FBG and F-P cavity multiplexing air pressure sensing system

Abstract

The invention discloses an FBG (Fiber Bragg Grating) and F-P (Fabry-Perot) cavity multiplexing air pressure sensing system, which comprises a linear tunable laser 1, a Mach-Zehnder interference device 2, a sensing array 3 and a processing unit 4, and is characterized in that an output optical coupler of the linear tunable laser 1 is connected with the Mach-Zehnder interference device 2; the linear tunable laser 1 and the Mach-Zehnder interference device 2 are directly connected with the processing unit 4, and the sensing array 3 is an FBG and F-P cavity sensor multiplexing array and is connected with the Mach-Zehnder interference device 2; according to the invention, multiplexing and distance positioning of the FBG and the F-P cavity sensor are realized, temperature compensation is carried out on pressure measurement while the spatial resolution can reach 20 [mu] m for temperature measurement, and the FBG-F-P cavity sensor can be widely applied to the fields of aviation, precision manufacturing and the like.

Description

(I)TECHNICAL FIELD

[0001] The application belongs to the technical field of optical fiber sensing, and is a FBG and F-P cavity multiplexing air pressure sensing system. (II)BACKGROUND

[0002] From the mercury air pressure gauge to the electronic air pressure gauge, the measurement accuracy and sensitivity of air pressure have been significantly improved. However, electronic air pressure gauges are difficult to exclude electromagnetic interference, so optical fiber sensing technology based on optics is attracting attention because of its excellent sensing performance and is gradually increasing in demand. Among them, the Fabry-Perot (F-P) cavity sensor changes the cavity length by making a thin film to perform air pressure sensing, and has the advantages of anti-electromagnetic interference, small size, high sensitivity, high multiplexing capacity, etc. However, the sensitivity of the F-P cavity sensor is mainly determined by the thickness and radius of the thin film, and it is more difficult to reduce the thickness. At the same time, organic materials are often susceptible to temperature, so temperature compensation is also needed. Fiber Bragg Grating (FBG) is not sensitive to air pressure without special treatment, and is sensitive to temperature, so it is very suitable for temperature compensation, but the multiplexing of FBG and F-P also requires corresponding demodulation methods, and the FBG and F-P cavity multiplexing technology also faces problems.

[0003] Conventional multiplexing technology usually multiplexes multiple sensors in the time domain, and then separates them by fast Fourier transform, which requires a high signal-to-noise ratio, and current instruments cannot meet this demand, so the cost is high, while using Optical Frequency Domain Reflectometry (OFDR) can detect under lower signal-to-noise ratio. Because it acts on the frequency domain and the frequency is one-to-one corresponding to the distance, the wavelength signals in the time domain can be independently extracted according to the distance, thereby realizing serial multiplexing. However, using OFDR technology for multiplexing still needs to deal with the phase noise problem caused by the laser, and the demodulation methods of FBG and F-P are different, so nonlinear compensation algorithms and specific demodulation methods are needed.

[0004] Patents CN114046927B, CN113701937B, CN112161738B, etc. are electronic air pressure sensors, which are difficult to exclude the influence of electronic interference, so it is difficult to ensure accuracy in the face of strong electric field or strong magnetic field.

[0005] Patents CN115046674A, CN105606277A, CN106323516B disclose F-P cavity air pressure sensors that achieve high sensitivity, but there is still room for improvement. And if more accurate data is needed, temperature compensation is still needed.

[0006] In summary, while thin organic films were used to improve sensor sensitivity, this resulted in temperature affecting the accuracy of pressure sensing. Therefore, pressure-insensitive sensors need to be connected in series with temperature compensation. However, current demodulation methods still require addressing the aliasing problem of multiple sensors and suffer from low signal-to-noise ratios. Therefore, exploring a high-sensitivity FP pressure sensor and a complete system capable of connecting FBG and FP in series has become an urgent problem to be solved. (III) Summary of the Invention

[0007] To address the aforementioned problems, this invention provides an FBG and FP cavity multiplexing pressure sensing system.

