Optical fiber weak magnetic field sensor based on double-FP-cavity composite structure

Abstract

The invention discloses an optical fiber weak magnetic field sensor based on a double-FP cavity composite structure, which is characterized by comprising a first FP cavity and a second FP cavity, and an optical resonant cavity is formed by connecting the first FP cavity and the second FP cavity in series. According to the invention, the sensitivity of the sensor is greatly improved by using a vernier effect generated by superposition of transmission spectrums of the two FP cavities.

Description

Technical Field

[0001] The present invention relates to the field of magnetic field measurement technology, and in particular to a weak magnetic field detection sensor system. Background Art

[0002] The current representative weak magnetic field detection sensor systems are mainly divided into the following categories:

[0003] 1. Optically pumped magnetometers are optically pumped magnetometers that combine magnetic resonance technology with optical pumping technology. The principle of weak magnetic field detection is mainly to use the Zeeman effect of atoms to achieve magnetic field measurement. When the working material is affected by the external magnetic field, the atoms will produce energy level transitions, and the frequency of the transition is proportional to the external magnetic field. When the high-frequency excitation source is irradiated on the working material, the atoms will produce optical orientation or polarization. When the excitation frequency is equal to the transition frequency, magnetic resonance will occur. Therefore, the magnitude of the external magnetic field can be indirectly calculated by measuring the frequency of magnetic resonance. In 2003, the China Space Remote Sensing Center successfully developed a helium optically pumped magnetometer with international leading performance, working in the range of 3-7×104T, and a sensitivity of 10-12T / Hz1 / 2. This sensor is a scalar sensor that can only measure the magnitude of the magnetic field but cannot determine the direction of the magnetic field. It has the advantages of high sensitivity, high signal-to-noise ratio, and high precision. The disadvantages are that the probe manufacturing process is complex, energy consumption is high, and the volume is large.

[0004] 2. Fluxgate magnetometer

[0005] The magnetic core with high magnetic permeability reaches magnetic saturation under the action of external magnetic field. By measuring the driving current when the magnetic core reaches magnetic saturation, the size of the magnetic field can be determined. This is a relatively mature technology and has been applied to space technology in the United States. It can detect weak magnetic fields of about 10-10T at the minimum. It has the advantages of small size, simple manufacturing process, low energy consumption, compact and stable structure and large measurement range. The disadvantage is that the temperature stability of the magnetic core material is not good, the noise and temperature drift caused by the manufacturing process are obvious, and it is susceptible to electromagnetic interference.

[0006] 3. Superconducting quantum interference device

[0007] Among the currently known magnetic field sensors, the superconducting quantum interference device has the highest sensitivity for magnetic field detection, and its sensitivity can reach 10-14-10-15T / Hz1 / 2. Its working principle is: in superconductors, the tunnel current generated by the tunnel effect is a function that changes with the magnetic field periodically, and this characteristic is used to measure the magnetic field. Due to its ultra-high sensitivity, it is often used as a gradient sensor. This sensor has great potential in biological magnetic exploration, geological exploration and other directions, but it also has very obvious disadvantages. The main one is that the system needs to work in a superconducting state, and in order to achieve the superconducting state, the temperature needs to be lowered to -269℃. In order to meet the low-temperature working conditions, the volume of the system will be too large, inconvenient to move, high in price and difficult to maintain and manage. Therefore, the sensor currently only has the conditions to work in the laboratory.

