Optical fiber low frequency weak magnetic detection sensor system based on sagnac auxiliary reference interferometer

Abstract

This invention belongs to the field of fiber optic sensing technology, specifically relating to a fiber optic low-frequency weak magnetic field detection sensing system based on a Sagnac-assisted reference interferometer. In the sensing system, the laser output light intensity is split into two optical signals via a 1×2 coupler. These two signals pass through a reference interferometer and a sensing interferometer, both with the same optical path difference and carrier modulation. The interferometers are based on a linear Sagnac structure with high sensitivity, high stability, and low polarization noise. Under identical external environmental conditions, only the sensing interferometer is modulated by a low-frequency weak magnetic field. According to the principle of auxiliary reference interferometry, this reduces the system's low-frequency background noise, enabling low-frequency weak magnetic field detection. This invention offers advantages in system structure, including low-frequency noise reduction, simple structure, high stability, and high sensitivity, and can be widely applied to interferometric fiber optic low-frequency weak signal sensing.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing technology, specifically relating to a fiber optic low-frequency weak magnetic field detection sensing system based on a Sagnac-assisted reference interferometer. Background Technology

[0002] Low-frequency weak magnetic field detection technology has wide applications in military anti-submarine warfare, biomedicine, earthquake early warning, resource exploration and other fields. The level of weak magnetic field measurement largely represents the level of technological development of a country's magnetic field measurement. The development and application of detection technology has always been a matter of great concern.

[0003] Magnetoelectric sensors and quantum sensors are both commonly used low-frequency weak magnetic field detection technologies, and both can achieve high sensitivity. However, mechanical electromagnetic magnetic field measurements such as giant magnetoresistive and fluxgate magnetometers generally have low accuracy, large size, and are susceptible to electromagnetic interference. Quantum magnetic field sensors offer high accuracy but are complex, require additional magnetic shielding, and are expensive. Therefore, the limitations of traditional magnetic field sensors in terms of sensitivity, size, power consumption, and complexity expose their inability to meet the needs of underwater, underground, and biological local magnetic field detection in harsh and complex environments. In contrast, fiber optic magnetic field sensors offer miniaturization, multi-parameter capability, and strong resistance to electromagnetic interference. Their high sensitivity and stability demonstrate great potential in the field of low-frequency weak magnetic field detection.

[0004] Interferometric fiber optic sensing systems are a sensing technology based on the principle of optical interference. By utilizing the transmission characteristics of light in optical fibers, they interact with physical quantities in the external environment, causing a change in the phase of the light. Due to their high sensitivity and large dynamic range, they are considered the most promising fiber optic sensing systems. Common interferometric fiber optic sensing systems include the Mach-Zehnder interferometer system, the Michelson interferometer system, the Fabry-Perot interferometer system, and the Sagnac interferometer system. However, traditional interferometric fiber optic sensing systems are susceptible to low-frequency background noise and external low-frequency interference, and have certain limitations in measuring weak low-frequency signals.

[0005] Existing technologies disclose reference interferometers based on the Michelson fiber interferometer system and the Mach-Zehnder fiber interferometer system for suppressing common-mode noise. However, these interferometers have complex structures, poor stability, and are prone to causing large differences between the reference interferometer and the sensing interferometer, resulting in inaccurate detection results for low-frequency weak signals. Summary of the Invention

[0006] To address the problems of complex sensing structures, high system maintenance costs, and susceptibility to electromagnetic interference in low-frequency weak magnetic field detection by magnetoelectric and quantum sensing magnetic field measurement systems, as well as the shortcomings of traditional interferometric system structures in low-frequency magnetic field signal detection, this invention proposes a fiber optic low-frequency weak magnetic field detection sensing system based on a Sagnac-assisted reference interferometer.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0008] A fiber optic low-frequency weak magnetic field detection sensing system based on a Sagnac-assisted reference interferometer includes a laser (1). The beam emitted by the laser (1) enters the sensing interferometer A and the reference interferometer B respectively through a 1×2 coupler (3). The sensing interferometer A and the reference interferometer B have the same optical path difference and are synchronously, at the same frequency and with the same modulation depth. The data processing module (16) collects the signals of the two interferometers and demodulates them.

[0009] The sensor interferometer A is composed of the following structure: the beam emitted by the laser (1) enters the first port (41) of the circulator, the second port (42) of the circulator is connected in sequence to a 2×2 coupler, a sensing fiber and a Faraday rotator, the 2×2 coupler has two ports connected to a phase modulator, and a photodetector is connected to the third port (43) of the circulator. The reference interferometer B is composed of the following structure: the beam emitted by the laser (1) enters the first port (101) of the circulator, the second port (102) of the circulator is connected in sequence to a 2×2 coupler, a sensing fiber and a Faraday rotator, the 2×2 coupler has two ports connected to a phase modulator, and a photodetector is connected to the third port (103) of the circulator.

