FPGA-based free precession vector atomic magnetometer excitation measurement system

Abstract

The application discloses a free precession type vector atomic magnetometer excitation measurement system based on FPGA and belongs to the technical field of magnetic field measurement, which comprises a laser unit, the laser unit is connected with an atomic magnetometer probe and a light pulse excitation unit respectively, a three-axis magnetic modulation excitation unit is connected with the atomic magnetometer probe through a three-axis magnetic coil, three-axis modulation coils are respectively arranged in X, Y and Z directions of the atomic magnetometer probe, a magnetic pulse excitation unit is connected with the atomic magnetometer probe through an excitation coil, the atomic magnetometer probe is connected with a three-axis vector free precession signal acquisition and processing unit, the three-axis magnetic modulation excitation unit, the magnetic pulse excitation unit, the light pulse excitation unit and the three-axis vector free precession signal acquisition and processing unit are all interactively connected with an upper computer, and the free precession type vector atomic magnetometer excitation measurement system based on FPGA has the advantages of good real-time performance, simple hardware structure, easy integration and low cost.

Description

Technical Field

[0001] This invention relates to the field of magnetic field measurement technology, and in particular to an FPGA-based free-precessing vector atomic magnetometer excitation measurement system. Background Technology

[0002] Vector measurements of magnetic fields can yield a wealth of useful information, which is of great significance for promoting scientific and technological progress and improving production efficiency. With the development of quantum precision measurement, atomic magnetometers have emerged. Their widespread application in the field of magnetic field vector measurement has spurred the development of high-performance, miniaturized, and low-cost technologies. The free-precession vector atomic magnetometer is a high-performance new type of magnetometer. Because it can directly extract the Larmor precession frequency from the time-domain signal without the need for closed-loop frequency tracking, it is simpler and more reliable in terms of hardware implementation and frequency extraction, and has greater potential for miniaturization. By applying the pump light, radio frequency magnetic field, and detection light separately, the influence of photobroadening and radio frequency broadening on the resonant linewidth is eliminated. During detection, the polarized atoms precess freely at the Larmor frequency without being affected by the pump light. Compared with the continuous pump mode, the system optical frequency shift is suppressed, improving the accuracy of magnetic field measurement. The magnetometer structure of a free-precessing vector atomic magnetometer can be based on that of a traditional atomic magnetometer. However, existing instruments and equipment struggle to effectively coordinate the excitation timing design and the three-axis vector solution timing design, as well as the real-time display function. Currently, there are many vector measurement methods based on atomic magnetometers, but they require very complex circuit design and cannot perform real-time magnetic field measurement. Summary of the Invention

[0003] The purpose of this invention is to provide an FPGA-based free-precession vector atomic magnetometer excitation and measurement system. This system can effectively excite the atomic magnetometer to generate a scalar free precession signal and a free precession signal modulated by the X, Y, and Z axes. Based on the excitation timing, the system performs vector processing on the four acquired Larmor precession frequencies and displays the magnitude and direction of the vector magnetic field amplitude in real time. This system has the advantages of good real-time performance, simple hardware structure, and convenient integration.

[0004] To achieve the above objectives, this invention provides an FPGA-based free-precession vector atomic magnetometer excitation and measurement system, comprising a laser unit, an atomic magnetometer probe, a host computer, an optical pulse excitation unit, a magnetic pulse excitation unit, a three-axis vector magnetic modulation excitation unit, and a three-axis vector free precession signal acquisition and processing unit. The laser unit is connected to both the atomic magnetometer probe and the optical pulse excitation unit. The three-axis vector magnetic modulation excitation unit is connected to the atomic magnetometer probe via three-axis modulation coils, which are respectively installed in the X, Y, and Z directions of the atomic magnetometer probe. The magnetic pulse excitation unit is connected to the atomic magnetometer probe via an excitation coil, and the atomic magnetometer probe is connected to the three-axis vector free precession signal acquisition and processing unit. The three-axis magnetic modulation excitation unit, the magnetic pulse excitation unit, the optical pulse excitation unit, and the three-axis vector free precession signal acquisition and processing unit are interactively connected to the host computer via the FPGA.