[0008] 4. Fiber Optic Magnetic Field Sensor

[0009] Fiber optic magnetic field sensors are divided into fiber optic Faraday sensors based on magneto-optical rotation effect and fiber optic weak magnetic field sensors based on magnetostriction. These two sensors are the most mature and most concerned fiber optic magnetic field sensor structure systems in the current fiber optic magnetic field sensor technology. The working principle of the fiber optic weak magnetic field sensor made by magnetostrictive effect is as follows: the magnetostrictive material is coupled to the outer surface of the optical fiber of the sensing arm of the interferometer. Under the action of the external magnetic field, the magnetostrictive material undergoes axial strain along the axis of the optical fiber. Through coupling, the structural parameters of the fiber Fabry-Perot cavity change, thereby changing the transmission characteristics of light in the optical fiber. By detecting changes in light wave information such as phase and intensity, the size of the external magnetic field is calculated. Compared with other types of sensors, this sensor has extremely high sensitivity and is second only to the superconducting quantum interferometer. The current experiment has reached 7×10 -14 T / Hz 1 / 2 .

[0010] In recent years, researchers have found that when two sets of spectra have similar free spectral ranges (FSR), the vernier effect can be achieved by cascading, which can effectively amplify the sensitivity of the sensor by dozens of times. This phenomenon has attracted attention in the field of fiber optic sensing. The principle of the vernier effect is the same as that of a vernier caliper. The vernier caliper consists of a main scale and a vernier attached to the main scale. The scale periods of the main scale and the vernier are slightly different. During the measurement process, the vernier slides along the main scale and reads the overlapping scales between the two scales for measurement. In the vernier effect, the two sets of spectra correspond to the main scale and the vernier respectively. After the spectrum of the vernier is offset, the measurement is performed by reading the position where the interference peaks in the spectrum are aligned. This year, researchers from domestic universities and research institutes have proposed sensors based on the FP cavity and vernier effect principles. This type of sensor uses the vernier effect caused by the dual resonant cavity to obtain extremely high detection sensitivity, which is at least one order of magnitude higher than the sensitivity of the same type of sensor without the vernier effect. Summary of the invention

[0011] The technical problem to be solved by the present invention is to provide an optical fiber weak magnetic field sensor which is small in size, resistant to electromagnetic interference, resistant to corrosion, highly sensitive and low in cost.

[0012] In order to solve the above technical problems, the present invention provides an optical fiber weak magnetic field sensor based on a dual FP cavity composite structure, and the technical solution is as follows:

[0013] The optical fiber weak magnetic field sensor comprises a first FP cavity and a second FP cavity, and the optical resonant cavity is formed by connecting the first FP cavity and the second FP cavity in series.

[0014] Furthermore, it also includes a first single-mode optical fiber, a second single-mode optical fiber, a first bracket, a second bracket, a double-sided reflector, a second base, a first base, a first optical fiber fixing terminal, a second optical fiber fixing terminal, and a magnetostrictive material sleeve.

[0015] The first single-mode optical fiber and the second single-mode optical fiber pass through the first base and the second base respectively, and are fixed on the first bracket made of alumina ceramic material through the first optical fiber fixing terminal and the second optical fiber fixing terminal; the end faces of the first single-mode optical fiber and the second single-mode optical fiber are coated with a high reflection film to form an optical fiber end face with high reflectivity; a high reflection mirror is installed at one end of the second bracket, and the high reflection mirror is two optical parallel plates parallel to each other and coated with a high reflection film on the surface, and forms two FP cavities with the two optical fiber end faces, namely the first FP cavity and the second FP cavity. The first bracket and the second bracket are fixed on the first base and the second base respectively, and the first base and the second base are fixed on the two ends of the magnetostrictive material sleeve respectively.

