Geomagnetic measurement rubidium optical pump magnetometer control system and method

Abstract

The invention belongs to the field of rubidium optical pump magnetometer control, and relates to a terrestrial magnetism measurement rubidium optical pump magnetometer control system and a terrestrial magnetism measurement rubidium optical pump magnetometer control method. The system comprises a 795nm pump laser, a 780nm detection laser, an alkali metal atom gas chamber, a non-magnetic heating module, a magnetic field coil, a photoelectric detector, a signal amplification conditioning module, an amplitude demodulation module, a high-speed frequency discrimination module, a phase error extraction module, a PID (Proportion Integration Differentiation) controller, a DDS (Direct Digital Synthesizer) reference signal and a high-precision constant current source, a magnetic field coil is arranged on the outer side of the non-magnetic heating module, light output by a 795nm pump laser and light output by a 780nm detection laser are emitted into an alkali metal atom gas chamber in an orthogonal mode, and light emitted by the alkali metal atom gas chamber enters a photoelectric detector and a signal amplification conditioning module. A frequency difference value is calculated according to the phase error through a PID controller and is fed back to a high-precision constant current source, and the high-precision constant current source outputs a current drive to a magnetic field coil to realize closed-loop stable control of the rubidium optical pump magnetometer.

Description

Technical Field

[0001] This invention belongs to the technical field of rubidium optical pump magnetometer control methods, specifically relating to a control system and method for a rubidium optical pump magnetometer for geomagnetic measurement. It is used for low-noise, high-precision driving, high-precision detection of output signals, and high-precision closed-loop control of the rubidium optical pump magnetometer for geomagnetic measurement. It is suitable for high-precision, small-volume prototype rubidium optical pump magnetometers for geomagnetic measurement, and is also suitable for the control of prototype alkali metal optical pump magnetometers such as cesium optical pumps. Background Technology

[0002] High-precision measurement and magnetic anomaly detection of the geomagnetic field have wide applications in military fields such as geomagnetic navigation and anti-submarine warfare, and there is also significant demand in civilian fields such as resource exploration. Alkali metal atomic optically pumped magnetometers have attracted widespread attention and research both domestically and internationally due to their advantages of high sensitivity, small size, and low power consumption. Currently, most research focuses on optically pumped magnetometers for weak magnetic field measurements, which typically have passive magnetic shielding devices around the sensor, making signal detection and closed-loop control relatively easy. However, geomagnetic optically pumped magnetometers operate in a geomagnetic environment, resulting in typically higher signal noise. Furthermore, when applied to geomagnetic matching navigation and anti-submarine warfare missions, high-precision, high-bandwidth output from the magnetometer sensor is required, especially in airborne applications where higher requirements are placed on size and power consumption. Therefore, a control method and system for geomagnetic optically pumped magnetometers is needed to achieve high-precision detection of geomagnetic signals and closed-loop system control. Simultaneously, the system must be small in size and low in power consumption; therefore, the design of each loop should be as simple and reusable as possible. Summary of the Invention

[0003] Purpose of the invention: This invention provides a control system and method for a rubidium optical pump magnetometer for geomagnetic measurement, which can solve the problem of low signal detection and closed-loop control accuracy and bandwidth caused by the excessively high operating frequency of the rubidium optical pump magnetometer. At the same time, the closed-loop circuit is simple to form, which is conducive to the miniaturization design of the prototype of the rubidium optical pump magnetometer for geomagnetic measurement.

