Sapphire fiber Fabry-Perot temperature sensing system and demodulation method based on microwave photonic interference optical fiber loop and constructed virtual reflecting surface

Abstract

The present invention discloses a sapphire fiber Fabry-Perot temperature sensing system and a demodulation method based on a microwave photonic interference optical fiber loop and constructing a virtual reflecting surface. The system includes a light source, an electro-optic modulator, a vector network analyzer, a circulator, a first coupler, a second coupler, a photoelectric conversion module, and an optical fiber sensor. The method includes the following steps: The port 23 of the second coupler receives and couples the reflected light of the k-th iteration to obtain the coupled reflected light of the k-th iteration, and divides the coupled reflected light of the k-th iteration into two parts. One part of the coupled reflected light of the k-th iteration is output through the port 21 to the photoelectric conversion module. The other part of the coupled reflected light serves as the input optical signal of the (k + 1)-th iteration and is output through the port 22 to the port 12 of the first coupler, so that the optical signal circulates in the loop formed by the circulator, the first coupler, and the second coupler. The present invention can increase the sensitivity by more than 2 times, and the sensitivity amplification factor is greater than the number of cycles of the optical fiber loop.

Description

Technical Field

[0001] The present invention relates to the field of sensors, in particular to a sapphire optical fiber Fabry-Perot temperature sensing system and a demodulation method based on a microwave photon interference optical fiber loop and a constructed virtual reflection surface. Background Art

[0002] Sapphire fiber Fabry-Perot interferometer sensor has the advantages of strong anti-electromagnetic interference ability, high signal quality and fast response speed. Due to the high melting point of sapphire fiber (2040℃), it can be used for ultra-high temperature environment temperature measurement. Typical sapphire fiber Fabry-Perot interferometer sensing systems are as follows: Figure 1 shown.

[0003] However, in the actual measurement process, the error caused by multimode interference caused by the large mode field diameter of sapphire optical fiber can be dozens of times the change in actual environmental parameters such as temperature, strain and pressure. In order to solve this problem, domestic and foreign experts and scholars have conducted extensive research on the correction of this error from the perspective of system structure.

[0004] To eliminate the problem of intermodal interference within sapphire optical fibers, a common approach is to shift the modulation and demodulation frequencies from the high-frequency optical interference field to the low-frequency microwave photon interference field. This involves using light waves as carrier waves and microwaves as modulating signals, both of which enter the sensor. When the sensor is affected by changes in the measured quantity, the interference signal reflected back from the sensor contains both light and microwaves. By demodulating only the microwave signal as it changes with the measured quantity, the intermodal interference of the light waves is avoided, reducing measurement errors and enabling accurate acquisition of environmental parameters such as temperature, strain, and pressure. Based on this principle, Clemson University in the United States, the University of Electronic Science and Technology of my country, and the Zhijiang Laboratory have conducted research on sapphire fiber Fabry-Perot sensing systems based on microwave photon interferometry.

[0005] In 2015, HUANG.J et al. reported a sapphire fiber microwave Michelson high temperature sensor based on microwave modulation, such as Figure 2 (a)- Figure 2 As shown in Figure (d), two sapphire optical fibers (85 cm and 70 cm, respectively) and two quartz multimode optical fibers are fused together using an arc method. This splits the optical path into two, each reflected from the sapphire fiber end face. Due to the optical path difference between the two light paths, the reflected light interferes with each other at the coupler, enabling temperature measurement from 100°C to 1400°C with a sensitivity of 64kHz / °C.

[0006] Since the wavelength of microwaves is relatively longer than that of light waves, the optical path difference caused by the measured change is very small relative to the microwave signal, which in turn leads to a very low sensitivity of the microwave photon Fabry-Perot sensor (only -64 kHz / °C), while the microwave frequency of the system is in the GHz range. To improve the sensitivity of the sensor, there are reports in the literature on methods to amplify the sensor sensitivity using the Vernier effect. And when the thermal expansion coefficient, thermo-optic coefficient, and refractive index of the fiber optic sensor itself are all immutable, the sensitivity can also be improved by increasing the length of the sensor.

[0007] ZUOWEI.XU et al. from Huazhong University of Science and Technology used a single-mode optical fiber with a length of 200 m as a temperature sensor, as Figure 3 (a)- Figure 3 (d) shows. Another single-mode optical fiber with a slightly different length was used as a reference section, and the two were cascaded. Using the Vernier effect, the sensitivity of the temperature sensor was increased from -19.068 kHz / °C to -556.856 kHz / °C.

[0008] As Figure 4 (a)- Figure 4 (d) shows, Chen.Z et al. from Zhejiang Lab used a 7.568 m single-mode optical fiber as a reference Fabry-Perot interferometer and another 7.854 m single-mode optical fiber as a temperature-sensing Fabry-Perot interferometer. The two Fabry-Perot interferometers were cascaded to form a microwave photon Vernier effect, and the temperature sensitivity of the temperature-sensing Fabry-Perot interferometer was increased to -266.1 kHz / °C in the temperature measurement range of 20 - 80 °C.

[0009] In 2023, Chen.S et al. from Xiamen University used an 11.43 m single-mode optical fiber as a reference arm and another 21.57 m single-mode optical fiber as a temperature-sensing arm. The two Fabry-Perot interferometers were connected in parallel to form a microwave photon Vernier effect, and the temperature sensitivity of the sensing arm was increased from -31.18 kHz / °C to 580.45 kHz / °C, as Figure 5 (a)- Figure 5 (d) shows.

[0010] However, there are currently problems with fiber optic Fabry-Perot sensing systems based on microwave photon interference:

[0011] Low sensor sensitivity. Since the wavelength of microwaves is relatively longer than that of light waves, the optical path difference caused by the measured change is very small relative to the microwave signal, which in turn leads to a very low sensor sensitivity. For example, the sensitivity of the Michelson interferometer-type sensor at Clemson University in the United States is only -64 kHz / °C, while the microwave frequency range of the system is in the GHz range.

[0012] The sensor has a large size. To make the change in optical path difference caused by the change in the measured temperature large enough, many research teams use a sensor with a sensitive element of decimeter order in length. For example, the Mach-Zehnder interferometer type sensor of Huazhong University of Science and Technology in China has a size of 200 m. Summary of the Invention

[0013] The object of the present invention is to provide a sapphire fiber Fabry-Perot temperature sensing system based on a microwave photonic interference fiber loop and a constructed virtual reflecting surface, including a light source, an electro-optic modulator, a vector network analyzer, a circulator, a first coupler, a second coupler, a photoelectric conversion module, and a fiber optic sensor;

[0014] The light source sends an optical signal to the electro-optic modulator;

[0015] The vector network analyzer sends a microwave scanning frequency signal to the electro-optic modulator;

[0016] The electro-optic modulator modulates the optical signal and the microwave scanning frequency signal to obtain an initial modulated optical signal, and transmits it to the first coupler;

[0017] The first coupler is provided with at least three input / output ports, which are respectively denoted as port 11, port 12, and port 13;

[0018] The 11th port of the first coupler receives and couples the initial modulated optical signal to obtain an initial coupled optical signal, and transmits the initial coupled optical signal to the circulator through the 13th port;

[0019] The 12th port of the first coupler receives and couples the input optical signal of the kth iteration to obtain the coupled optical signal of the kth iteration, and transmits the coupled optical signal of the kth iteration to the circulator through the 13th port; the initial value of k is 2; k = 2,..., K; K is the total number of iterations;

[0020] The circulator transmits the initial coupled optical signal to the fiber optic sensor, then receives the initial reflected light from the fiber optic sensor, and transmits the initial reflected light to the second coupler;

[0021] The circulator transmits the coupled optical signal of the kth iteration to the fiber optic sensor, then receives the reflected light of the kth iteration from the fiber optic sensor, and transmits the reflected light of the kth iteration of the fiber optic sensor to the second coupler;

[0022] The second coupler is provided with at least three input / output ports, which are respectively denoted as port 21, port 22, and port 23;

[0023] The port 23 of the second coupler receives and couples the initial reflected light to obtain the initial coupled reflected light, and divides the initial coupled reflected light into two parts; one part of the coupled reflected light is output through the port 21 to the photoelectric conversion module; the other part of the coupled reflected light is used as the input optical signal for the k-th iteration and is output through the port 22 to the port 12 of the first coupler;