[0016] The size of the magnetostrictive material sleeve in the sensor changes with the change of the weak magnetic field, resulting in changes in the cavity lengths of the first FP cavity and the second FP cavity. The change in the weak magnetic field is obtained by demodulating the offset of the vernier effect spectrum generated by superimposing the reflection spectra of the first FP cavity and the second FP cavity.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The present invention proposes an optical fiber magnetic field sensor based on a series double FP cavity composite structure and a vernier effect, which utilizes the vernier effect generated by the superposition of two FP cavity reflection spectra to greatly improve the sensitivity of the sensor. Compared with the vernier effect sensor composed of the traditional birefringence principle and two independent resonant cavity structures, the series double FP cavity composite structure can respond to the change of the magnetic field at the same time. Compared with the vernier effect sensor composed of the traditional birefringence principle and two independent resonant cavity structures, the sensitivity can be doubled, and the alumina ceramic bracket structure in the sensor can be used as a temperature compensation mechanism, and its thermal expansion coefficient is similar to that of magnetostrictive materials. In the absence of compensation measures, the error caused by the thermal expansion effect can be reduced by about one order of magnitude. Compared with weak magnetic field sensors with non-optical fiber structures such as optical pump magnetometers, fluxgate magnetometers, and superconducting quantum interferometers, the present invention has strong anti-electromagnetic interference ability, strong anti-corrosion and anti-pollution ability, and can be used in working scenes with large temperature differences. In addition, the optical fiber is a passive device with good self-independence, will not destroy the state of the measured object, and can adapt to work under flammable, explosive or toxic environmental conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The included drawings are used to provide a further understanding of the embodiments of the present invention, which constitute a part of the specification, are used to illustrate the embodiments of the present invention, and together with the text description, explain the principles of the present invention. Obviously, the drawings in the following description are only some embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0020] Figure 1 A schematic diagram of the structure of an optical fiber weak magnetic field sensor based on a double FP cavity composite structure provided according to a specific embodiment of the present invention is shown;

[0021] Figure 2 A schematic diagram showing changes in the magnetostrictive material sleeve, the second bracket and the double-sided reflector when the magnetic field is enhanced according to a specific embodiment of the present invention is shown;

[0022] Figure 3 A schematic diagram showing changes in the magnetostrictive material sleeve, the second bracket and the double-sided reflector when the magnetic field weakens according to a specific embodiment of the present invention is shown;

[0023] Figure 4 The figure shows the simulation of the sensor output spectrum with the Vernier effect characteristics and the schematic diagram of the reflection spectrum of the FP cavity.

[0024] The above drawings include the following reference numerals:

[0025] 1. First single-mode optical fiber; 2. Second single-mode optical fiber; 3. First alumina ceramic bracket; 4. Second bracket; 5. Double-sided reflector; 6. First FP cavity; 7. Second FP cavity; 8. Second base; 9. First base; 10. First optical fiber fixed terminal; 11. Second optical fiber fixed terminal; 12. Magnetostrictive material sleeve DETAILED DESCRIPTION

[0026] It should be noted that, in the absence of conflict, the embodiments in this application and the features in the embodiments can be combined with each other. The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present invention and its application or use. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0027] Figure 1 A schematic structural diagram of an optical fiber weak magnetic field sensor based on a dual FP cavity composite structure provided according to a specific embodiment of the present invention is shown, comprising a first single-mode optical fiber 1, a second single-mode optical fiber 2, a first bracket 3, a second bracket 4, a double-sided reflector 5, a first FP cavity 6, a second FP cavity 7, a second base 8, a first base 9, a first optical fiber fixed terminal 10, and a second optical fiber fixed terminal 11. The above components are installed in a magnetostrictive material sleeve 12, and the first base 9 and the second base 8 are respectively fixed at both ends of the magnetostrictive material sleeve 12.

[0028] The optical fiber weak magnetic field sensor is evolved from the Fabry-Perot interferometer. The first single-mode optical fiber 1 and the second single-mode optical fiber 2 pass through the first base 9 and the second base 8 respectively, and are fixed on the first bracket 3 made of alumina ceramic material through the first optical fiber fixed terminal 10 and the second optical fiber fixed terminal 11. The end faces of the first single-mode optical fiber 1 and the second single-mode optical fiber 2 are coated with a high reflection film to form an optical fiber end face with high reflectivity. The end face of the second bracket 4 made of alumina ceramic material is equipped with a high reflection double-sided reflector 5. The high reflection mirror 5 is two optical parallel plates parallel to each other and coated with a high reflection film on the surface. The distance from the two high reflectivity optical fiber end faces is about 100 microns, forming two high-fineness optical fiber FP cavities with high reflectivity, namely the first FP cavity 6 and the second FP cavity 7. The vernier effect spectrum generated by the superposition of the resonance spectrum of the first FP cavity 6 and the second FP cavity 7 is used to measure the weak magnetic field.