[0005] Preferably, the laser unit includes laser 1 and laser 2. Laser 1 outputs detection light to the atomic magnetometer probe, and laser 2 outputs pump light, which is input to the atomic magnetometer probe after passing through an AOM light intensity modulator.

[0006] Preferably, the magnetic pulse excitation unit includes an FPGA and a DAC1, wherein the FPGA controls the DAC1 to generate a magnetic pulse excitation signal to supply the excitation coil.

[0007] Preferably, the optical pulse excitation unit includes an FPGA and a DAC2, wherein the FPGA controls the DAC2 to generate an optical pulse excitation signal to supply the AOM optical intensity modulator.

[0008] Preferably, the three-axis vector magnetic modulation excitation unit includes an FPGA, DAC3, DAC4, and DAC5. The FPGA controls DAC3, DAC4, and DAC5 to generate X-axis, Y-axis, and Z-axis vector magnetic modulation signals, which are transmitted to the X-axis, Y-axis, and Z-axis modulation coils of the three-axis modulation coils respectively. DAC3 is connected to the Z-axis modulation coil, DAC4 is connected to the X-axis modulation coil, and DAC5 is connected to the Y-axis modulation coil.

[0009] Preferably, the three-axis vector free precession signal acquisition and processing unit includes an FPGA, a comparator, and a transimpedance amplifier. The atomic magnetometer probe generates four different frequencies of free precession signals, which are first amplified by the transimpedance amplifier, then shaped by the comparator, and then sent to the FPGA.

[0010] Preferably, the FPGA includes a three-axis vector magnetic field calculation module and a high-precision multi-channel digital phase-shifting frequency meter module. The high-precision multi-channel digital phase-shifting frequency meter module extracts the Larmor precession frequencies of four free precession signals and processes them in the three-axis vector magnetic field calculation module to obtain three-axis vector magnetic field information.

[0011] Preferably, the host computer sends instructions to the FPGA, and the FPGA uploads the three-axis vector magnetic field information to the host computer.

[0012] Preferably, the optical pulse excitation signal is a square wave signal with an adjustable duty cycle, the magnetic excitation signal is a high-frequency sine wave signal, and the signal waveforms of the X-axis, Y-axis, and Z-axis vector magnetic modulation signals are square wave synchronous pulse signals.

[0013] Preferably, the high-precision multi-channel digital phase-shifting frequency meter calls the PLL phase-locked loop inside the FPGA to convert the 50MHz base clock into a 1.5GHz clock, increasing the number of clock cycles to be tested.

[0014] Therefore, the present invention employs the above-mentioned FPGA-based free-precessing vector atomic magnetometer excitation and measurement system, which has the following beneficial effects:

[0015] (1) Compared with the traditional excitation system, the excitation system of the present invention effectively improves the bandwidth of the free precession atomic magnetometer; the measurement system uses FPGA to design a multi-channel phase-shifting frequency meter to improve the sensitivity index of the free precession optical pump atomic magnetometer, and simplifies the circuit structure and reduces the circuit volume. The three-axis vector solution system based on FPGA fully utilizes the high-speed processing characteristics of FPGA to solve the magnitude of the vector magnetic field in one step to the ns level.

[0016] (2) This invention can effectively excite atoms to generate free precession signals, designs a detection timing sequence, and realizes real-time measurement of the amplitude and direction of the three-axis vector magnetic field according to the detection timing sequence;

[0017] (3) Compared with the prior art, the present invention has the advantages of good real-time performance, simple hardware structure, easy integration and low cost.

[0018] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of an FPGA-based free-precessing vector atomic magnetometer excitation and measurement system according to the present invention.

[0020] Figure 2 This is a three-axis vector excitation timing diagram of an FPGA-based free-precession vector atomic magnetometer excitation measurement system according to the present invention.

[0021] Figure 3 This is a timing diagram of a three-axis vector measurement system for a FPGA-based free-precessing vector atomic magnetometer excitation measurement system according to the present invention.

