strain, s T_T,i is the sensitivity of the temperature sensing unit in the i-th sensor to temperature, S ε_T,i is the sensitivity of the strain sensing unit to temperature, Δλ ε,i is the total center wavelength shift of the strain sensing unit, Δλ T,i is the total center wavelength shift of the temperature sensing unit affected by temperature.

[0039] Preferably, in the present application, the temperature measurement range is 0-200℃, and the strain measurement range is 0-15000με.

[0040] Preferably, in the present application, the strain measurement sensitivity after temperature compensation is 18με. (Four) Description of the drawings

[0041] Figure 1 is the system structure diagram of the present application. It is composed of a sweep laser 1, an optical coupler 2, an auxiliary interference device 3, a main interference device 4, a multiplexing device 5, a data acquisition card 6 and a data processing computer 7;

[0042] Figure 2 is the auxiliary interference device structure diagram of the present application. It is composed of optical couplers 31-32, a delay optical fiber 33 and a balanced photoelectric detector 34;

[0043] Figure 3 is the main interference device structure diagram of the present application. It is composed of optical couplers 41-42, a polarization controller 43, a three-port optical fiber ring 44 and a balanced photoelectric detector 45;

[0044] Figure 4 The structure diagram of the multiplexing device of the application is shown in the figure. Figure 4 The three-port fiber circulator 53 in the middle is the same as the three-port fiber circulator 44. Figure 3 The three-port fiber circulator 44 in the middle is the same as the three-port fiber circulator 53. (V) Specific embodiments

[0045] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application, and the described embodiments are only some of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative efforts belong to the protection scope of the application.

[0046] The embodiments of the application provide a novel multi-point strain sensing system based on a cascaded Bragg grating and an optical FMCW interference technology with temperature compensation.

[0047] The sweep laser 1 adopts a sweep laser, which provides a continuous sweep signal for the system, and the signal is transmitted into the auxiliary interference device 3 and the main interference device 4 through the optical coupler 2, and at the same time, a trigger signal for starting data acquisition is provided to the data acquisition card 6 through the data processing computer 7, the optical signal in the main interference device is reflected back after being transmitted into the multiplexing device 5, and then the signals of the auxiliary interference device 3 and the main interference device 4 are acquired by the data acquisition card 6 and transmitted into the data processing computer 7 for processing.

[0048] In the application, the sweep laser 1 has a sweep range of 1515-1565 nm, a sweep rate of 40.01 nm / s (5 THz / s), and an output power of 8 mW, and the laser line width is 1.5 MHz.

[0049] In the application, the sampling bandwidth is 125 MHz, the sampling frequency is 4.55 MS / s, and the corresponding sampling time interval is 20 ms.

[0050] The signal transmitted into the auxiliary interference device 3 through the coupler 2 of the auxiliary interference device 3 is sequentially transmitted into the data acquisition card 6 through the optical coupler 31, the delay optical fiber 33, the optical coupler 32 and the balanced photoelectric detector 34, and the splitting ratio of the optical couplers 31-32 is 50:50.

[0051] In the application, the splitting ratio of the optical coupler 2 is 10:90, the splitting ratio of the optical couplers 31-32 is 50:50, and the acquisition range of the balanced photoelectric detector 34 is 1200-1700 nm, and the bandwidth is 400 MHz.

[0052] The signal transmitted into the main interference device 4 is transmitted into the optical coupler 41 of the main interference device 4, is divided into two paths, and then is connected to the polarization controller 43 and the three-port fiber ring 44 respectively, the polarization controller 43 and the three-port fiber ring 44 are combined into one path through the optical coupler 42, and then are connected to the balanced photoelectric detector 45.

[0053] The collection range of the balanced photoelectric detector 45 in the application is 1200-1700nm, the bandwidth is 400MHz, the light splitting ratio of the optical coupler 41 is 1:99, and the light splitting ratio of the optical coupler 42 is 50:50.

[0054] The light splitting ratio of the optical couplers 51-1, 51-2 and 51-M in the application is 50:50.

[0055] Embodiment: The tail end of an optical fiber with a total length of 16m is processed, and one Bragg grating is placed at 5.2112m, 5.7221m, 10.0136m, 10.5245m, 12.3638m and 13.0752m respectively, each Bragg grating is a measurement unit, and adjacent two measurement units form a sensing channel. Each group of sensors is placed in a constant temperature tank, and the sensitivity of two measurement units to temperature is measured at 10℃ steps from 30℃ to 90℃, and the sensitivities of six measurement units to temperature are 10.28pm / ℃, 10.12pm / ℃, 9.88pm / ℃, 10.36pm / ℃, 10.21pm / ℃ and 10.23pm / ℃ respectively, and the average sensitivity is 10.18pm / ℃.