[0004] Technical solution: A control system for a rubidium optically pumped magnetometer for geomagnetic measurement includes: a 795nm pump laser, a 780nm probe laser, an alkali metal atom gas cell, a non-magnetic heating module, a magnetic field coil, a photodetector and signal amplification and conditioning module, an amplitude demodulation module, a high-speed frequency discriminator module, a phase error extraction module, a PID controller, a DDS reference signal, and a high-precision constant current source. A non-magnetic heating module is installed outside the alkali metal atom gas chamber, and a magnetic field coil is installed outside the non-magnetic heating module. The light output from the 795nm pump laser and the 780nm probe laser is orthogonally incident into the alkali metal atom gas chamber. The light emitted from the alkali metal atom gas chamber enters the photodetector and signal amplification and conditioning module. The output of the photodetector and signal conditioning and amplification module is electrically connected to the input of the amplitude demodulation module, the input of the high-speed frequency discriminator module, and one input of the phase error extraction module, respectively. The output of the phase error extraction module is connected to the input of the PID controller. The output of the PID controller is connected to the input of the DDS reference signal module. The first output of the DDS reference signal module is connected to the other input of the phase error extraction module. The second output of the phase error extraction module is connected to the input of the high-precision constant current source. The output of the high-precision constant current source is connected to the magnetic field coil.

[0005] Furthermore, the alkali metal atom gas chamber is filled with Rb alkali metal atoms, buffer gas N2, and anti-relaxation gas H2. H2 reacts with Rb atoms to form an Rb-H anti-relaxation film, which increases the lifetime of Rb atoms.

[0006] Furthermore, the size of the Rb alkali metal atom gas chamber is .

[0007] Furthermore, the non-magnetic heating module consists of two multi-directional symmetrically wound resistance wires. The two resistance wires are symmetrically installed on both sides of the atomic gas chamber to achieve a near-non-magnetic heating effect where the magnetic fields cancel each other out when the current is applied to the two heating resistance wires. At the same time, the drive uses an AC current of up to 1~2MHz to reduce low-frequency noise and the magnetometer's sensitivity to the heating drive signal.

[0008] A control method for a rubidium optically pumped magnetometer for geomagnetic measurement, the method being executed by means of the aforementioned control system for a rubidium optically pumped magnetometer for geomagnetic measurement, the method comprising: The reference operating frequency of the time-pumped magnetometer was obtained in its initial state by frequency sweeping. : The reference operating frequency of the optical pump magnetometer The initial frequency used as the DDS reference signal; The reference working phase of the optical pump magnetometer is obtained by phase scanning. ; The reference working phase of the optical pump magnetometer As the excitation phase of a high-precision constant current source; The photodetector and signal amplification and conditioning module convert the optical signal after passing through the alkali metal atom gas cell into an electrical signal, and then amplify and condition the electrical signal. The amplitude demodulation module performs amplitude demodulation on the amplified and conditioned signal to obtain the signal. ; The high-speed frequency discrimination module performs high-speed frequency discrimination on the amplified and conditioned signal to obtain the frequency. ; The phase error extraction module mixes the amplified and conditioned signal with the DDS reference signal, and then extracts the phase difference of the magnetometer's electrical signal through a three-stage low-pass filter. ; PID controller based on phase difference Calculate the frequency difference It is then sent to the DDS reference signal module; The high-precision constant current source uses the DDS reference signal as a reference and superimposes the excitation phase. The current that maintains the resonance of iodine metal atom signals is supplied to the magnetic field coil to achieve closed-loop stable control of the rubidium optical pump magnetometer.

[0009] Furthermore, the amplitude demodulation module is specifically used to: perform bandpass filtering on the converted digital electrical signal from the magnetometer, with the bandpass set near the measurement frequency range of the optically pumped magnetometer; then perform digital inversion processing, that is, invert the portion of the magnetometer's sinusoidal high-frequency AC signal that is less than zero to above zero; and then pass it through a low-pass filter with a cutoff frequency as low as Hz to extract the magnetometer amplitude. .

[0010] Furthermore, the high-speed frequency discrimination module performs frequency discrimination on the converted magnetometer electrical signal. First, it passes the signal through a low-pass filter with a cutoff frequency on the order of MHz to remove the quantization error caused by the high-speed sampling of the A / D device. Then, it uses a clock signal with a frequency multiplied by a PLL, reaching up to 300MHz, for gated frequency discrimination to obtain the signal frequency. Because the gyromagnetic ratio of Rb atoms is 6998 At this point, the magnitude of the measured magnetic field can be directly calculated. B , can be represented as: .