[0010] The sensing fiber is a magneto-refractive-doped silica fiber. The sensing interferometer A is placed in a low-frequency weak magnetic field environment, while the reference interferometer B is placed in a shielded magnetic field environment.

[0011] The phase modulator is a PZT piezoelectric ceramic, with a single-mode optical fiber wound on it. A phase modulation carrier signal is generated by a function generator to synchronously modulate the phase of the sensing interferometer A and the reference interferometer B at the same frequency and modulation depth, while ensuring that the modulation depth C = 2.63 rad.

[0012] The data processing module (16) includes a multiplier, a low-pass filter, a subtractor, a divider, and an arctangent algorithm.

[0013] The photodetector receives the interference signals output by sensing interferometer A and reference interferometer B, I s It is the output light intensity of the sensor interferometer, I r It is the output light intensity of the reference interferometer.

[0014]

[0015] Among them, A s Let B be the DC component of sensor interferometer A. s For the AC component of sensor interferometer A, A r For the DC component of the reference interferometer B, B r The AC component of the reference interferometer B is used, and C is the modulation depth. The carrier angular frequency, The phase change is caused by the sensor signal and environmental noise. The phase change is caused by environmental noise. For the phase noise of the light source of sensor interferometer A, To reference the phase noise of the light source in interferometer B, I s With baseband carrier and second harmonic carrier The mixture is then passed through a low-pass filter and output.

[0016]

[0017] The same for I r Signal processing yields:

[0018]

[0019] Where J1(C) and J2(C) are the coefficients of the Bessel function.

[0020] The data processing module (16) performs the following signal processing:

[0021] ,

[0022] In the reference interferometer algorithm, the modulation depth needs to be controlled so that J1(C) = J2(C), at which point C = 2.63 rad. To make J1(C) = J2(C), for G = H, since the correlation parameters of sensing interferometer A and reference interferometer B are essentially the same in the sensing system, the phase noise generated by the light source is essentially equal. Under the above conditions, after signal processing, a pair of orthogonal signals can be obtained:

[0023]

[0024] The modulation depth is controlled at C = 2.63 rad. Dividing L1 and L2 yields:

[0025] And perform arctangent calculation.

[0026] The amplitudes of the sine and cosine correlation components output by the interferometer are normalized. Since the correlation parameters of the sensing interferometer and the reference interferometer are essentially the same in the sensing system, the noise levels in the system are roughly equal. Therefore, theoretical analysis shows that a low-frequency, weak magnetic field sensing signal can be obtained under strict control of the background noise of the reference interferometer.

[0027] This scheme utilizes the magnetorefractive effect of rare-earth-doped optical fibers for magnetic field sensing. A reference interferometer and a sensing interferometer are constructed using a linear Sagnac sensing structure, exhibiting high stability and sensitivity. The optical path difference between the sensing and reference interferometers is the same. The sensing interferometer is placed within a low-frequency, weak magnetic field, while the reference interferometer is placed outside the magnetic field, with all other external environmental conditions remaining the same. To achieve accurate demodulation of the low-frequency, weak signal, phase-generated carrier technology is used to modulate the signal, which is then demodulated using an algorithm based on the reference interferometer. This invention reduces the impact of low-frequency interference from the external environment, low-frequency noise from circuits, and low-frequency noise from the light source on signal demodulation.

[0028] The beneficial effects of this invention are:

[0029] 1. This invention uses magnetostrictive doped silica fiber, which has the advantages of simple sensing structure, low cost, low power consumption and high stability compared with magnetic field measurement methods using Faraday effect, magnetostrictive effect and magnetostrictive effect of magnetohydrodynamics. It is also easy to realize the miniaturization and networking of the sensing system.

[0030] 2. The fiber optic low-frequency weak magnetic field detection sensing system based on the Sagnac-assisted reference interferometer reduces the low-frequency noise of the system, making the detection results of low-frequency weak signals more accurate.

[0031] 3. Based on the linear Sagnac auxiliary reference interference structure, the Mach-Zehnder fiber optic interference system has advantages such as simple structure, high stability and high sensitivity compared with the Michelson fiber optic interference system. Compared with the ring Sagnac interference structure, the linear fiber optic sensor does not need to shield ordinary optical fibers, which greatly improves its practicality. Attached Figure Description

[0032] Figure 1 Fiber Optic Low-Frequency Magnetism Weakening Measurement Sensing System Based on Sagnac Reference Interferometer

[0033] Figure 2 Reference Interferometric Demodulation Algorithm Block Diagram Detailed Implementation

[0034] The present invention will be further described below with reference to the accompanying drawings and examples.