[0008] To achieve a high-sensitivity FP cavity pressure sensing system that can be multiplexed in the frequency domain, has high spatial resolution, and can achieve temperature compensation through FBG, the present invention achieves this through the following technical solution:

[0009] The FBG and FP cavity multiplexed pneumatic sensing system includes a linearly tunable laser 1, a Mach-Zehnder interferometer 2, a sensor array 3, and a processing unit 4. The output optical coupler of the linearly tunable laser 1 is connected to the Mach-Zehnder interferometer 2. Both the linearly tunable laser 1 and the Mach-Zehnder interferometer 2 are directly connected to the processing unit 4. The sensor array 3 is an FBG and FP cavity sensor multiplexed array and is connected to the Mach-Zehnder interferometer 2.

[0010] The linearly tunable laser 1 is configured with an output light linewidth of 100 kHz, a sweep frequency width ΔF = 5 THz, a sweep rate of 40 nm / s (5 THz / s), a wavelength range of 1530 nm - 1570 nm, an output power of 10 mW, and a corresponding sweep time of 1 s.

[0011] The processing unit 4 is configured with a sampling rate of 2MS / s, a sampling number of 200,000, and a corresponding sampling time of 1s.

[0012] The function of the Mach-Zehnder interferometer 2 is to generate a beat frequency optical signal carrying position information through interference, and to convert the beat frequency optical signal into an electrical signal. The Mach-Zehnder interferometer 2 includes an auxiliary interferometer 21 and a main interferometer 22, connected to the linearly tunable laser 1 via an optical coupler. The auxiliary interferometer 21 is mainly used to generate an auxiliary interference signal, including an optical coupler, a delay fiber, and a photodetector. The auxiliary interference signal is mainly used to analyze the instantaneous optical frequency of the linearly tunable laser 1, and resampling can be performed using this instantaneous optical frequency to improve the spatial resolution of the system. The main interferometer 22 is mainly used to connect to the sensing array 3 and generate sensing signals, including an optical coupler, a delay fiber, a three-port fiber circulator, and a balanced photodetector. The sensing signal is the main interference signal generated by the interference of the optical signals reflected from the FBG and FP cavity after passing through the optical circulator. The main interference signal contains temperature information carried by the FBG and pressure information carried by the FP cavity.

[0013] The sensor array 3 is mainly used to generate back-propagating sensing optical signals carrying temperature and pressure information and transmit them to the Mach-Zehnder interferometer. The sensor array 3 includes N groups of FBG and FP cavity sensor groups 3-1, 3-2...3-N. Adjacent groups of FBG and FP cavity sensor groups are connected in parallel through M optical couplers with a 50:50 splitting ratio, where M=N-1. Each FBG and FP cavity sensor group includes one FBG and one FP cavity sensor, which are connected in series.

[0014] The center wavelength of the FBG must be within the wavelength range of the linearly tunable laser 1, and because of frequency domain multiplexing, the center wavelengths of the FBG can be the same.

[0015] Preferably, the FP cavity sensor adopts a thin-film structure with a film thickness of 10-15 μm. The fabrication process is as follows: a tapered optical fiber is fabricated using a fiber optic fusion splicer; the tapered optical fiber is then cut at the center of the tapered section using a length cutter to obtain a liquid-filled optical fiber. A small amount of UV adhesive is applied to the liquid-filled optical fiber, and the FP cavity and the liquid-filled optical fiber are aligned using the fiber optic fusion splicer. After rapid contact and separation, the newly obtained FP cavity sensor is cured with ultraviolet light to obtain the desired FP cavity pressure sensor.

[0016] Preferably, in this invention, the temperature sensitivity of the FBG sensor is about 10 pm / ℃, and the pressure sensitivity of the FP cavity sensor after compensation is 68.32~125.65 pm / Mpa.