[0024] The port 23 of the second coupler receives and couples the reflected light of the k-th iteration to obtain the coupled reflected light of the k-th iteration, and divides the coupled reflected light of the k-th iteration into two parts; one part of the coupled reflected light of the k-th iteration is output through the port 21 to the photoelectric conversion module; the other part of the coupled reflected light is used as the input optical signal for the (k + 1)-th iteration and is output through the port 22 to the port 12 of the first coupler;

[0025] The photoelectric conversion module performs photoelectric conversion on the received coupled reflected light to obtain the reflected light time-domain signal and transmits it to the vector network analyzer;

[0026] During the operation of the sapphire fiber Fabry - Perot system, each time the optical signal circulates and transmits more once in the loop composed of the circulator, the first coupler, and the second coupler, the temperature sensitivity of the fiber optic sensor doubles; the sensor temperature sensitivity S corresponding to the reflected light time-domain signal of the k-th iteration vir,k = kS0; S0 is the original temperature sensitivity of the fiber optic sensor;

[0027] After the vector network analyzer receives the time-domain reflection peak signal of the far-end reflecting surface of the fiber optic sensor, it constructs a virtual reflecting surface to form a virtual Fabry - Perot cavity with the far-end reflecting surface of the fiber optic sensor, and the distance between the virtual reflecting surface and the far-end reflecting surface of the fiber optic sensor is the Fabry - Perot cavity length L vir ;

[0028] The time-domain signal of the virtual Fabry - Perot cavity is subjected to time-frequency domain conversion using complex Fourier transform, so that the time-domain signals of the two reflecting surfaces of the virtual Fabry - Perot cavity interfere in the microwave frequency domain to generate an interference spectrum signal. At this time, the temperature sensitivity S2 of the fiber optic sensor = k2S0; the parameter k2 is greater than k;

[0029] Furthermore, it also includes an erbium-doped fiber amplifier for amplifying the modulated optical signal, a radio frequency amplifier for amplifying the microwave scanning frequency signal, and a polarizer for polarizing the optical signal.

[0030] The demodulation method of the sapphire fiber Fabry - Perot temperature sensing system based on the microwave photon interference fiber loop and constructing a virtual reflecting surface includes the following steps:

[0031] 1) The light source sends an optical signal to the electro-optic modulator;

[0032] The vector network analyzer sends a microwave sweep frequency signal to the electro-optic modulator;

[0033] 2) The electro-optic modulator modulates the optical signal and the microwave sweep frequency signal to obtain an initial modulated optical signal, and transmits it to the first coupler;

[0034] 3) The port 11 of the first coupler receives and couples the initial modulated optical signal to obtain an initial coupled optical signal, and transmits the initial coupled optical signal to the circulator through port 13;

[0035] 4) The circulator transmits the initial coupled optical signal to the fiber optic sensor, then receives the initial reflected light from the fiber optic sensor, and transmits the initial reflected light to the second coupler;

[0036] 5) The port 23 of the second coupler receives and couples the initial reflected light to obtain an initial coupled reflected light, and divides the initial coupled reflected light into two parts; one part of the coupled reflected light is output to the photoelectric conversion module through port 21; the other part of the coupled reflected light serves as the input optical signal for the (k + 1)-th iteration and is output to port 12 of the first coupler through port 22;

[0037] 6) The photoelectric conversion module performs photoelectric conversion on the received coupled reflected light to obtain a reflected light time-domain signal, and transmits it to the vector network analyzer;

[0038] 7) The port 12 of the first coupler receives and couples the input optical signal for the k-th iteration to obtain the coupled optical signal for the k-th iteration, and transmits the coupled optical signal for the k-th iteration to the circulator through port 13; the initial value of k is 2; k = 2,..., K; K is the total number of iterations;

[0039] 8) The circulator transmits the coupled optical signal for the k-th iteration to the fiber optic sensor, then receives the reflected light for the k-th iteration from the fiber optic sensor, and transmits the reflected light for the k-th iteration to the second coupler;

[0040] 9) The port 23 of the second coupler receives and couples the reflected light for the k-th iteration to obtain the coupled reflected light for the k-th iteration, and divides the coupled reflected light for the k-th iteration into two parts; one part of the coupled reflected light for the k-th iteration is output to the photoelectric conversion module through port 21; the other part of the coupled reflected light serves as the input optical signal for the (k + 1)-th iteration and is output to port 12 of the first coupler through port 22;

[0041] 10) The photoelectric conversion module performs photoelectric conversion on the received coupled reflected light to obtain a reflected light time-domain signal, and transmits it to the vector network analyzer;

[0042] During the operation of the sapphire fiber Fabry-Perot system, each time the optical signal makes an additional round-trip transmission in the loop formed by the circulator, the first coupler, and the second coupler, the temperature sensitivity of the fiber optic sensor doubles; the temperature sensitivity S of the sensor corresponding to the reflected optical time-domain signal of the k-th iteration vir,k = kS0; S0 is the original temperature sensitivity of the fiber optic sensor;

[0043] After the vector network analyzer receives the time-domain reflection peak signal of the distal reflecting surface of the fiber optic sensor, a virtual reflecting surface is constructed to form a virtual Fabry-Perot cavity with it, and the distance between the two is the Fabry-Perot cavity length L vir ;

[0044] The time-domain signal of the virtual Fabry-Perot cavity is converted from time-domain to time-frequency domain using complex Fourier transform, so that the time-domain signals of the two reflecting surfaces of the virtual Fabry-Perot cavity interfere in the microwave frequency domain to generate an interference spectrum signal. At this time, the temperature sensitivity S2 of the fiber optic sensor = k2S0; k2 is greater than k;

[0045] 11) Return to step 7), and let the input optical signal circulate in the loop formed by the circulator, the first coupler, and the second coupler until the iteration number k is greater than or equal to K, thereby expanding the sensitivity of the fiber optic sensing system.

[0046] Furthermore, before transmitting the modulated optical signal to the first coupler, an erbium-doped fiber amplifier is also used to amplify the modulated optical signal.

[0047] Furthermore, the microwave scanning frequency signal emitted by the vector network analyzer first reaches the RF amplifier, and after being amplified by the RF amplifier, it is then transmitted to the electro-optic modulator.

[0048] Furthermore, the optical signal emitted by the broadband light source first reaches the polarizer, and after being processed by the polarizer into a polarized optical signal, it is then transmitted to the electro-optic modulator.

[0049] Furthermore, the fiber optic sensor includes a sapphire fiber optic sensor.

[0050] Furthermore, the calculation steps of the temperature sensitivity S of the fiber optic sensor vir,k include:

[0051] a1) Construct the sensor reflected optical time-domain signal X(t) in the time domain, that is:

[0052]

[0053] In the formula, g is the gain of the PD; R is the sensor reflectivity; M is the modulation depth of the microwave signal; I is the intensity of the propagating light; Ω min and Ω maxare the minimum and maximum values of the microwave frequency scan respectively; W is the system equivalent optical path inside the light source, electro-optic modulator and vector network analyzer; OPD loop is the equivalent optical path of the fiber optic loop structure; n sf and L sf are the refractive index and length of the sapphire optical fiber respectively, c is the speed of light; t is time;

[0054] a2) Each time the reflected optical signal circulates in the system, the relationship between the optical intensity of the reflected optical signal and the fiber optic loop loss and the sensor reflection loss is as follows:

[0055]

[0056]

[0057] In the formula, I is the propagating optical intensity, Q is the sum of the fiber optic loop loss and the sensor reflection loss, P is the gain of the EDFA in the loop; I0 is the propagating optical intensity at the first cycle;

[0058] a3) Establish the expression of the reflected optical time-domain signal received by the vector network analyzer at the k-th and K-th cycles, that is:

[0059]

[0060]

[0061] In the formula, K is the total number of cycles; X k (t) is the reflected optical time-domain signal received at the k-th cycle; X K (t) is the reflected optical time-domain signal received at the K-th cycle;

[0062] a4) Introduce the external temperature change △T. At this time, the expression of the reflected optical time-domain signal X ΔT (t) received by the vector network analyzer at the k-th cycle is as follows:

[0063]

[0064] In the formula, α is the thermal expansion coefficient α; ξ is the thermo-optic coefficient;

[0065] a5) Calculate the difference in the reflection peak positions between the 1st cycle and the k-th cycle before and after the temperature change respectively, that is:

[0066]

[0067]

[0068]

[0069] where, ΔL k is the difference in the reflection peak positions of the k-th cycle before and after the temperature change; ΔL1 is the difference in the reflection peak positions of the first cycle before and after the temperature change; ΔOPD sf is the change value of the optical path of the sapphire fiber optic sensor caused by temperature;

[0070] a6) Calculate the temperature sensitivity S of the fiber optic sensor at the k-th cycle vir,k = kS0.