[0022] Figure 4 This is a schematic diagram of the working principle of a high-precision multi-channel digital phase-shifting frequency meter for an FPGA-based free-precessing vector atomic magnetometer excitation measurement system. Detailed Implementation

[0023] The following detailed description of embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0024] Example

[0025] like Figure 1 As shown, this invention provides an FPGA-based free-precession vector atomic magnetometer excitation and measurement system, including a laser unit, an atomic magnetometer probe, a host computer, an optical pulse excitation unit, a magnetic pulse excitation unit, a three-axis vector magnetic modulation excitation unit, and a three-axis vector free precession signal acquisition and processing unit. The laser unit is connected to both the atomic magnetometer probe and the optical pulse excitation unit. The laser unit includes laser 1 and laser 2. Laser 1 outputs detection light to the atomic magnetometer probe, and laser 2 outputs pump light, which is modulated by an AOM optical intensity modulator and then input to the atomic magnetometer probe. The magnetic pulse excitation unit is connected to the atomic magnetometer probe via an excitation coil. The magnetic pulse excitation unit includes an FPGA and a DAC1. The FPGA controls DAC1 to generate a magnetic pulse excitation signal to supply the excitation coil to form a pulse, applying magnetic excitation to the atoms and causing the atoms in the atomic magnetometer probe to enter a free precession state. The optical pulse excitation unit includes an FPGA and a DAC2. The FPGA controls DAC2 to generate an optical pulse excitation signal to supply the AOM optical intensity modulator, which is used to control the pump light emitted by laser 2 to apply optical excitation to the atoms.

[0026] The three-axis vector magnetic modulation excitation unit is connected to the atomic magnetometer probe via a three-axis modulation coil. The three-axis modulation coil is installed in the X, Y, and Z directions of the atomic magnetometer probe. The atomic magnetometer probe is connected to the three-axis vector free precession signal acquisition and processing unit. The three-axis magnetic modulation excitation unit, magnetic pulse excitation unit, optical pulse excitation unit, and three-axis vector free precession signal acquisition and processing unit are interconnected with the host computer via FPGA. The three-axis vector magnetic modulation excitation unit includes an FPGA, DAC3, DAC4, and DAC5. The FPGA controls DAC3, DAC4, and DAC5 to generate X-axis, Y-axis, and Z-axis vector magnetic modulation signals, which are transmitted to the X-axis, Y-axis, and Z-axis modulation coils of the three-axis modulation coils, respectively. DAC3 is connected to the Z-axis modulation coil, applying a magnetic modulation signal in the Z-axis direction to the atoms, causing the atoms in the atomic magnetometer probe to enter a free precession state containing Z-axis vector information. DAC4 is connected to the X-axis modulation coil, applying a magnetic modulation signal in the X-axis direction to the atoms, causing the atoms in the atomic magnetometer probe to enter a free precession state containing X-axis vector information. DAC5 is connected to the Y-axis modulation coil, applying a magnetic modulation signal in the Y-axis direction to the atoms, causing the atoms in the atomic magnetometer probe to enter a free precession state containing Y-axis vector information. Figure 2 As shown, the magnetic modulation period is 4ms, and a 1ms magnetic excitation is applied in each process to make the atoms generate four different free precession signals to achieve three-axis vector calculation.

[0027] The difference between a free-precessing vector atomic magnetometer and a traditional vector atomic magnetometer lies in the different application methods of optical excitation pulses, magnetic excitation pulses, and three-axis vector magnetic field modulation signals, as well as the different forms of the final output signals. For example, a type that simultaneously applies optical and magnetic pulse excitation and sequentially applies three-axis vector magnetic modulation signals.

[0028] One cycle can be divided into four parts: optical pulse excitation, radio frequency magnetic field pulse excitation, three-axis vector magnetic modulation signal, and signal detection. First, the scalar field measurement method of the free-precessing atomic magnetometer is used, which is set to move along the Z, Y, and X axes respectively. The magnetic field to be measured, B0, is also along the Z axis. In the pumping phase, the atom is polarized by the pump light. In the steady state, the macroscopic magnetic moment M will be along the Z axis. This process can be described by the Bloch equation:

[0029]