[0017] The processing unit 4 is a personal computer equipped with a data acquisition card. It acquires the electrical signals using the data acquisition card, generates data, and then processes the data using software. The processing includes: obtaining the instantaneous photofrequency of the linearly tunable laser 1 based on the acquired auxiliary interference signal and performing phase unwrapping; and obtaining the phase-noise-removed main interference signal by resampling the main interference signal based on the instantaneous photofrequency and performing cubic spline interpolation.

[0018] Furthermore, in a system employing OFDR technology, the position information of the sensors can be correlated one-to-one with their frequencies, allowing the acquisition of each sensor's frequency once its position information is determined. Based on the phase-noise-removed main interference signal and the frequencies of each sensor, filtering can demodulate the sensing information of the FBG and FP cavity sensors, respectively.

[0019] Furthermore, since the structure is front FBG and rear FP, it is necessary to use one-dimensional interpolation to restore the acquired FP cavity sensor signal.

[0020] Preferably, the sweep frequency range used in this invention is 5THz, the refractive index is 1.46, the maximum sensing distance after using the nonlinear compensation algorithm is 41.6m, the maximum spatial resolution is 20μm, and the maximum number of multiplexed FP cavity sensors is 50,000.

[0021] The beneficial effects of this invention are: it realizes the multiplexing of FBG and FP cavity sensors in the frequency domain and distance domain, and solves the spectral aliasing problem faced by FBG and FP cavity sensors in the wavelength domain; it uses a nonlinear compensation algorithm to enable the sensing distance to exceed the length of the auxiliary interferometer, and the spatial resolution reaches 20 micrometers; the multiplexed sensor array can simultaneously sense temperature and pressure under large-scale and high-precision conditions, with significant advantages in pressure sensitivity and temperature compensation function. (iv) Description of the attached drawings

[0022] Figure 1 This is a system structure diagram of the present invention. It consists of a linearly tunable laser 1, a Mach-Zehnder interferometer 2, a sensor array 3, and a processing unit 4; wherein the Mach-Zehnder interferometer 2 includes an auxiliary interferometer 21 and a main interferometer 22.

[0023] Figure 2 This is the range domain diagram of the master interferometer after resampling according to the present invention.

[0024] Figure 3 The images show the demodulated FBG signal and the FP cavity sensor signal after the present invention. (V) Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. The described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] This invention provides a novel temperature-compensated pressure sensing system based on OFDR technology using FBG and FP cavity sensors with high spatial resolution.

[0027] The linearly tunable laser 1 uses swept-frequency linearly tunable light to provide a continuous swept-frequency signal to the system. The signal passes through the Mach-Zehnder interferometer 2 and the sensor array 3, and at the same time provides a trigger signal to the processing unit 4 to start data acquisition. The signal from the sensor array 3 is reflected back and generates an interference signal in the Mach-Zehnder interferometer 2. The interference signal is then acquired and processed by the data acquisition unit 6.

[0028] Example: The tail end of a 16.7m long optical fiber was processed, and FBGs were deployed at 7.79923m, 10.1719m, and 13.064m respectively. Individual FP cavity sensors were deployed at 8.56112m, 11.0614m, and 13.7852m, with cavity lengths of 139μm, 142μm, and 148μm respectively.

[0029] Tapered optical fibers are fabricated using an optical fiber fusion splicer. These tapered fibers are then cut at the center of the tapered section using a length cutter to obtain a liquid-filled optical fiber. A small amount of UV adhesive is applied to the liquid-filled optical fiber, and the FP cavity and the liquid-filled optical fiber are aligned using the optical fiber fusion splicer. After rapid contact, they are separated, and the newly obtained FP cavity sensor is cured with UV light. The resulting film thicknesses are 14.7 μm, 12.8 μm, and 10.2 μm, respectively.

[0030] The sensing system was placed in a constant temperature bath, with temperatures ranging from 25℃ to 50℃ in 5℃ increments. The temperature sensitivities of the three FP cavity sensors were measured to be 2.45 pm / ℃, 2.87 pm / ℃, and 2.62 pm / ℃, respectively, with an average sensitivity of 2.65 pm / ℃. The temperature sensitivities of the FBG sensor were 10.01 pm / ℃, 10.02 pm / ℃, and 9.97 pm / ℃, respectively, with an average sensitivity of 10 pm / ℃.