[0071] Furthermore, the calculation steps of the temperature sensitivity of the fiber optic sensor include:

[0072] s1) Construct a virtual reflection surface, and the function peak X vir (t) of this virtual reflection surface is as follows:

[0073] [[ID=2C]]

[0074] where, L vir is the optical path length of the constructed virtual reflection surface;

[0075] The time-domain spectrum X(t) of this virtual reflection surface is as follows:

[0076]

[0077] where, K is the total number of cycles; A

[0078] The distance between the virtual reflection surface and the distal reflection surface of the fiber optic sensor, that is, the constructed virtual Fabry-Perot cavity length OPD vir is as follows:

[0079] OPD vir = L vir (11)

[0080] s2) Use a window function g(t) to select the reflection peaks of the fiber optic sensor end face and the virtual reflection surface to obtain the time-domain signal g k (t)·X(t); g k (t) is the window function of the k-th iteration;

[0081] Perform complex Fourier transform on the time-domain signal g k (t)·X(t) to obtain the microwave interference spectrum, that is:

[0082] W k = W vir * G(Ω)exp(-jΩτ0) (12)

[0083] where, W k is the reconstructed interference spectrum of the k-th cycle; W virInterference spectrum of the virtual reflecting surface; G(Ω) is the inverse Fourier transform function of the gate function g(t), τ0 is the transmission delay; Ω is the Fourier transform frequency domain parameter;

[0084] s3) Calculate the resonance frequency f of the Fabry interference spectrum formed by the virtual Fabry cavity vir,m,k and the free spectral range FSR, that is:

[0085]

[0086]

[0087] In the formula, m is the resonance order;

[0088] s4) When the external environmental temperature changes by △T, the time-domain spectrum X of the reflection signal of the k-th cycle of the fiber optic temperature sensor k (t,ΔT) is as follows:

[0089]

[0090] s5) Keep the position of the virtual reflecting surface unchanged, update the time-domain spectrum of the reflection signal, and obtain:

[0091]

[0092] s6) Calculate the resonance frequency f of the virtual Fabry interference spectrum caused by the change in the external environmental temperature vir,m,k and the free spectral range FSR, that is:

[0093]

[0094]

[0095] s7) Calculate the temperature sensitivity of the resonance frequency, that is:

[0096]

[0097] In the formula, Δf vir,k is the change in the resonance frequency of the virtual Fabry interference spectrum caused by the change in the external environmental temperature;

[0098] Among them, the amplification factor K2 is as follows:

[0099]

[0100] In the formula, S vir,k 、S vir,1 are the temperature sensitivities of the resonance frequency in the k-th cycle and the first cycle.

[0101] The technical effect of the present invention is beyond doubt. The beneficial effects of the present invention are as follows:

[0102] 1) A method for amplifying the sensor sensitivity using an optical fiber loop is proposed. Theoretically, the sensitivity can be increased by more than 2 times, and the amplification factor of the sensitivity is equal to the number of cycles of the optical fiber loop.

[0103] 2) A method for amplifying the sensor sensitivity by constructing a virtual reflecting surface is proposed. Based on the optical fiber loop, the amplification factor of the sensitivity can be further increased to a level greater than the number of cycles of the optical fiber loop.

[0104] 3) The reflective sapphire fiber Fabry - Perot sensor is used as the sensor of this system, reducing the size of the sensor to less than 20 cm.

[0105] 4) The sapphire fiber Fabry - Perot sensing system based on microwave - photon interference is used for temperature measurement, eliminating the interference problem of the inter - mode interference signal existing inside the sapphire fiber in the high - frequency optical interference field and improving the temperature measurement accuracy.

[0106] 4) An optical fiber loop is added in the microwave - photon interference, enhancing the temperature sensing sensitivity and reducing the size of the sensor.

[0107] 6) By constructing a virtual reflecting surface, the temperature sensing sensitivity is further enhanced. Description of the Drawings

[0108] Figure 1 is a typical sapphire fiber Fabry - Perot interference sensing system;

[0109] Figure 2 (a)- Figure 2 (d) is a sapphire fiber Fabry - Perot high - temperature sensor based on microwave - photon interference;

[0110] Figure 3 (a)- Figure 3 (d) is an optical fiber loop temperature sensor that amplifies the sensitivity using the Vernier effect;

[0111] Figure 4 (a)- Figure 4 (d) is a cascaded fiber Fabry - Perot temperature sensor that amplifies the sensitivity using the Vernier effect.

[0112] Figure 5 (a)- Figure 5 (d) is a fiber Fabry - Perot temperature sensor that amplifies the sensitivity using the Vernier effect;

[0113] Figure 6 is the schematic diagram of the system hardware involved in the method of the present invention;

[0114] Figure 7 is the 5 - cycle time - domain peak;

[0115] Figure 8 The change of time domain spectrum after the external environment temperature changes by 1000℃;

[0116] Figure 9 (a)-(b) are the time domain peaks of the 1st and 5th cycle reflections;

[0117] Figure 10 The relationship between the time domain peak shift and temperature when the temperature change step is 100℃;

[0118] Figure 11 is the relationship between temperature sensitivity and the number of fiber loop cycles;

[0119] Figure 12 The time domain reflection peaks of the reflection end face of the sapphire optical fiber sensor and the constructed virtual reflection surface in the first cycle and the fifth cycle;

[0120] Figure 13 (a)-(b) are the interference spectra of the two reflecting surfaces in the first cycle;

[0121] Figure 14 The time domain spectrum of the virtual Fabry-Perot cavity changes after two cycles of temperature change of 1000℃.

[0122] Figure 15 (a)-(b) are the frequency domain changes of the reflection surface of the sapphire fiber Fabry-Perot sensor in the first and fifth cycles;

[0123] Figure 16 (a)-(b) show the changes in the time domain spectrum and virtual Fabry-Perot interference spectrum of the first and fifth cycles when the temperature change step is 100°C, from 0°C to 1000°C;

[0124] Figure 17 The changes of the resonant frequency around 5.5 GHz in the first and fifth cycles;

[0125] Figure 18 is the relationship between the time domain peak shift and the resonant frequency shift and temperature;

[0126] Figure 19 is the relationship between the temperature sensitivity magnification K1 of the optical fiber loop and the temperature sensitivity magnification K2 of the virtual reflection surface constructed by combining the optical fiber loop;

[0127] Figure 20 This is the relationship between the temperature sensitivity magnification K1 when only the optical fiber loop is in effect and the temperature sensitivity magnification K2 when the virtual reflection surface is constructed in combination with the optical fiber loop. DETAILED DESCRIPTION

[0128] The present invention will be further described below in conjunction with embodiments, but it should not be understood that the above-mentioned subject scope of the present invention is limited to the following embodiments. Without departing from the above-mentioned technical idea of the present invention, various substitutions and changes made according to common general technical knowledge and conventional means in the art should be included within the protection scope of the present invention.

[0129] Embodiment 1:

[0130] Refer to Figures 6 to 20 , a sapphire fiber Fabry-Perot temperature sensing system based on a microwave photonic interference fiber loop and constructing a virtual reflecting surface, including a light source (ASE), an electro-optic modulator (EOM), a vector network analyzer (VNA), an optical circulator, a first optical coupler, a second optical coupler, a photoelectric conversion module (PD), and an optical fiber sensor;

[0131] The light source sends an optical signal to the electro-optic modulator;

[0132] The vector network analyzer sends a microwave scanning frequency signal to the electro-optic modulator;

[0133] The electro-optic modulator modulates the optical signal and the microwave scanning frequency signal to obtain an initial modulated optical signal, and transmits it to the first optical coupler;

[0134] The first optical coupler is provided with at least three input / output ports, which are respectively denoted as port 11, port 12, and port 13;

[0135] Port 11 of the first optical coupler receives and couples the initial modulated optical signal to obtain an initial coupled optical signal, and transmits the initial coupled optical signal to the optical circulator through port 13;

[0136] Port 12 of the first optical coupler receives and couples the input optical signal of the k-th iteration to obtain the coupled optical signal of the k-th iteration, and transmits the coupled optical signal of the k-th iteration to the optical circulator through port 13; the initial value of k is 2; k = 2,..., K; K is the total number of iterations;

[0137] The optical circulator transmits the initial coupled optical signal to the optical fiber sensor, then receives the initial reflected light from the optical fiber sensor, and transmits the initial reflected light to the second optical coupler;