[0035] Example 1:

[0036] like Figure 1 As shown, the fiber optic low-frequency weak magnetic field measurement sensing system based on the Sagnac reference interferometer includes a laser 1, a fiber optic isolator 2, a 1×2 coupler 3, fiber optic circulators 4 and 10, 2×2 couplers 5 and 11, a phase modulator 6 and 12, sensing fibers 7 and 13, Faraday rotator mirrors 8 and 14, a photodetector 9 and 15, and a data processing module 16.

[0037] The laser 1 is connected to the fiber optic isolator 2, and port 31 of the 1×2 coupler 3 is connected to the fiber optic isolator 2. The optical signal is split into two paths by the 1×2 coupler: one path has its output port 32 of the 1×2 coupler 3 connected to port 41 of the fiber optic circulator 4, port 42 of the circulator 4 connected to input port 52 of the 2×2 coupler 5, port 51 connected to the phase modulator 6, the phase modulator 6 connected to port 53 of the output of the 2×2 coupler 5, port 54 of the 2×2 coupler 5 connected to the sensing fiber 7, and the end of the sensing fiber 7 connected to the Faraday rotator mirror 8. After reflection by the Faraday rotator mirror 8, the optical signal is output from port 43 of the fiber optic circulator 4 and then... The electrical detector 9 is connected to form a sensing interferometer A; the output port 33 of another 1×2 coupler 3 is connected to port 101 of fiber optic circulator 10, port 102 of fiber optic circulator 10 is connected to port 111 of 2×2 coupler 11, port 112 is connected to phase modulator 12, phase modulator 12 is connected to port 114 of output of 2×2 coupler 11, port 113 of 2×2 coupler 11 is connected to sensing fiber 13, and the end of sensing fiber 13 is connected to Faraday rotator mirror 14. After reflection by Faraday rotator mirror 14, port 103 of output of fiber optic circulator 10 is connected to photodetector 15 to form a reference interferometer B.

[0038] In this embodiment, the sensing interferometer A is placed in a low-frequency weak magnetic field environment, and the sensing fiber 7 modulates the magnetic field signal into a refractive index signal; the reference interferometer B is placed in an external environment that shields the magnetic field but has the same other conditions as the sensing interferometer A.

[0039] The sensing fibers 7 and 13 are magnetorefractive fibers. Under the influence of a magnetic field, the refractive index of the magnetorefractive fiber will change, thereby realizing magnetic field sensing.

[0040] The phase modulators 6 and 12 are PZT piezoelectric ceramics. Single-mode optical fibers are wound around the PZT piezoelectric ceramics. A high-frequency carrier signal generated by a function generator is applied to the PZT piezoelectric ceramics, thereby modulating the phase of the interference output signal. The phase modulators 6 and 12 modulate the phase of the sensing interferometer A and the reference interferometer B, respectively.

[0041] In this embodiment, the optical path difference between the sensing interferometer A and the reference interferometer B is the same, and they are subjected to phase modulation with synchronization, same frequency, and same modulation depth.

[0042] In this embodiment, the interference signal generated by the sensing interferometer A is detected by the photodetector 9, and the interference signal generated by the reference interferometer is detected by the photodetector 15.

[0043] In this embodiment, the signal processing is based on the demodulation algorithm of the reference interferometer. It utilizes the principle that the OPD of the two interferometers is equal to cancel the phase noise of the system, reduce the system noise floor, and expand the dynamic range.

[0044] Example 2:

[0045] In this embodiment, the fiber optic low-frequency magnetic field weakening measurement and sensing system based on the Sagnac reference interferometer from Embodiment 1 is used.

[0046] In this embodiment, the magnetic field generator outputs a low-frequency weak magnetic field, which acts on the sensing fiber 7 in the sensing interferometer A, modulating the magnetic field signal into a refractive index signal, and generating a phase difference in the sensing interferometer A.

[0047] The phase modulator is activated, introducing the same phase carrier signal into both the sensing interferometer A and the reference interferometer B. The signal is detected by photodetectors 9 and 15, and the data is then processed by the data processing module 16. This data processing module is based on a reference interferometry algorithm, such as... Figure 2 .

[0048] In this embodiment, the data processing module 16 includes a multiplier, a low-pass filter, a subtractor, a divider, and an arctangent algorithm.

[0049] The photodetector receives the interference signals output by the sensing interferometer and the reference interferometer, I s It is the output light intensity of the sensor interferometer, I r It refers to the output light intensity of the reference interferometer.