[0056] In the application, the strain is measured, the environmental temperature is kept unchanged at 20℃, and the strain interval of the measurement unit is 1000με, and the strain amount is from 0με to 7000με. As the strain sensing unit, the sensitivities of FBG2, FBG4 and FBG6 are 1.163pm / με, 1.001pm / με and 1.076pm / με respectively, and the average sensitivity is 1.08pm / με, and the temperature sensing unit FBG1, FBG3 and FBG5 are not sensitive to strain.

[0057] The application carries out 20 repeated experiments under the condition that the temperature is 20℃ and the strain is 5000με, demodulates the data of each strain measurement, and the root mean square of 20 strain measurements is less than 17.97pm, which is equivalent to a shift of 18με, so it can be determined that the sensitivity of the strain measurement is 18με. The stability and accuracy of the multi-point strain measurement system built are very high.

Claims

1. A multipoint strain sensing system with temperature compensation based on Bragg grating and optical FMCW interferometer, comprising a frequency-swept laser (1), a first optical coupler (2), an auxiliary interference device (3), a main interference device (4), a multiplexing device (5), a data acquisition card (6) and a data processing computer (7), characterized in that: The two output ends of the sweep laser (1) are connected with a first optical coupler (2) and a data processing computer (7) respectively, two ports of the first optical coupler (2) are connected with an auxiliary interference device (3) and a main interference device (4) respectively, the main interference device (4) is connected with a multiplexing device (5) through a three-port fiber ring, the output ends of the auxiliary interference device (3) and the main interference device (4) are connected with the input end of a data acquisition card (6), the output end of the data acquisition card (6) is connected with the input end of the data processing computer (7), the sweep range of the sweep laser (1) is 1515-1565 nm, Δf=6.25THz, the sweep rate is 40.01 nm / s, the output power is 8 mW, and the laser linewidth is 1.5 MHz; the multiplexing device comprises M optical couplers (51-1, 51-2, …, 51-M), N FBG sensors (52-1, 52-2, …, 52-N), N is even and M=N-1, FBG is in series in each group, and N / 2 groups are in parallel.

2. The multi-point strain sensing system with temperature compensation based on Bragg grating and optical FMCW interferometer according to claim 1, characterized in that: The main interference device comprises a second optical coupler (41), a third optical coupler (42), a polarization controller (43), a three-port fiber ring (44) and a balanced photodetector (45), the signal transmitted from the first optical coupler (2) into the main interference device (4) is transmitted into the second optical coupler (41), is divided into two paths and is connected with the polarization controller (43) and the three-port fiber ring (44) respectively, the polarization controller (43) and the three-port fiber ring (44) are combined into one path through the third optical coupler (42) and are connected with the balanced photodetector (45); the light splitting ratio of the second optical coupler (41) is 1:99, and the light splitting ratio of the third optical coupler (42) is 50:

50.

3. The multi-point strain sensing system with temperature compensation based on Bragg grating and optical FMCW interferometer according to claim 1, characterized in that: The strain measurement has a temperature compensation function, and the sensitivity after compensation is 18 με.

4. The multi-point strain sensing system with temperature compensation based on Bragg grating and optical FMCW interferometer according to claim 1, characterized in that: The auxiliary interference device comprises a fourth optical coupler (31), a delay fiber (33), a fifth optical coupler (32) and a balanced photodetector (34); the signal transmitted into the auxiliary interference device (3) is sequentially transmitted through the fourth optical coupler (31), the delay fiber (33), the fifth optical coupler (32) and the balanced photodetector (34) and then is transmitted into the data acquisition card (6).

5. The multi-point strain sensing system with temperature compensation based on Bragg grating and optical FMCW interferometer according to claim 1, characterized in that: The maximum sensing distance is 100 m, and the maximum spatial resolution is 17.12 mm.

6. The multi-point strain sensing system with temperature compensation based on Bragg grating and optical FMCW interferometer according to claim 1, characterized in that: The temperature measurement range is 0-200℃, the strain measurement range is 0-15000 με, and simultaneous temperature and strain measurement is realized. The temperature measurement range is 0-200℃, the strain measurement range is 0-15000 με, and simultaneous temperature and strain measurement is realized.

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