[0138] The optical circulator transmits the coupled optical signal of the k-th iteration to the optical fiber sensor, then receives the reflected light of the k-th iteration from the optical fiber sensor, and transmits the reflected light of the k-th iteration of the optical fiber sensor to the second optical coupler;

[0139] The second optical coupler is provided with at least three input / output ports, which are respectively denoted as port 21, port 22, and port 23;

[0140] The port 23 of the second coupler receives and couples the initial reflected light to obtain the initial coupled reflected light, and divides the initial coupled reflected light into two parts; one part of the coupled reflected light is output through the port 21 to the photoelectric conversion module; the other part of the coupled reflected light serves as the input optical signal for the second iteration and is output through the port 22 to the port 12 of the first coupler;

[0141] The port 23 of the second coupler receives and couples the reflected light of the k-th iteration to obtain the coupled reflected light of the k-th iteration, and divides the coupled reflected light of the k-th iteration into two parts; one part of the coupled reflected light of the k-th iteration is output through the port 21 to the photoelectric conversion module; the other part of the coupled reflected light serves as the input optical signal for the (k + 1)-th iteration and is output through the port 22 to the port 12 of the first coupler;

[0142] The photoelectric conversion module performs photoelectric conversion on the received coupled reflected light to obtain the reflected light time-domain signal and transmits it to the vector network analyzer;

[0143] During the operation of the sapphire fiber Fabry-Perot system, each time the optical signal circulates and transmits more once in the loop formed by the circulator, the first coupler, and the second coupler, the temperature sensitivity of the fiber optic sensor doubles; the sensor temperature sensitivity S corresponding to the reflected light time-domain signal of the k-th iteration vir,k = kS0; S0 is the original temperature sensitivity of the fiber optic sensor;

[0144] After receiving the time-domain reflection peak signal of the remote reflecting surface of the fiber optic sensor, the vector network analyzer constructs a virtual reflecting surface to form a virtual Fabry-Perot cavity with the remote reflecting surface of the fiber optic sensor, and the distance between the virtual reflecting surface and the remote reflecting surface of the fiber optic sensor is the Fabry-Perot cavity length L vir ;

[0145] The time-domain signal of the virtual Fabry-Perot cavity is subjected to time-frequency domain conversion using complex Fourier transform, so that the time-domain signals of the two reflecting surfaces of the virtual Fabry-Perot cavity interfere in the microwave frequency domain to generate an interference spectrum signal. At this time, the temperature sensitivity S2 of the fiber optic sensor = k2S0; the parameter k2 > k > 0;

[0146] Embodiment 2:

[0147] See Figures 6 to 20 , for the sapphire fiber Fabry-Perot temperature sensing system based on microwave photon interference fiber loop and constructing a virtual reflecting surface, the technical content is the same as that of Embodiment 1. Further, it further includes an erbium-doped fiber amplifier for amplifying the modulated optical signal.

[0148] Embodiment 3:

[0149] See Figures 6 to 20, a sapphire fiber Fabry-Perot temperature sensing system based on a microwave photonic interference fiber loop and a constructed virtual reflecting surface, with the technical content being the same as any one of Embodiments 1-2. Further, it further includes a radio frequency amplifier for amplifying the microwave scanning frequency signal.

[0150] Embodiment 4:

[0151] See Figures 6 to 20 , a sapphire fiber Fabry-Perot temperature sensing system based on a microwave photonic interference fiber loop and a constructed virtual reflecting surface, with the technical content being the same as any one of Embodiments 1-3. It further includes a polarizer for polarizing the optical signal.

[0152] Embodiment 5:

[0153] The demodulation method of the sapphire fiber Fabry-Perot temperature sensing system based on a microwave photonic interference fiber loop and a constructed virtual reflecting surface according to any one of Embodiments 1-4 includes the following steps:

[0154] 1) The light source sends an optical signal to the electro-optic modulator;

[0155] The vector network analyzer sends a microwave scanning frequency signal to the electro-optic modulator;

[0156] 2) The electro-optic modulator modulates the optical signal and the microwave scanning frequency signal to obtain an initial modulated optical signal, and transmits it to the first coupler;

[0157] 3) The 11th port of the first coupler receives and couples the initial modulated optical signal to obtain an initial coupled optical signal, and transmits the initial coupled optical signal to the circulator through the 13th port;

[0158] 4) The circulator transmits the initial coupled optical signal to the fiber optic sensor, then receives the initial reflected light from the fiber optic sensor, and transmits the initial reflected light to the second coupler;

[0159] 5) The 23rd port of the second coupler receives and couples the initial reflected light to obtain an initial coupled reflected light, and divides the initial coupled reflected light into two parts; one part of the coupled reflected light is output to the photoelectric conversion module through the 21st port; the other part of the coupled reflected light serves as the input optical signal for the second iteration and is output to the 12th port of the first coupler through the 22nd port;

[0160] 6) The photoelectric conversion module performs photoelectric conversion on the received coupled reflected light to obtain a reflected light time-domain signal, and transmits it to the vector network analyzer;

[0161] 7) The port 12 of the first coupler receives and couples the input optical signal of the k-th iteration to obtain the coupled optical signal of the k-th iteration, and transmits the coupled optical signal of the k-th iteration to the circulator through port 13; the initial value of k is 2; k = 2, …, K; K is the total number of iterations;

[0162] 8) The circulator transmits the coupled optical signal of the k-th iteration to the fiber optic sensor, then receives the reflected light of the k-th iteration from the fiber optic sensor, and transmits the reflected light of the k-th iteration to the second coupler;

[0163] 9) The port 23 of the second coupler receives and couples the reflected light of the k-th iteration to obtain the coupled reflected light of the k-th iteration, and divides the coupled reflected light of the k-th iteration into two parts; one part of the coupled reflected light of the k-th iteration is output through port 21 to the photoelectric conversion module; the other part of the coupled reflected light is used as the input optical signal of the (k + 1)-th iteration and is output through port 22 to port 12 of the first coupler;

[0164] 10) The photoelectric conversion module performs photoelectric conversion on the received coupled reflected light to obtain the reflected light time-domain signal, and transmits it to the vector network analyzer;

[0165] During the operation of the sapphire fiber Fabry-Perot system, every time the optical signal circulates and transmits one more time in the loop formed by the circulator, the first coupler, and the second coupler, the temperature sensitivity of the fiber optic sensor doubles; the sensor temperature sensitivity S corresponding to the reflected light time-domain signal of the k-th iteration vir,k = kS0; S0 is the original temperature sensitivity of the fiber optic sensor;

[0166] After the vector network analyzer receives the time-domain reflection peak signal of the remote reflecting surface of the fiber optic sensor, it constructs a virtual reflecting surface to form a virtual Fabry-Perot cavity with it, and the distance between the two is the Fabry-Perot cavity length L vir ;

[0167] The time-domain signal of the virtual Fabry-Perot cavity is converted from time-domain to time-frequency domain by using complex Fourier transform, so that the time-domain signals of the two reflecting surfaces of the virtual Fabry-Perot cavity interfere in the microwave frequency domain to generate an interference spectrum signal. At this time, the temperature sensitivity S2 of the fiber optic sensor = k2S0; k2 is greater than k;

[0168] 11) Return to step 7), and let the input optical signal circulate in the loop formed by the circulator, the first coupler, and the second coupler until the number of iterations k is greater than or equal to K, so as to increase the sensitivity of the fiber optic sensing system.

[0169] Example 6:

[0170] The demodulation method of any one of Examples 1-4 is based on a microwave photon interference fiber loop and a sapphire fiber Fabry-Perot temperature sensing system with a constructed virtual reflection surface. The technical content is the same as that of Example 5. Furthermore, before the modulated optical signal is transmitted to the first coupler, the modulated optical signal is amplified using an erbium-doped fiber amplifier.

[0171] Example 7:

[0172] Any one of Examples 1-4 is a demodulation method for a sapphire fiber Fabry-Perot temperature sensing system based on a microwave photon interference fiber loop and a constructed virtual reflection surface. The technical content is the same as any one of Examples 5-6. Furthermore, the microwave scanning frequency signal emitted by the vector network analyzer first reaches the RF amplifier, is amplified by the RF amplifier, and then transmitted to the electro-optical modulator.

[0173] Example 8:

[0174] Any one of Examples 1-4 is a demodulation method for a sapphire fiber Fabry-Perot temperature sensing system based on a microwave photon interference fiber loop and a constructed virtual reflection surface. The technical content is the same as any one of Examples 5-7. Furthermore, the optical signal emitted by the broadband light source first reaches the polarizer, is processed by the polarizer into a polarized light signal, and then transmitted to the electro-optical modulator.