[0050]

[0051] Among them, A s Let B be the DC component of sensor interferometer A. s For the AC component of sensor interferometer A, A r For the DC component of the reference interferometer B, B r The AC component of the reference interferometer B is used, and C is the modulation depth. The carrier angular frequency, The phase change is caused by the sensor signal and environmental noise. The phase change is caused by environmental noise. For the phase noise of the light source of sensor interferometer A, To reference the phase noise of the light source in interferometer B, I s With baseband carrier and second harmonic carrier The mixture is then passed through a low-pass filter and output.

[0052]

[0053] The same for I r Signal processing yields:

[0054]

[0055] Where J1(C) and J2(C) are the coefficients of the Bessel function.

[0056] For G=H, perform the following signal processing according to the reference interferometer algorithm;

[0057]

[0058] In the reference interferometry algorithm, the modulation depth needs to be controlled so that J1(C) = J2(C). The amplitudes of the sine and cosine correlation components output by the interferometer are normalized. Since the correlation parameters of sensing interferometer A and reference interferometer B are essentially the same in the sensing system, the phase noise generated by the light source is essentially equal, i.e. Under the above conditions, after signal processing, a pair of orthogonal signals can be obtained:

[0059]

[0060] Preferably, the modulation depth is controlled at C = 2.63 rad, and L1 is obtained by dividing L2:

[0061]

[0062] And perform arctangent calculations, since the reference interferometer is not placed in a magnetic field. Compared to It is very small. Therefore, theoretical analysis shows that, under strict control of the background noise of the reference interferometer, a low-frequency weak magnetic field sensing signal can be obtained.

[0063] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the invention. Therefore, all technical solutions obtained through equivalent substitution or transformation fall within the protection scope of the present invention.

Claims

1. A fiber optic low frequency weak magnetic field detection sensor system based on Sagnac auxiliary reference interferometer, comprising a laser (1), characterized in that: The laser beam emitted by the laser (1) enters the sensing interferometer A and the reference interferometer B respectively through the 1×2 coupler (3). The optical path difference between the sensing interferometer A and the reference interferometer B is the same, and they are synchronously, at the same frequency, and with the same modulation depth. The data processing module (16) collects the signals from the two interferometers and demodulates them. The photodetector receives the interference signals output by the sensing interferometer A and the reference interferometer B. s It is the output light intensity of the sensor interferometer, I r It is the output light intensity of the reference interferometer. A s Let B be the DC component of sensor interferometer A. s For the AC component of sensor interferometer A, A r For the DC component of the reference interferometer B, B r The AC component of the reference interferometer B is used, and C is the modulation depth. The carrier angular frequency, The phase change is caused by the sensor signal and environmental noise. Phase changes caused by environmental noise For the phase noise of the light source of sensor interferometer A, To reference the phase noise of the light source in interferometer B, I s With baseband carrier and second harmonic carrier The mixture is then passed through a low-pass filter and output. The same applies to I r The signal processing gives: Where J1(C) and J2(C) are the coefficients of the Bessel function; The data processing module (16) performs the following signal processing; , J1(C) = J2(C), G = H, resulting in a pair of quadrature signals: The modulation depth is controlled at C = 2.63 rad. Dividing L1 and L2 yields: And the inverse tangent calculation, get low frequency field sensing signal; The sensor interferometer A has the following structure: the beam emitted by the laser (1) enters the first port (41) of the circulator, the second port (42) of the circulator is connected in sequence to the 2×2 coupler, the sensing fiber and the Faraday rotating mirror, the 2×2 coupler has two ports connected to the phase modulator, and the photodetector is connected to the third port (43) of the circulator. The reference interferometer B is composed of the following structure: the beam emitted by the laser (1) enters the first port (101) of the circulator, the second port (102) of the circulator is connected in sequence to the 2×2 coupler, the sensing fiber and the Faraday rotating mirror, the 2×2 coupler has two ports connected to the phase modulator, and the photodetector is connected to the third port (103) of the circulator. The sensing fiber is a magnetorefractive-doped quartz fiber. The sensing interferometer A is placed in a low-frequency weak magnetic field environment, and the reference interferometer B is placed in a shielded magnetic field environment. The phase modulator is a PZT piezoelectric ceramic, with a single-mode optical fiber wound on it. A phase modulation carrier signal is generated by a function generator to synchronously modulate the phase of the sensing interferometer A and the reference interferometer B at the same frequency and modulation depth, while ensuring that the modulation depth C = 2.63 rad.

2. The fiber optic low-frequency weak magnetic field detection sensing system based on a Sagnac-assisted reference interferometer according to claim 1, characterized in that: The data processing module (16) includes a multiplier, a low-pass filter, a subtractor, a divider, and an arctangent algorithm.

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