[0030] After the atom is polarized to a steady state, M x =M y =0,M z =M0, at this point, the pump light is turned off, the radio frequency magnetic field RF is turned on, and a radio frequency signal of 2B1cos(ω0t) is applied along the Y-axis. Since the duration of the radio frequency signal is very short, relaxation can be ignored. This process requires adjusting B1 and t2 to make M zTo maximize the signal, a π / 2 pulse is generated when the initial value is 0. This process is solved in a rotating coordinate system, yielding the following results:

[0031] M x =-M0sin(γB1t);

[0032] M y =0;

[0033] M z =M0cos(γB1t);

[0034] When the radio frequency field is applied and a steady state is reached, M can be obtained. y =M z =0,M x =M0, using this as the initial value for the detection phase, and then removing the radio frequency pulse to enter the detection phase, considering the relaxation term, returning to the laboratory coordinate system, we can obtain:

[0035]

[0036] M z =0;

[0037] Ultimate M x Or M y The expression for the free precession signal is the same as the expression for the free precession signal.

[0038] Based on the scalar field measurement achieved by a free-precessing atomic magnetometer, triaxial vector measurement under geomagnetic conditions is realized by sequentially applying triaxial vector magnetic modulation signals. The excitation timing design in the excitation system is based on the principle of the free-precessing vector atomic magnetometer. The excitation principle in this embodiment is as follows:

[0039] like Figure 3 As shown, the three-axis vector magnetic modulation signal can effectively extract magnetic field vector information. First, the solution is... x When obtaining magnetic field information along the x-axis, based on the timing relationship of the three-axis vector magnetic modulation signals, the measurement system registers ω0 when the x-axis magnetic modulation signal is at its rising edge, and registers ω1 when the x-axis solution signal is at its falling edge. Finally, the magnetic field information of the x-axis is obtained through the three-axis vector free precession signal acquisition and processing unit. Next, when the y-axis solution signal is at its falling edge, ω2 is registered, and the magnetic field information of the y-axis is obtained through the three-axis vector free precession signal acquisition and processing unit. Finally, when the z-axis solution signal is at its falling edge, ω3 is registered, and the magnetic field information of the z-axis is obtained through the three-axis vector free precession signal acquisition and processing unit. This process is repeated periodically to obtain the three-axis vector magnetic field information.

[0040] When only radio frequency magnetic field pulse excitation is applied without a three-axis vector magnetic modulation signal, the magnetic field magnitude B0 is:

[0041]

[0042] At this time, the Larmor precession frequency ω0 in the acquired free precession signal is:

[0043]

[0044] When a radio frequency magnetic field pulse excitation and an X-axis magnetic modulation signal are applied simultaneously, B1 is:

[0045]

[0046] At this time, the precession frequency of the Larmor precession signal is ω1.

[0047]

[0048] Combining equations ω0 and ω1, we can obtain the magnitude of the magnetic field in the X direction as B. x ;

[0049]

[0050] Similarly, when a radio frequency magnetic field pulse excitation and a Y-axis magnetic modulation signal are applied simultaneously, B can be calculated. y

[0051]

[0052] Similarly, when a radio frequency magnetic field pulse excitation and a Z-axis magnetic modulation signal are applied simultaneously, B can be calculated. z

[0053]

[0054] The three-axis vector free precession signal acquisition and processing unit includes an FPGA, a comparator, and a transimpedance amplifier. After the excitation phase is completed, the detection phase begins. The detection light from laser 1 enters the atomic magnetometer probe, and the atomic-sensitive detection light in the gas chamber generates four different frequency free precession signals. After passing through the transimpedance amplifier, the comparator converts the free precession attenuation signal into a square wave signal, which is then sent to the FPGA. The FPGA internally includes a three-axis vector magnetic field calculation module and a high-precision multi-channel digital phase-shifting frequency meter module. The high-precision multi-channel digital phase-shifting frequency meter module extracts the Larmor precession frequencies of the four free precession signals, which are then processed in the three-axis vector magnetic field calculation module to obtain the three-axis vector magnetic field information. The host computer sends instructions to the FPGA, and the FPGA uploads the three-axis vector magnetic field information to the host computer.