[0175] Example 9:

[0176] Any one of Examples 1-4 is a demodulation method for a sapphire fiber Fabry-Perot temperature sensing system based on a microwave photon interference fiber loop and a constructed virtual reflection surface. The technical content is the same as any one of Examples 5-8. Furthermore, the fiber optic sensor includes a sapphire fiber optic sensor.

[0177] Example 10:

[0178] The demodulation method of the sapphire fiber Fabry-Perot temperature sensing system based on the microwave photon interference fiber loop and the construction of the virtual reflection surface in any one of embodiments 1-4 has the same technical content as any one of embodiments 5-9. Furthermore, the temperature sensitivity S of the fiber optic sensor is vir,k The calculation steps include:

[0179] a1) The time domain signal X(t) of the reflected light from the sensor built in the time domain is:

[0180]

[0181] Where g is the gain of PD; R is the sensor reflectivity; M is the modulation depth of microwave signal; I is the intensity of propagated light; Ω min and Ω max are the minimum and maximum values of microwave frequency sweep respectively; W is the system equivalent optical path of the light source, electro-optic modulator and vector network analyzer; OPD loopis the equivalent optical path of the fiber optic loop structure; n sf and L sf are the refractive index and length of the sapphire fiber respectively, c is the speed of light; t is time;

[0182] a2) Each time the reflected optical signal circulates in the system, the relationship between the optical intensity of the reflected optical signal, the fiber optic loop loss, and the sensor reflection loss is as follows:

[0183]

[0184]

[0185] In the formula, I is the propagated optical intensity, Q is the sum of the fiber optic loop loss and the sensor reflection loss, P is the gain of the EDFA in the loop; I0 is the propagated optical intensity at the first cycle;

[0186] a3) Establish the expression of the reflected optical time-domain signal received by the vector network analyzer at the k-th and K-th cycles, that is:

[0187]

[0188]

[0189] In the formula, K is the total number of cycles; X k (t) is the reflected optical time-domain signal received at the k-th cycle; X K (t) is the reflected optical time-domain signal received at the K-th cycle;

[0190] a4) Introduce the external temperature change △T. At this time, the expression of the reflected optical time-domain signal X ΔT (t) received by the vector network analyzer at the k-th cycle is as follows:

[0191]

[0192] In the formula, α is the thermal expansion coefficient α; ξ is the thermo-optic coefficient;

[0193] a5) Calculate the difference in the reflection peak positions between the 1st cycle and the k-th cycle before and after the temperature change respectively, that is:

[0194]

[0195]

[0196]

[0197] In the formula, ΔL kis the difference in the position of the reflection peak at the k-th cycle before and after the temperature change; ΔL1 is the difference in the position of the reflection peak at the 1st cycle before and after the temperature change; ΔOPD sf is the change value of the optical path of the sapphire fiber sensor caused by temperature;

[0198] a6) Calculate the temperature sensitivity S of the fiber sensor at the k-th cycle vir,k = kS0.

[0199] Example 11:

[0200] For the demodulation method of the sapphire fiber Fabry - Perot temperature sensing system based on microwave - photon interference fiber loop and constructing a virtual reflection surface in any one of Examples 1 - 4, the technical content is the same as that in any one of Examples 5 - 10. Further, the calculation steps of the temperature sensitivity of the fiber sensor include:

[0201] s1) Construct a virtual reflection surface, and the function peak X vir (t) is as follows:

[0202]

[0203] where L vir is the optical path length of the constructed virtual reflection surface;

[0204] The time - domain spectrum X(t) of this virtual reflection surface is as follows:

[0205]

[0206] where K is the total number of cycles;

[0207] The distance between the virtual reflection surface and the distal reflection surface of the fiber sensor, that is, the constructed virtual Fabry - Perot cavity length OPD vir is as follows:

[0208] OPD vir = L vir (11)

[0209] s2) Use a window function g(t) to select the reflection peaks of the fiber sensor end face and the virtual reflection surface, and obtain the time - domain signal g k (t)·X(t); g k (t) is the window function of the k - th iteration;

[0210] Perform a complex Fourier transform on the time - domain signal g k (t)·X(t) to obtain the microwave interference spectrum, that is:

[0211] W k = W vir *G(Ω)exp(-jΩτ0) (12)

[0212] Wherein, W k is the interference spectrum of the k-th cycle reconstructed; W vir is the interference spectrum of the virtual reflecting surface; G(Ω) is the inverse Fourier transform function of the gate function g(t), τ0 is the transmission delay; Ω is the Fourier transform frequency domain parameter;

[0213] s3) Calculate the resonance frequency f vir,m,k and the free spectral range FSR of the Fabry-Perot interference spectrum formed by the virtual Fabry-Perot cavity, that is:

[0214]

[0215]

[0216] Wherein, m is the resonance order;

[0217] s4) When the external environmental temperature changes by ΔT, the time-domain spectrum X k (t, ΔT) of the reflection signal of the fiber optic temperature sensor for the k-th cycle is as follows:

[0218]

[0219] s5) Keep the position of the virtual reflecting surface unchanged, update the time-domain spectrum of the reflection signal, and obtain:

[0220]

[0221] s6) Calculate the resonance frequency f vir,m,k and the free spectral range FSR of the virtual Fabry-Perot interference spectrum caused by the change of the external environmental temperature, that is:

[0222]

[0223]

[0224] s7) Calculate the temperature sensitivity of the resonance frequency, that is:

[0225]

[0226] Wherein, Δf vir,k is the change amount of the resonance frequency of the virtual Fabry-Perot interference spectrum caused by the change of the external environmental temperature;

[0227] Among them, the magnification K2 is as follows:

[0228]

[0229] Wherein, S vir,k 、S vir,1is the temperature sensitivity of the resonance frequency at the k-th cycle and the first cycle.

[0230] Example 12:

[0231] A sapphire fiber Fabry-Perot demodulation method based on a microwave photonic interference fiber loop and constructing a virtual reflecting surface is as follows:

[0232] As Figure 6 shown, the system hardware structure of the present invention includes the following:

[0233] ASE light source. Broadband light with a wide output wavelength range enters the fiber optic sensor.

[0234] Polarizer: Changes the output optical signal of the light source into an arbitrary polarization optical signal, so that the microwave signal in the system can better modulate the optical signal and load the optical signal, and at the same time reduces the polarization correlation in the system.

[0235] Electro-optic modulator. Makes the intensity of the optical carrier change with the modulated microwave signal, thereby realizing that the optical signal carries the microwave signal.

[0236] RF amplifier: Amplifies the intensity of the microwave signal output by the vector network analyzer VNA to be more matched with the intensity of the microwave signal input to the electro-optic modulator.

[0237] Vector network analyzer: The core of this system, namely the vector network analyzer VNA, outputs microwave radio frequency signals, receives the returned microwave radio frequency signals, and processes and stores the signals.

[0238] [[ID=2*]]Opto-electronic detection module: Used to convert the reflected optical signal carrying the temperature information of the environment to be measured into an electrical signal and output it to the VNA for subsequent processing.

[0239] Sapphire fiber Fabry-Perot sensor: Used to sense temperature changes in the environment to be measured.

[0240] The steps of the sapphire fiber Fabry-Perot demodulation method are as follows:

[0241] It should be noted that there seems to be a typo in the original text where "[[ID=2*]]" is used. It should probably be a correct ID number. This has been translated as is for now.The electro-optic modulator simultaneously receives the optical signal generated by the ASE broadband light source and the microwave scanning frequency signal emitted by the VNA, modulates the microwave signal onto the optical signal and outputs it. Since the intensity of the modulated optical signal is not high, it is necessary to amplify the optical intensity through an erbium-doped fiber amplifier (EDFA). The optical signal output by the EDFA is input from port 1 of the first 1×2 coupler, then output from port 3 to the input end of the circulator, and then enters the sapphire fiber sensor. The reflected light of the sapphire fiber sensor is output from the output end of the circulator to port 3 of the second coupler. The second coupler then divides the light into two parts: the first part is output from port 1 to the PD for photoelectric conversion and enters the VNA for synchronous scanning measurement of the reflection spectrum (i.e., the S21 parameter), which is the output of the first cycle of the loop at this time; the second part remains in the loop, is output from port 2 to the EDFA for amplification, enters port 2 of the first coupler, and is used as the input light to be output to the sapphire fiber sensor again by the coupler and the circulator, which is the output of the second cycle of the loop at this time, and so on for repeated cycles, and continuous cycling occurs within the fiber loop.