[0011] Furthermore, the phase error extraction module performs correlation demodulation and phase detection on the converted magnetometer electrical signal, multiplies and mixes the magnetometer electrical signal with the DDS reference signal, and then performs a low-pass filter operation to obtain the phase error of the magnetometer electrical signal. This invention employs a three-stage low-pass filter. The filter settings comprehensively consider the frequency range and control bandwidth of the magnetometer signal, using a stepwise reduction method from a high sampling rate and high cutoff frequency to a lower sampling rate and low cutoff frequency. In particular, the lower sampling rate here is on the order of MHz, which serves two purposes: first, to sample the phase error signal with high precision, and second, to improve the response speed, thereby achieving high-precision, high-bandwidth phase error extraction. The multiplication and mixing operation can be mathematically expressed as:

[0012] In the formula After passing through a three-stage low-pass filter, the second harmonic and other high-frequency components can be filtered out, such as the gas chamber heating drive signal sensed by the magnetometer, resulting in... Within a certain range of difference Infinitely close to the phase difference between the magnetometer signal and the DDS reference signal .

[0013] Furthermore, the frequency difference of the PID control output satisfies: ,in, For real time i Output frequency difference value at any time. for i Time phase difference, for i -1 represents the phase difference at the previous moment. The phase difference between the two previous time points. K , , , All coefficients are constants; Utilizing the frequency difference output by the PID controller The magnitude of the magnetic field in the environment where the magnetometer is located can be calculated. In the formula, 10M represents the clock frequency of the DDS reference signal as 10MHz. If other clock frequency values ​​are used, their values ​​can be changed accordingly. The frequency difference is represented in FPGA using a 32-bit digital format.

[0014] Furthermore, the reference operating frequency of the light-pumped magnetometer in the initial state was obtained by frequency sweeping. With reference working phase Specifically: Set an initial excitation phase value for the high-precision constant current source. The frequency of the DDS reference signal remains unchanged; by changing the frequency, the amplitude of the magnetometer signal at that frequency can be obtained. Compare signal amplitudes Obtain the extreme value of amplitude The corresponding operating frequency ; The excitation phase of a high-precision constant current source is obtained by phase scanning. Specifically: Set the DDS reference signal frequency to the operating frequency. By continuously changing the initial value of the excitation phase of the high-precision constant current source, the amplitude of the magnetometer signal at this phase is obtained. Compare signal amplitudes Obtain the extreme value of amplitude The corresponding excitation phase .

[0015] Beneficial effects: A control method for a rubidium optically pumped magnetometer for geomagnetic measurement is provided. First, in open-loop mode, the initial optimal operating point of the rubidium optically pumped magnetometer is digitally and automatically scanned, including the reference operating frequency and reference operating phase. After writing the optimal operating point, it enters closed-loop operation. When the external magnetic field of the rubidium optically pumped magnetometer changes, the alkali metal atom resonance frequency changes accordingly with a fixed proportional coefficient. The detection laser signal after passing through the gas cell is sensed by the photodetector, and its frequency is the alkali metal atom resonance frequency. The phase difference between the alkali metal atom resonance frequency and the DDS reference signal can be obtained through the phase error extraction module. In particular, a filter design with a high sampling rate and high cutoff frequency to a lower sampling rate and low cutoff frequency is adopted to achieve high-precision and high-bandwidth phase error extraction. Finally, the frequency difference value calculated by the PID controller is fed back to the high-precision constant current source. The high-precision constant current source drives the current output to the magnetic field coil to achieve closed-loop stable control of the rubidium optically pumped magnetometer. Specifically, the system employs both a high-speed frequency discrimination method with magnetometer signal gating and a frequency difference calculation method based on the PID controller output. Both methods can calculate the magnitude of the magnetic field measured by the magnetometer, allowing for the selection of the more accurate method for sensor data output and analysis based on actual conditions. All signal processing and control are digitized within the FPGA, resulting in a simple structure, high module reusability, and ease of operation, meeting the miniaturization and high-precision requirements of rubidium optically pumped magnetometers for geomagnetic measurements. Furthermore, this invention can also be reused in cesium optically pumped magnetometers; since the gyromagnetic ratio of cesium alkali metal atoms is lower than that of rubidium, only the relevant frequencies need to be reduced. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. The drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] The accompanying drawings are provided to further illustrate the embodiments of the present invention. Since the main focus is on the control system and method of the rubidium optical pump magnetometer for geomagnetic measurement, some optical components that are widely used by those skilled in the art to form the optical path, such as collimating mirrors, polarizing mirrors, and reflecting mirrors, are not shown.