[0055] The optical pulse excitation signal is a square wave signal with an adjustable duty cycle, the magnetic excitation signal is a high-frequency sine wave signal, and the signal waveforms of the X-axis, Y-axis, and Z-axis vector magnetic modulation signals are square wave synchronous pulse signals.

[0056] The excitation and detection phases are divided into four stages, which cycle periodically. The first stage begins after the scalar field excitation phase, when the detection light from laser 1 enters the atomic magnetometer probe. The atoms in the gas chamber then generate a freely precessing signal with oscillations and decay. The second stage begins after the excitation phase carrying X-axis magnetic field information, when the detection light from laser 1 enters the atomic magnetometer probe. The atoms in the gas chamber then generate a freely precessing signal with oscillations and decay. The third stage begins after the excitation phase carrying Y-axis magnetic field information, when the detection light from laser 1 enters the atomic magnetometer probe. The atoms in the gas chamber then generate a freely precessing signal with oscillations and decay. The fourth stage begins after the excitation phase carrying Z-axis magnetic field information, when the detection light from laser 1 enters the atomic magnetometer probe. The detection light from laser 1 enters the atomic magnetometer probe. The atoms in the gas chamber sense the detection light, generating a free precession signal carrying Z-axis magnetic field information that oscillates and decays. This signal is then output as a free precession voltage signal via a transimpedance amplifier. After preprocessing by a comparator module, square wave signals are obtained for the scalar field Larmor precession frequency, the X-axis magnetic field direction, the Y-axis magnetic field direction, and the Z-axis magnetic field direction. These signals are then fed into the FPGA, where gating is set as the detection stage. The FPGA handles timing, with the end of each π / 2 pulse serving as the gating activation flag. A high-precision multi-channel digital phase-shift frequency meter calls the FPGA's internal PLL to convert the 50MHz base clock to a 1.5GHz clock. Figure 4 As shown, the clock signal under test input to the high-precision comparator module is counted within the gate time to obtain cnt_c, and the high-frequency standard clock signal obtained by multiplying the FPGA system clock is counted to obtain cnt_l. For the known frequency f of the high-frequency standard clock signal... s Then the clock frequency f to be measured t It can be represented as:

[0057]

[0058] A high-precision multi-channel digital phase-shift frequency meter designed based on FPGA increases the number of clock cycles to be measured, thereby increasing the base clock frequency and improving the frequency measurement accuracy. Ultimately, four clock frequencies f to be measured are obtained. tThe data consists of four frequencies: scalar field Larmor precession frequency ω0, Larmor precession frequency ω1 (containing X-axis magnetic field information), Larmor precession frequency ω2 (containing Y-axis magnetic field information), and Larmor precession frequency ω3 (containing Z-axis magnetic field information). These four frequencies are then fed into the FPGA's three-axis vector calculation module. Based on vector calculation principles, the three-axis vector magnetic field information is obtained. This information can be uploaded to a host computer for real-time display and can be saved and exported for subsequent processing and analysis. Simultaneously, the host computer can send commands to the FPGA to exchange parameters such as waveform, frequency, phase, and duty cycle of the optical-magnetic excitation signal and the three-axis vector magnetic modulation signal. It also receives the amplitude and magnitude of the three-axis vector magnetic field uploaded by the FPGA for real-time display and has the function of saving and exporting magnetic field amplitude data for subsequent processing and analysis.

[0059] Therefore, this invention employs the aforementioned FPGA-based free-precession vector atomic magnetometer excitation and measurement system. This system effectively excites the atomic magnetometer to generate a scalar field free precession signal and X, Y, and Z three-axis vector solution field free precession signals. Based on the timing requirements of the three-axis vector modulation signals, four different Larmor free precession frequencies are extracted in real time and then calculated using vector measurement principles, ultimately achieving real-time measurement of the three-axis vector magnetic field amplitude. Existing instruments and equipment struggle to meet the timing requirements of free-precession vector mode excitation, detection, and three-axis vector magnetic modulation due to inconsistent clocks. While many vector measurement methods based on atomic magnetometers exist, they impose very complex circuit design requirements and cannot perform real-time magnetic field measurement. This invention offers advantages such as good real-time performance, simple hardware structure, easy integration, low cost, and high bandwidth.