[0242] Meanwhile, in order to obtain higher temperature sensitivity, a fictitious reflecting surface is constructed beside the reflection peak of the time-domain signal sensor after the k-th cycle, so that interference is formed with this reflecting surface in the microwave domain.

[0243] The principle of the present invention:

[0244] In the sapphire fiber Fabry-Perot demodulation method based on microwave photon interference fiber loop and constructing a virtual reflecting surface, the time-domain signal of the sensor in the time domain is:

[0245]

[0246] where g is the gain of the PD, R is the sensor reflectivity, M is the modulation depth of the microwave signal, I0 is the propagating light intensity, Ω min and Ω max are respectively the minimum and maximum values of the microwave frequency scan, W is the system equivalent optical path inside the ASE, EOM and VNA, OPD loop is the equivalent optical path of the fiber loop structure, nsf and Lsf are respectively the refractive index and length of the sapphire fiber, and c is the speed of light.

[0247] The propagating light intensity after each cycle will gradually decrease with transmission loss and reflection of the sapphire fiber sensor. Assuming that the total loss of the loop sensor is fixed, the attenuated intensity is determined by the total loss of the loop and the sensor. The relationship between the light intensity after the k-th cycle and time t is:

[0248]

[0249] Where I is the propagated light intensity, Q is the sum of the fiber loop loss and the sensor reflection loss, and P is the gain of the EDFA in the loop. Solving for I as a function of t:

[0250]

[0251] Where I0 is the propagated light intensity at the first cycle. It can be seen that the coupled optical signal will decay exponentially. Combining the two equations, the k-th cycle time-domain signal collected by the VNA is:

[0252]

[0253] The obtained time-domain signal is:

[0254]

[0255] Where kmax is the total number of cycles. It can be seen that the time-domain signal of the fiber loop will consist of sinc function peaks, and the peak values of the sinc function peaks will decay exponentially. The position of the sinc function peak is determined by the equivalent optical path of the loop and the sapphire fiber sensor. After the external temperature changes by △T, the equivalent optical path of the sapphire fiber sensor will also change. Then the k-th cycle time-domain signal collected by the VNA at this time is:

[0256]

[0257] It can be seen from the equation that the differences in the positions of the sinc function peaks between the first cycle and the k-th cycle before and after the temperature change are respectively:

[0258]

[0259]

[0260] It can be seen that the difference in the position of the sinc function peak in the k-th cycle is exactly k times the difference in the position of the sinc function peak in the first cycle:

[0261]

[0262] A single reflecting surface cannot form a microwave frequency-domain Fabry-Perot interference. In order to make it form a Fabry-Perot interference and thus obtain a higher temperature sensitivity, a virtual reflecting surface is constructed to interfere with this reflecting surface. The virtual reflecting surface is placed far away from the end face of the sapphire fiber. On the time-domain spectrum, it is shown that the virtual reflection peak appears on the right side of the reflection peak of the sapphire fiber end face.

[0263] Construct a sinc function peak identical to it at a certain distance beside its reflection peak:

[0264]

[0265] where Lvir is the optical path length of the constructed virtual reflecting surface. The time-domain spectrum obtained is:

[0266]

[0267] where kmax is the total number of cycles. The distance between the constructed virtual reflecting surface and the end face of the actual sapphire fiber sensor (for convenience, the refractive index of the transmission medium of the virtual Fabry-Perot cavity is set to 1), that is, the length of the constructed virtual Fabry-Perot cavity can be expressed as:

[0268] OPD vir = L vir

[0269] Select the reflection peaks of the end face of the sapphire fiber sensor and the constructed virtual reflecting surface using a window function g(t). The time-domain signal after the action of the gate function becomes g k (t)·X(t). Apply the complex Fourier transform to the selected time-domain signal to the frequency domain to realize the reconstruction of the microwave interference spectrum, which is expressed as:

[0270] W k = W vir *G(Ω)exp(-jΩτ0)

[0271] where Wk is the interference spectrum of the k-th cycle reconstructed, Wvir is the interference spectrum of the virtual reflecting surface, G(Ω) is the inverse Fourier transform function of the gate function g(t), and τ0 is the transmission delay.

[0272] The resonant frequency f vir,m,k and the free spectral range FSR of the Fabry interference spectrum formed by the virtual Fabry-Perot cavity can be expressed as:

[0273]

[0274]

[0275] where m is the resonant order. When the external environmental temperature changes by △T, the time-domain spectrum of the reflection signal of the k-th cycle of the sapphire fiber temperature sensor is:

[0276]

[0277] To make the change in the length of the virtual Fabry-Perot cavity only depend on the change in the end face of the sapphire fiber sensor, keep the position of the constructed virtual reflecting surface unchanged (that is, Xvir(t) is independent of △T). The time-domain spectrum at this time is:

[0278]

[0279] The variations of the resonant frequency fvir,m,k and the free spectral range FSR of the virtual Fabry-Perot interference spectrum caused by the change in the external environmental temperature are expressed as:

[0280]

[0281]

[0282] Then the temperature sensitivity of the resonant frequency can be expressed as:

[0283]

[0284] It can be seen that the temperature sensitivity of the resonant frequency fvir,m,k is related to the number of cycles k, the refractive index nsf of the sapphire optical fiber, the length Lsf of the sapphire optical fiber, and the virtual Fabry-Perot cavity length OPDvir.

[0285] The magnification factor of the temperature sensitivity of the k-th cycle compared to the first cycle is:

[0286]

[0287] Figure 19 For the relationship between the magnification factor K1 of the temperature sensitivity of only the optical fiber loop and the magnification factor K2 of the temperature sensitivity of the structure of the virtual reflecting surface by combining the optical fiber loop, it can be seen that as the number of cycles increases, K2 gradually increases and shows an exponential growth; while K1 also gradually increases, but the growth curve is a linear curve, and the growth rate is lower than that of K1. This means that by constructing a virtual reflecting surface at the distal end of the sapphire optical fiber temperature sensor, which appears as constructing a virtual reflection peak beside the time-domain reflection peak after the optical fiber loop circulates in the time domain, and constructing the reflection peak of the sapphire optical fiber and the virtual reflection peak into a virtual Fabry-Perot cavity for interference in the frequency domain, a higher magnification factor of temperature sensitivity can be obtained compared to the time domain.

[0288] Considering that the refractive index nsf of the sapphire optical fiber is 1.75, the length is 0.2 m (the equivalent optical path is 1.75×0.2 = 0.35 m), the microwave frequency bandwidth is 0 - 8.5 GHz (corresponding to the actual microwave frequency bandwidth of the VNA), the gain g of the photodetector PD is 1, the reflectivity of the sapphire optical fiber is 0.07, the propagating light intensity I0 is 1 W, the modulation depth M is 1, the equivalent optical path OPDloop of the optical fiber loop is 1 m, the speed of light c is 3×10^8, the total loss P of the optical fiber loop loss and the sensor reflection is -40 dB, the EDFA gain is 20 dB (assuming that the light intensity after 5 cycles is lower than the input threshold of the EDFA and cannot be effectively amplified), and the internal optical path of the system is 16 m, the final 5-cycle time-domain peak is obtained as Figure 7 shown.

[0289] It can be seen that the five reflection peaks in the time domain are evenly distributed, and the interval between each reflection peak is OPDloop + nsfLsf / 2c, which is 9 ns (i.e., the optical path is 5.4 m). Moreover, the intensity of the reflection peak decreases exponentially as the number of cycles increases.

[0290] After the external environmental temperature changes by 1000 °C, check the change of the time-domain spectrum, as Figure 8 shown.

[0291] From Figures 8 - 9 it can be seen that the first-cycle peak of the time-domain spectrum moves 0.07 ns to the right, and the fifth-cycle peak moves 0.35 ns to the right, which is exactly a 5-fold relationship, and this is also consistent with the theoretical calculation: the moving amount of the reflection time-domain peak in the k-th cycle is k times that of the first-cycle reflection time-domain peak.

[0292] Examine the change of the time-domain spectrum during the process from 0 °C to 1000 °C when the temperature change step is 100 °C. Track the time-domain spectra of five cycles to obtain the relationship between the moving amount of the time-domain peak and the temperature, as Figure 10 shown.

[0293] It can be seen that the temperature sensitivities of the first five cycles are 0.07 ps / °C, 0.14 ps / °C, 0.21 ps / °C, 0.28 ps / °C, and 0.35 ps / °C respectively. The temperature sensitivity gradually increases as the number of cycles increases. Obtain the relationship between the magnification of the temperature sensitivity and the number of fiber-loop cycles, as Figure 11 shown.