[0018] Figure 1 A schematic diagram of the control system of the rubidium optical pump magnetometer for geomagnetic measurement provided in an embodiment of the present invention is shown.

[0019] The above-mentioned figures include the following components: 1-795nm pump laser, 2-780nm detector laser, 3-alkali metal atom gas cell, 4-non-magnetic heating module, 5-magnetic field coil, 6-photodetector and signal amplification and conditioning module, 7-amplitude demodulation module, 8-high-speed frequency discrimination module, 9-phase error extraction module, 10-PID controller, 11-DDS reference signal module, 12-high-precision constant current source. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] The features and illustrative embodiments of various aspects of the present invention will now be described in detail. Numerous specific details are set forth in the following detailed description to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced without requiring some of these specific details. The following description of embodiments is merely intended to provide a better understanding of the invention by illustrating examples of the invention. The invention is by no means limited to any specific setups and methods set forth below, but covers any improvements, substitutions, and modifications to structures, methods, and devices without departing from the spirit of the invention. Well-known structures and techniques are not shown in the drawings and the following description to avoid unnecessarily obscuring the invention.

[0022] In the description of this invention, it should be noted that the directions or positional relationships indicated by terms such as "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer" are based on the directions or positional relationships shown in the accompanying drawings and are only for the convenience of describing and simplifying the invention, and should not be construed as limiting the invention. Furthermore, the use of ordinal numbers (e.g., "first and second," etc.) is for distinguishing objects and is not limited to this order, and should not be construed as indicating or implying relative importance.

[0023] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly, encompassing both direct connection and indirect connection via an intermediate medium. Those skilled in the art can understand the specific meaning of these terms in this invention based on the specific circumstances.

[0024] It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other, and the various embodiments can be referenced and cited in each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0025] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0026] This invention provides a control system for a rubidium optically pumped magnetometer for geomagnetic measurement, comprising a 795nm pump laser, a 780nm probe laser, an alkali metal atom gas cell, a non-magnetic heating module, a magnetic field coil, a photodetector and signal amplification and conditioning module, an amplitude demodulation module, a high-speed frequency discrimination module, a phase error extraction module, a PID controller, a DDS reference signal, and a high-precision constant current source. The photodetector converts the optical signal after passing through the alkali metal atom gas cell into an electrical signal, and then amplifies and conditions the electrical signal for amplitude adjustment and high-speed frequency discrimination. Simultaneously, the phase error extraction module mixes the amplified and conditioned signal with the DDS reference signal, and extracts the phase difference of the magnetometer's electrical signal through a three-stage low-pass filter. The PID controller calculates the frequency compensation control word and sends it to the DDS reference signal module. Based on the initial frequency value of this DDS reference signal, the frequency compensation code value output by the PID controller is superimposed with a fixed initial phase value. The high-precision constant current source outputs a current to maintain the resonance of the iodine metal atom signal, thereby achieving closed-loop stable control of the rubidium optically pumped magnetometer.