[0060] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. An FPGA-based free-precessing vector atomic magnetometer excitation and measurement system, characterized in that: The system includes a laser unit, an atomic magnetometer probe, a host computer, an optical pulse excitation unit, a magnetic pulse excitation unit, a three-axis vector magnetic modulation excitation unit, and a three-axis vector free precession signal acquisition and processing unit. The laser unit is connected to both the atomic magnetometer probe and the optical pulse excitation unit. The three-axis vector magnetic modulation excitation unit is connected to the atomic magnetometer probe via three-axis modulation coils, which are respectively installed in the X, Y, and Z directions of the atomic magnetometer probe. The magnetic pulse excitation unit is connected to the atomic magnetometer probe via an excitation coil, and the atomic magnetometer probe is connected to the three-axis vector free precession signal acquisition and processing unit. The three-axis vector magnetic modulation excitation unit, the magnetic pulse excitation unit, the optical pulse excitation unit, and the three-axis vector free precession signal acquisition and processing unit are interconnected with the host computer via an FPGA. The three-axis vector magnetic modulation excitation unit includes an FPGA, DAC3, DAC4, and DAC5. The FPGA controls DAC3, DAC4, and DAC5 to generate X-axis, Y-axis, and Z-axis vector magnetic modulation signals, which are transmitted to the X-axis, Y-axis, and Z-axis modulation coils of the three-axis modulation coils respectively. DAC3 is connected to the Z-axis modulation coil, DAC4 is connected to the X-axis modulation coil, and DAC5 is connected to the Y-axis modulation coil. The three-axis vector free precession signal acquisition and processing unit includes an FPGA, a comparator, and a transimpedance amplifier. The atomic magnetometer probe generates four different frequencies of free precession signals, which are first amplified by the transimpedance amplifier, then shaped by the comparator, and then sent to the FPGA.

2. The FPGA-based free-precessing vector atomic magnetometer excitation and measurement system according to claim 1, characterized in that: The laser unit includes laser 1 and laser 2. Laser 1 outputs detection light to the atomic magnetometer probe, and laser 2 outputs pump light, which is input to the atomic magnetometer probe after passing through an AOM light intensity modulator.

3. The FPGA-based free-precessing vector atomic magnetometer excitation and measurement system according to claim 1, characterized in that: The magnetic pulse excitation unit includes an FPGA and a DAC1. The FPGA controls the DAC1 to generate a magnetic pulse excitation signal to supply the excitation coil.

4. The FPGA-based free-precessing vector atomic magnetometer excitation and measurement system according to claim 2, characterized in that: The optical pulse excitation unit includes an FPGA and a DAC2. The FPGA controls the DAC2 to generate an optical pulse excitation signal to supply the AOM optical intensity modulator.

5. The FPGA-based free-precessing vector atomic magnetometer excitation and measurement system according to claim 1, characterized in that: The FPGA includes a three-axis vector magnetic field calculation module and a high-precision multi-channel digital phase-shifting frequency meter module. The high-precision multi-channel digital phase-shifting frequency meter module extracts the Larmor precession frequencies of four free precession signals, which are then processed by the three-axis vector magnetic field calculation module to obtain the three-axis vector magnetic field information.

6. The FPGA-based free-precessing vector atomic magnetometer excitation and measurement system according to claim 1, characterized in that: The host computer sends instructions to the FPGA, and the FPGA uploads the three-axis vector magnetic field information to the host computer.

7. The FPGA-based free-precessing vector atomic magnetometer excitation and measurement system according to claim 1, characterized in that: The optical pulse excitation signal is a square wave signal with an adjustable duty cycle, the magnetic excitation signal is a high-frequency sine wave signal, and the signal waveforms of the X-axis, Y-axis, and Z-axis vector magnetic modulation signals are square wave synchronous pulse signals.

8. The FPGA-based free-precessing vector atomic magnetometer excitation and measurement system according to claim 5, characterized in that: The high-precision multi-channel digital phase-shifting frequency meter calls the internal PLL phase-locked loop of the FPGA to convert the 50MHz base clock into a 1.5GHz clock, increasing the number of clock cycles to be tested.

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