[0294] The temperature sensitivity increases as the number of fiber-loop cycles increases and shows a linear growth, which is consistent with the theoretical calculation.

[0295] In order to enable the single reflecting surface of the sapphire fiber to form interference, a virtual reflecting surface is constructed beside its time-domain reflection peak to interfere with it. Set the length of the constructed virtual Fabry-Perot cavity to 0.2 m, and obtain the time-domain reflection peaks of the reflecting end face of the sapphire fiber sensor and the constructed virtual reflecting surface in the first cycle and the fifth cycle, as Figure 12 shown.

[0296] After selecting the two reflecting surfaces in the first cycle using a Hanning window, perform complex Fourier transform to the frequency domain to view its interference spectrum, as Figure 13 shown.

[0297] The free spectral range FSR of the interference spectra of the first and fifth cycles is obtained as 1.5 GHz, and according to the theoretical calculation, FSR is 1.5 GHz, which is consistent with the theory.

[0298] Examine the change of the time-domain spectra of the virtual Fabry-Perot cavity for two cycles after the temperature changes by 1000 °C, asFigure 14 as shown

[0299] After a temperature change of 1000 °C, the reflecting surfaces of the sapphire fiber Fabry - Perot sensor in the first and fifth cycles moved 0.07 ns and 0.35 ns respectively on the time - domain spectrum. Fix the positions of the virtual reflecting surfaces before and after the temperature change, select the two reflecting surfaces on the time - domain spectrum using a Hanning window and then transform to the frequency domain, as Figure 15 as shown

[0300] It can be seen that after the temperature change, the interference peak of the virtual Fabry - Perot cavity in the first cycle moved 0.274 GHz. The interference peak of the virtual Fabry - Perot cavity in the fifth cycle moved 1.757 GHz.

[0301] Examine the changes in the time - domain spectrum and the virtual Fabry - Perot interference spectrum during the process of changing from 0 °C to 1000 °C with a temperature change step of 100 °C, as Figure 16 as shown

[0302] Examine the change in the resonant frequency near 5.5 GHz, as Figure 17 as shown

[0303] Track the resonant frequencies of the virtual Fabry - Perot cavity interference spectrum in the 1st - 5th cycles, as Figure 18 as shown. The temperature sensitivities of the resonant frequencies of the virtual Fabry - Perot cavity interference spectrum in the 1st - 5th cycles are 275 kHz / °C, 348 kHz / °C, 476 kHz / °C, 749 kHz / °C, and 1763 kHz / °C respectively. In the resonant - frequency shift amount - temperature curve graph, the amplification factor K2 of the temperature sensitivity in the fifth cycle compared to that in the first cycle is 6.4 times, which is close to the theoretical calculation K2 = 6.33.

[0304] Obtain the relationship between the amplification factor of sensitivity and the number of cycles, as Figure 19 as shown

[0305] Figure 20 It is the relationship between the amplification factor of temperature sensitivity K1 when only the optical - fiber loop acts and the amplification factor of temperature sensitivity K2 when constructing a virtual reflecting surface and combining with the optical - fiber loop action. It can be seen that as the number of cycles increases, K2 gradually increases, and the growth rate is higher than that of K1. That is to say, by constructing a virtual Fabry - Perot cavity, a higher amplification factor of temperature sensitivity can be obtained compared to the time - domain.

Claims

1. A sapphire fiber Fabry - Perot temperature sensing system based on a microwave - photonic - interference fiber loop and constructing a virtual reflecting surface, characterized in that, It includes a light source, an electro-optical modulator, a vector network analyzer, a circulator, a first coupler, a second coupler, a photoelectric conversion module, and an optical fiber sensor; The light source sends an optical signal to the electro-optical modulator; The vector network analyzer sends a microwave sweep frequency signal to the electro-optical modulator; The electro-optical modulator modulates the optical signal and the microwave sweep frequency signal to obtain an initial modulated optical signal, and transmits the initial modulated optical signal to the first coupler; The first coupler is provided with at least three input / output ports, which are respectively designated as port 11, port 12 and port 13; Port 11 of the first coupler receives and couples the initial modulated optical signal to obtain an initial coupled optical signal, and transmits the initial coupled optical signal to the circulator through port 13; The port 12 of the first coupler receives and couples the input optical signal of the kth iteration to obtain a coupled optical signal of the kth iteration, and transmits the coupled optical signal of the kth iteration to the circulator through the port 13; the initial value of k is 2; k=2, ..., K; K is the total number of iterations; The circulator transmits the initial coupled optical signal to the optical fiber sensor, then receives the initial reflected light from the optical fiber sensor, and transmits the initial reflected light to the second coupler; The circulator transmits the k-th iteration coupled optical signal to the optical fiber sensor, then receives the k-th iteration reflected light from the optical fiber sensor, and transmits the k-th iteration reflected light from the optical fiber sensor to the second coupler; The second coupler is provided with at least three input / output ports, which are respectively designated as port 21, port 22 and port 23; Port 23 of the second coupler receives and couples the initial reflected light to obtain initial coupled reflected light, and divides the initial coupled reflected light into two parts; one part of the coupled reflected light is output to the photoelectric conversion module through port 21; the other part of the coupled reflected light is used as the input optical signal of the second iteration and is output to port 12 of the first coupler through port 22; Port 23 of the second coupler receives and couples the reflected light of the kth iteration to obtain coupled reflected light of the kth iteration, and divides the coupled reflected light of the kth iteration into two parts; one part of the coupled reflected light of the kth iteration is output to the photoelectric conversion module through port 21; the other part of the coupled reflected light is used as the input optical signal of the k+1th iteration and is output to port 12 of the first coupler through port 22; The photoelectric conversion module performs photoelectric conversion on the received coupled reflected light to obtain a reflected light time domain signal, and transmits the signal to the vector network analyzer; During the operation of the sapphire fiber Fabry-Perot system, every time the optical signal makes an additional round-trip transmission within the loop formed by the circulator, the first coupler, and the second coupler, the temperature sensitivity of the fiber optic sensor doubles; the sensor temperature sensitivity S corresponding to the reflected optical time-domain signal of the k-th iteration vir,k = kS0; S0 is the original temperature sensitivity of the fiber optic sensor; After the vector network analyzer receives the time-domain reflection peak signal of the distal reflecting surface of the fiber optic sensor, a virtual reflecting surface is constructed to form a virtual Fabry-Perot cavity with the distal reflecting surface of the fiber optic sensor. The distance between the virtual reflecting surface and the distal reflecting surface of the fiber optic sensor is the Fabry-Perot cavity length L vir ; The time domain signal of the virtual Fabry-Perot cavity is converted into the time-frequency domain using complex Fourier transform, so that the time domain signals of the two reflecting surfaces of the virtual Fabry-Perot cavity interfere in the microwave frequency domain, generating an interference spectrum signal. At this time, the temperature sensitivity of the optical fiber sensor is S2=k2S0; the parameter k2 is greater than k.

2. The sapphire fiber Fabry-Perot temperature sensing system based on a microwave photonic interference optical fiber loop and constructing a virtual reflecting surface according to claim 1, wherein It also includes an erbium-doped fiber amplifier for amplifying modulated optical signals, a radio frequency amplifier for amplifying microwave swept frequency signals, and a polarizer for performing polarization processing on optical signals.