[0031] In this invention, pressure is measured while maintaining an ambient temperature of 30°C. A pressure change from 0 MPa to 0.05 MPa with a strain interval of 0.01 MPa is applied to the measuring unit. As strain sensing units, the sensitivities of the FP cavity sensor are 68.32 nm / MPa, 98.52 nm / MPa, and 125.65 nm / MPa, respectively, while the FBG is insensitive to pressure.

[0032] This invention conducted 20 repeated experiments at a temperature of 30℃ and a pressure of 0.05 MPa. The data from each pressure measurement were demodulated, and the root mean square (RMS) of the 20 pressure measurements was less than 319.65 pm, equivalent to an offset of 0.001 MPa. Therefore, the sensitivity of the strain measurement can be determined to be 0.001 MPa. The stability and accuracy of the constructed pressure measurement system are very high.

Claims

1. A FBG and F-P cavity multiplexed air pressure sensing system, characterized in that: The linear tunable laser, the Mach-Zehnder interference device, the sensing array and the processing unit are connected, the linear tunable laser is connected with the Mach-Zehnder interference device through an optical coupler, the linear tunable laser and the Mach-Zehnder interference device are directly connected with the processing unit, and the sensing array comprises FBG and F-P cavity sensors and is connected with the Mach-Zehnder interference device. The linear tunable laser 1 is configured to output light with a line width of 100 kHz, a frequency width of a sweep frequency of 5 THz, a sweep rate of 40 nm / s (5 THz / s), a wavelength range of 1530-1570 nm, an output power of 10 mW, and a corresponding sweep time of 1 s; and the processing unit 4 is configured to have a sampling rate of 2 MS / s, a sampling point number of 200,000 and a corresponding sampling time of 1 s. 2.The FBG and F-P cavity multiplexing air pressure sensing system of claim 1, wherein: The Mach-Zehnder interference device comprises an auxiliary interference device and a main interference device; the main interference device comprises an optical coupler, a polarization controller, a three-port fiber ring and a balanced photodetector; the function of the main interference device is to generate a main interference signal by mutual interference of back light of the sensing array and light passing through the polarization controller through the three-port fiber ring and convert the main interference signal into an electrical signal by the balanced photodetector for collection by the processing unit; and the auxiliary interference device comprises an optical coupler, a delay fiber and a photodetector; the function of the auxiliary interference device is to generate an auxiliary interference signal by the delay fiber and convert the auxiliary interference signal into an electrical signal by the photodetector for collection by the processing unit for resampling of the main interference signal.

3. The resampling method of claim 2, wherein: The instantaneous optical frequency of the linear tunable light source is obtained by phase unwrapping operation based on the auxiliary interference device signal; the main interference signal without phase noise is generated after three spline interpolations of the instantaneous optical frequency; and the spatial resolution of the main interference signal without phase noise is up to 20 μm according to the configuration of the linear tunable laser 1.

4. The F-P cavity sensing system based on OFDR technology according to claim 1, characterized in that: The sensing array is composed of FBG with a central wavelength of 1540-1560 nm and F-P cavity sensors made by a special process, and the FBG and the F-P cavity sensors are connected by the optical coupler; the FBG is configured to have a temperature sensitivity of 10 pm / ℃; and the F-P cavity sensor is configured to have a structure with a film thickness of 3-5 um and a cavity length of 135-151 μm, and a pressure sensitivity of 282.32-362.65 pm / Mpa.

5. The process as claimed in claim 4, wherein: A point liquid fiber is obtained by cutting a drawn tapered fiber at the center of the drawn tapered fiber through a fixed-length cutter; The F-P cavity and the point liquid fiber are aligned by the optical fiber fusion splicer, quickly contacted, separated and ultraviolet cured to obtain the required F-P cavity pressure sensor.

Images