[0029] The working principle of the optical fiber weak magnetic field sensor provided by the present invention is as follows:

[0030] When a beam of coherent light is incident on the FP cavity, most of the incident light energy is reflected back and forth multiple times in the resonant cavity, thus forming optical resonance. Assume that the reflectivity of the two optical parallel plates is R and the incident light intensity is I 0 , the wavelength of the incident light is λ, and according to the principle of multi-beam interference, the reflected light intensity I R and the transmitted light intensity I T They are

[0031]

[0032] Where φ is the optical phase, we have:

[0033]

[0034] The two output lights of equation (2) are converged to form interference. At this time, the phase difference between the two FP cavity optical signals can be calculated by the following formula:

[0035]

[0036] L 1 and L 2 are the distances from the fiber end faces to the reflectors in the two FP cavities respectively, and the output light intensity of the sensor can be expressed as:

[0037]

[0038] P a and P r are the output light intensities of the two FP cavities respectively.

[0039] When the cavity lengths of the two FP cavities are slightly different, their resonant frequencies will also be different, that is, the free spectral range (FSR) will be different. Specifically, the ratio of the resonant frequency of the first FP cavity to the resonant frequency of the second FP cavity is N / (N+1), that is, the Nth interference peak in the reflection spectrum of the first FP cavity overlaps with the (N+1)th interference peak in the reflection spectrum of the second FP cavity, and the phase difference between the two optical signals at adjacent overlapping positions is Δφ=π. The spectrum generated after superposition is the output spectrum of the sensor (hereinafter referred to as the output spectrum). The position where the interference peaks overlap in the spectrum is regarded as the reference peak, and the wavelength position of the Nth reference peak can be expressed as:

[0040]

[0041] The spacing of the reference peaks is defined as the FSR of the output spectrum, which is:

[0042] FSR E =N×FSR a =(N+1)×FSR r (6)

[0043] Where FSR a and FSR r The FSR of the two FP cavity spectra is respectively used to understand how the envelope in the output spectrum is generated. The reflection spectra of the two FP cavities and the output spectra of the sensor are theoretically simulated. The simulation results are shown in Figure 4 As shown in the figure, the transmission intensity of the optical signal with matching resonant frequencies in the reflection spectra of the two FP cavities is least affected by superposition, and a high extinction ratio is retained in the spectrum. When the spectra of the two FP cavities shift, the matching resonant frequencies also change, causing the vernier effect spectrum to shift.

[0044] Figure 4 A schematic diagram of a sensor output spectrum simulation with a vernier effect characteristic and a reflection spectrum of a FP cavity provided according to a specific embodiment of the present invention is shown.

[0045] When the external magnetic field changes, the axial size of the magnetostrictive material sleeve will change, thereby driving the second bracket 4 and the double-sided reflector 5 on the bracket to move. Figure 2 As shown, when the external magnetic field increases, the axial size of the magnetostrictive material sleeve 12 will increase, thereby driving the second bracket 4 and the double-sided reflector 5 on the base to move. The cavity length of the series Fabry-Perot cavity (first FP cavity 6, second FP cavity 7) composed of the double-sided reflector 5 and the optical fiber end faces on both sides also changes with the movement of the double-sided reflector 5. When the double-sided reflector 5 moves to the left, the cavity length of the first FP cavity 6 decreases, while the cavity length of the second FP7 cavity increases. Figure 3As shown, when the external magnetic field weakens, the axial size of the magnetostrictive material sleeve 12 will decrease, thereby driving the second bracket 4 and the double-sided reflector 5 on the base to move. The cavity length of the series Fabry-Perot cavity (the first FP cavity 6 and the second FP cavity 7) composed of the double-sided reflector 5 and the optical fiber end faces on both sides also changes with the movement of the double-sided reflector 5. When the double-sided reflector 5 moves to the right, the cavity length of the first FP cavity 6 increases, and the cavity length of the second FP7 cavity decreases.