[0027] Furthermore, the alkali metal atom chamber is filled with Rb alkali metal atoms, buffer gas N2, and anti-relaxation gas H2. H2 reacts with Rb atoms to form an Rb-H anti-relaxation film, increasing the lifetime of Rb atoms. Specifically, in this patent embodiment, the size of the Rb alkali metal atom chamber is... It is the smallest Rb atom gas chamber in the current research data, which further requires the control system to be small in size and high in precision.

[0028] Furthermore, a non-magnetic heating module is installed outside the alkali metal atomic gas chamber. It consists of two multi-directional symmetrically wound resistance wires. The two resistance wires are symmetrically installed on both sides of the atomic gas chamber to achieve a near-non-magnetic heating effect where the magnetic fields cancel each other out when the current is applied to the two heating resistance wires. At the same time, the drive uses an AC current of up to 1~2MHz to reduce low-frequency noise and the magnetometer's sensitivity to the heating drive signal.

[0029] Furthermore, the photodetector and signal amplification and conditioning module convert the detection laser light signal passing through the atomic gas cell into an analog electrical signal. The analog electrical signal is filtered by a high-pass filter to remove DC and low-frequency signals, especially power frequency signals that are easily affected by the environment. At the same time, the signal is amplified and conditioned by a certain factor and then input to an A / D device (a commonly used device in the field, not shown in this patent) to convert it into a digital electrical signal.

[0030] Furthermore, amplitude demodulation, high-speed frequency discrimination, phase error extraction, PID controller, DDS reference signal, etc. are all completed in FPGA. They all process the digital electrical signals of the magnetometer, which is fast and accurate.

[0031] This invention also provides a control method for a rubidium optically pumped magnetometer for geomagnetic measurement. The method is executed using the aforementioned system. The control method includes: a 795nm pump laser; an alkali metal atom gas chamber, a photodetector, a signal amplification and conditioning module, and a phase error extraction module sequentially arranged along the beam propagation direction emitted from a 780nm probe laser; a non-magnetic heating module and a magnetic field coil arranged outside the alkali metal atom gas chamber; and an amplitude demodulation module and a high-speed frequency discrimination module arranged within the photodetector and signal conditioning and amplification module. The photodetector transmits the signal through the alkali metal atom gas chamber. The optical signal after passing through the metal atom gas cell is converted into an electrical signal. The electrical signal is then amplified and conditioned for amplitude demodulation and high-speed frequency discrimination. Simultaneously, the phase error extraction module mixes the amplified and conditioned signal with the DDS reference signal and extracts the phase difference of the magnetometer's electrical signal through a three-stage low-pass filter. The PID controller calculates the frequency compensation value and sends it to the DDS reference signal module. Using this DDS reference signal as a reference, a fixed initial phase value is superimposed. A high-precision constant current source outputs a constant current to maintain the resonance of the iodine metal atom signal, thus achieving closed-loop stable control of the rubidium optically pumped magnetometer.

[0032] Furthermore, the amplitude demodulation module performs bandpass filtering on the converted digital electrical signal from the magnetometer, setting the passband to near the measurement frequency range of the optically pumped magnetometer. Then, it performs digital inversion processing, flipping the less-than-zero portion of the magnetometer's sinusoidal high-frequency AC signal to above zero. Finally, it passes the signal through a low-pass filter with a cutoff frequency as low as Hz to extract the magnetometer amplitude. .