3. The demodulation method of the sapphire fiber Fabry-Perot temperature sensing system based on a microwave photonic interference fiber loop and constructing a virtual reflecting surface according to any one of claims 1-2, characterized in that, The following steps are involved: 1) The light source sends an optical signal to the electro-optical modulator; The vector network analyzer sends a microwave sweep frequency signal to the electro-optical modulator; 2) The electro-optical modulator modulates the optical signal and the microwave scanning frequency signal to obtain an initial modulated optical signal, and transmits it to the first coupler; 3) The port 11 of the first coupler receives and couples the initial modulated optical signal to obtain an initial coupled optical signal, and transmits the initial coupled optical signal to the circulator through port 13; 4) The circulator transmits the initial coupled optical signal to the fiber optic sensor, then receives the initial reflected light from the fiber optic sensor, and transmits the initial reflected light to the second coupler; 5) The port 23 of the second coupler receives and couples the initial reflected light to obtain an initial coupled reflected light, and divides the initial coupled reflected light into two parts; one part of the coupled reflected light is output through port 21 to the photoelectric conversion module; the other part of the coupled reflected light serves as the input optical signal for the second iteration and is output through port 22 to port 12 of the first coupler; 6) The photoelectric conversion module performs photoelectric conversion on the received coupled reflected light to obtain a reflected light time-domain signal, and transmits it to the vector network analyzer; 7) The port 12 of the first coupler receives and couples the input optical signal for the k-th iteration to obtain the coupled optical signal for the k-th iteration, and transmits the coupled optical signal for the k-th iteration to the circulator through port 13; the initial value of k is 2; k = 2, …, K; K is the total number of iterations; 8) The circulator transmits the coupled optical signal for the k-th iteration to the fiber optic sensor, then receives the reflected light for the k-th iteration from the fiber optic sensor, and transmits the reflected light for the k-th iteration to the second coupler; 9) The port 23 of the second coupler receives and couples the reflected light for the k-th iteration to obtain the coupled reflected light for the k-th iteration, and divides the coupled reflected light for the k-th iteration into two parts; one part of the coupled reflected light for the k-th iteration is output through port 21 to the photoelectric conversion module; the other part of the coupled reflected light serves as the input optical signal for the (k + 1)-th iteration and is output through port 22 to port 12 of the first coupler; 10) The photoelectric conversion module performs photoelectric conversion on the received coupled reflected light to obtain a reflected light time-domain signal, and transmits it to the vector network analyzer; During the operation of the sapphire fiber Fabry-Perot system, every time the optical signal makes an additional round-trip transmission in the loop formed by the circulator, the first coupler, and the second coupler, the temperature sensitivity of the fiber optic sensor doubles; the sensor temperature sensitivity S corresponding to the reflected optical time-domain signal of the k-th iteration vir,k = kS0; S0 is the original temperature sensitivity of the fiber optic sensor; After the vector network analyzer receives the time-domain reflection peak signal of the remote reflecting surface of the fiber optic sensor, a virtual reflecting surface is constructed to form a virtual Fabry-Perot cavity with it, and the distance between the two is the Fabry-Perot cavity length L vir ; The time-domain signal of the virtual Fabry-Perot cavity is subjected to time-frequency domain conversion using complex Fourier transform, so that the time-domain signals of the two reflecting surfaces of the virtual Fabry-Perot cavity interfere in the microwave frequency domain to generate an interference spectrum signal. At this time, the temperature sensitivity S2 of the fiber optic sensor is S2 = k2S0; k2 is greater than k; 11) Return to step 7), and let the input optical signal circulate in the loop formed by the circulator, the first coupler, and the second coupler until the number of iterations k is greater than or equal to K, thereby expanding the sensitivity of the fiber optic sensing system.

4. The demodulation method of the sapphire fiber Fabry-Perot temperature sensing system based on a microwave photonic interference optical fiber loop and constructing a virtual reflecting surface according to claim 3, characterized in that, Before transmitting the modulated optical signal to the first coupler, the modulated optical signal is also amplified by an erbium-doped fiber amplifier.

5. The demodulation method of the sapphire fiber Fabry-Perot temperature sensing system based on a microwave photonic interference optical fiber loop and constructing a virtual reflecting surface according to claim 3, characterized in that, The microwave scanning frequency signal sent by the vector network analyzer first reaches the RF amplifier, and after being amplified by the RF amplifier, it is then transmitted to the electro-optical modulator.

6. The demodulation method of the sapphire fiber Fabry-Perot temperature sensing system based on a microwave photonic interference optical fiber loop and constructing a virtual reflecting surface according to claim 3, characterized in that, The optical signal emitted by the broadband light source first reaches the polarizer, and after being processed by the polarizer into a polarized optical signal, it is then transmitted to the electro-optical modulator.

7. The demodulation method of the sapphire fiber Fabry-Perot temperature sensing system based on a microwave photonic interference fiber loop and constructing a virtual reflecting surface according to claim 3, characterized in that The fiber optic sensor includes a sapphire fiber optic sensor.

8. The demodulation method of the sapphire fiber Fabry-Perot temperature sensing system based on a microwave photonic interference fiber loop and constructing a virtual reflection surface according to claim 3, characterized in that, The temperature sensitivity S of the fiber optic sensor vir,k The calculation steps are as follows: 1) Construct the sensor reflected optical time-domain signal X(t) in the time domain, that is: where g is the gain of the PD; R is the sensor reflectivity; M is the modulation depth of the microwave signal; I is the intensity of the propagating light; Ω min and Ω max are the minimum and maximum values of the microwave frequency sweep, respectively; W is the system equivalent optical path inside the light source, electro-optic modulator, and vector network analyzer; OPD loop is the equivalent optical path of the fiber loop structure; n sf and L sf are the refractive index and length of the sapphire fiber, respectively; c is the speed of light; t is the time; 2) Each time the reflected optical signal circulates in the system, the relationship between the optical intensity of the reflected optical signal, the fiber loop loss, and the sensor reflection loss is as follows: In the formula, I is the propagated optical intensity, Q is the sum of the fiber loop loss and the sensor reflection loss, P is the gain of the EDFA in the loop; I0 is the propagated optical intensity at the first cycle; 3) Establish the expression of the reflected optical time-domain signal received by the vector network analyzer at the k-th and K-th cycles, that is: where K is the total number of cycles; X k (t) is the received reflected optical time-domain signal at the k-th cycle; X K (t) is the received reflected optical time-domain signal at the K-th cycle; 4) Introduce the external temperature change △T. At this time, the expression of the reflected optical time-domain signal X(t) received by the vector network analyzer in the k-th cycle is as follows: ΔT (t) is as follows: In the formula, α is the thermal expansion coefficient; ξ is the thermo-optic coefficient; 5) Calculate the difference in the positions of the reflection peaks at the first cycle and the k-th cycle before and after the temperature change, that is: where ΔL k is the difference in the reflection peak position of the k-th cycle before and after the temperature change; ΔL1 is the difference in the reflection peak position of the first cycle before and after the temperature change; ΔOPD sf is the change value of the optical path of the sapphire fiber optic sensor caused by temperature; 6) Calculate the temperature sensitivity S of the fiber optic sensor at the k-th cycle vir,k = kS0.

9. The demodulation method of the sapphire fiber Fabry-Perot temperature sensing system based on a microwave photonic interference fiber loop and constructing a virtual reflection surface according to claim 8, characterized in that, The calculation steps for the temperature sensitivity of the fiber optic sensor include: 1) Construct a virtual reflecting surface, and the function peak X vir (t) is as follows: where L vir is the optical path length of the constructed virtual reflection surface; The time-domain spectrum X(t) of this virtual reflecting surface is as follows: In the formula, K is the total number of cycles; The distance between the virtual reflecting surface and the reflecting surface at the far end of the fiber optic sensor, that is, the constructed virtual Fabry-Perot cavity length OPD vir As shown below: OPD vir = L vir (11) 2) Select the reflection peaks of the fiber optic sensor end face and the virtual reflection surface using a window function g(t) to obtain the time-domain signal g k (t)·X(t); g k (t) is the window function for the k-th iteration; Perform the complex Fourier transform on the time-domain signal g k (t)·X(t) to obtain the microwave interference spectrum, that is: W k = W vir * G(Ω) exp(-jΩτ0) (12) where W k is the interference spectrum of the k-th reconstruction cycle; W vir is the interference spectrum of the virtual reflector; G(Ω) is the inverse Fourier transform function of the gate function g(t), τ0 is the transmission delay; Ω is the Fourier transform frequency domain parameter; 3) Calculate the resonance frequency f of the Fabry-Perot interference spectrum formed by the virtual Fabry-Perot cavity vir,m,k and the free spectral range FSR, that is: In the formula, m is the resonance order; 4) When the external environmental temperature changes by ΔT, the time-domain spectrum X k (t, ΔT) of the reflected signal in the k-th cycle of the fiber optic temperature sensor is as follows: 5) Keep the position of the virtual reflecting surface unchanged and update the time-domain spectrum of the reflected signal to obtain: 6) Calculate the resonant frequency f of the virtual Fabry-Perot interference spectrum caused by the change in the external environmental temperature vir,m,k and the free spectral range FSR, that is: 7) Calculate the temperature sensitivity S2 of the resonance frequency, that is: where Δf vir,k is the change in the resonance frequency of the virtual Fabry-Perot interference spectrum caused by the change in the external environmental temperature; Among them, the magnification K2 is as follows: where S vir,k and S vir,1 are the temperature sensitivities of the resonance frequencies at the k-th cycle and the first cycle, respectively.

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