[0046] The change of the length of the first FP cavity and the second FP cavity causes the resonant spectral lines in the reflection spectra of the two FP cavities to shift. When the length of the first FP cavity and the second FP cavity increases and decreases respectively, the shift directions of the resonant spectra of the first FP cavity and the second FP cavity are opposite, resulting in a larger shift of the vernier effect spectrum generated by the superposition of the two resonant spectra. Compared with the vernier effect principle sensor with only one cavity involved in sensing, at least 2 times the sensitivity can be obtained;

[0047] The offset of the vernier effect output spectrum composed of the series FP cavity resonance spectrum can be amplified to several to dozens of times based on the offset of the FP cavity resonance spectrum. Therefore, the measurement sensitivity of the FP cavity can be improved by several to dozens of times. The change of the magnetic field can be obtained by demodulating the offset information of the output spectrum.

[0048] In addition, it should be noted that the use of terms such as "first" and "second" to limit components is only for the convenience of distinguishing the corresponding components. If not otherwise stated, the above terms have no special meaning and therefore cannot be understood as limiting the scope of protection of the present invention.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. An optical fiber weak magnetic field sensor based on a dual FP cavity composite structure, characterized in that: The optical resonant cavity comprises a first FP cavity (6) and a second FP cavity (7), wherein the first FP cavity (6) and the second FP cavity (7) are connected in series.

2. The optical fiber weak magnetic field sensor based on a dual FP cavity composite structure according to claim 1, characterized in that: It also includes a first single-mode optical fiber (1), a second single-mode optical fiber (2), a first bracket (3), a second bracket (4), a double-sided reflector (5), a second base (8), a first base (9), a first optical fiber fixing terminal (10), a second optical fiber fixing terminal (11) and a magnetostrictive material sleeve (12). The first single-mode optical fiber (1) and the second single-mode optical fiber (2) pass through a first base (9) and a second base (8) respectively, and are fixed to the first bracket (3) through the first optical fiber fixing terminal (10) and the second optical fiber fixing terminal (11); the end faces of the first single-mode optical fiber (1) and the second single-mode optical fiber (2) are coated with a high reflection film to form optical fiber end faces with high reflectivity; one end of the second bracket (4) is equipped with a high reflection mirror (5), the high reflection mirror (5) is two optical parallel plates parallel to each other and coated with a high reflection film on the surface, and forms two FP cavities with the two optical fiber end faces, namely a first FP cavity (6) and a second FP cavity (7); the first bracket (3) and the second bracket (4) are fixed to the first base (9) and the second base (8) respectively, and the first base (9) and the second base (8) are fixed to the two ends of a magnetostrictive material sleeve (12) respectively.

3. The optical fiber weak magnetic field sensor based on the dual FP cavity composite structure according to claim 2 is characterized in that: The size of the magnetostrictive material sleeve (12) changes with the change of the weak magnetic field, resulting in changes in the cavity lengths of the first FP cavity (6) and the second FP cavity (7). The change amount of the weak magnetic field is obtained by demodulating the offset of the vernier effect spectrum generated by superimposing the transmission spectra of the first FP cavity (6) and the second FP cavity (7).

4. The optical fiber weak magnetic field sensor based on the dual FP cavity composite structure according to claims 2 and 3 is characterized in that: The first bracket (3) and the second bracket (4) are made of alumina ceramic material.

5. The optical fiber weak magnetic field sensor based on a dual FP cavity composite structure according to any one of claims 1 to 4, characterized in that: The length of the first FP cavity (6) and the second FP cavity (7) is 100 microns.

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