[0033] Furthermore, the high-speed frequency discrimination module performs frequency discrimination on the converted magnetometer electrical signal. First, it passes the signal through a low-pass filter with a cutoff frequency on the order of MHz to mainly remove the quantization error during high-speed sampling by the A / D device. Then, it uses a clock signal with a frequency multiplied by a PLL, reaching up to 300MHz, for gated frequency discrimination to obtain the signal frequency. Because the gyromagnetic ratio of Rb atoms is 6998 At this point, the magnitude of the measured magnetic field can be directly calculated. B , can be represented as:

[0034] Furthermore, the phase error extraction module performs correlation demodulation and phase detection on the converted magnetometer electrical signal, multiplies and mixes the magnetometer electrical signal with the DDS reference signal, and then performs a low-pass filtering operation to obtain the phase error of the magnetometer electrical signal. In particular, this invention uses a three-stage low-pass filter. The filter settings comprehensively consider factors such as the frequency range of the magnetometer signal and the control bandwidth, and adopt a stepwise reduction method from a high sampling rate and high cutoff frequency to a lower sampling rate and low cutoff frequency. In particular, the lower sampling rate here is on the order of MHz, which serves two purposes: first, to sample the phase error signal with high precision, and second, to improve the response speed, thereby achieving high-precision and high-bandwidth phase error extraction. The multiplication and mixing operation can be mathematically expressed as:

[0035] In the formula After passing through a three-stage low-pass filter, the second harmonic and other high-frequency components can be almost completely filtered out, such as the gas chamber heating drive signal sensed by the magnetometer, resulting in... Within a certain range of difference Infinitely close to the phase difference between the magnetometer signal and the DDS reference signal .

[0036] Furthermore, the PID controller employs an incremental PID controller to convert the phase difference obtained above... Calculations were performed to obtain the frequency difference. , can be represented as:

[0037] In the formula For real time i Output frequency difference value at any time. for i Time phase difference, for i -1 represents the phase difference at the previous moment. The phase difference between the two previous time points. The integral time constant is... is the differential time constant.

[0038] Specifically, data can be obtained from the system transfer function analysis and multiple debugging sessions during implementation. T and , The coefficient relationship between them, i.e. , , ,in , , All coefficients are constants. To determine the critical oscillation period under pure proportional control of the system, the PID controller parameter adjustments can be normalized to the tuning of a single parameter, which can be expressed as:

[0039] because , , The coefficients are fixed and unaffected by fluctuations in the system signal magnitude; therefore, in actual system operation, they only need to be adjusted according to the signal magnitude. This value simplifies the tuning of three PID parameters into a single parameter tuning. In particular, within a certain signal magnitude range, the system exhibits greater robustness. K The actual value may remain unchanged.

[0040] Furthermore, the DDS reference signal generates the reference sinusoidal high-frequency AC signal required for the magnetometer to maintain operation. ,in , The resonance frequency of alkali metal atoms. The initial operating frequency of the time-pumped magnetometer is the reference operating frequency, which can also be understood as the atomic resonance frequency point in the open-loop operating state of the magnetometer, ranging from 140 to 700 kHz, corresponding to a magnetic field measurement range of 20 to 100. Frequency is typically obtained through frequency sweeping. In particular, frequency sweeping is based on digital automatic scanning. During the frequency sweeping operation, an initial excitation phase value is set. The frequency of the DDS reference signal is changed by continuously alternating the frequency control word. This signal can then be applied to the magnetometer's excitation magnetic field coil via a high-precision constant current source, thereby obtaining the magnetometer signal amplitude at that frequency. Compare signal amplitudes The operating frequency corresponding to the extreme value of amplitude .

[0041] Excitation phase The phase is also acquired during the initial state through a phase scan, which is also done via digital automatic scanning. During the phase scan operation, the DDS reference signal frequency is set to the operating frequency obtained above. The initial phase value changes continuously back and forth. This phase is obtained by applying the excitation phase of a high-precision constant current source to the excitation magnetic field coil, thus acquiring the magnetometer signal amplitude at this phase. Compare signal amplitudes The excitation phase corresponding to the amplitude extremum is obtained The above. and These are important parameters for the initial operating point of the optically pumped magnetometer. It's important to note that this only illustrates the method for finding the initial operating point. In actual frequency and phase sweeps, the operation must be performed according to the actual signal conditions. This may require multiple alternating frequency and phase sweeps, and the order can also be varied to obtain a result closer to the optimal operating point. and .

[0042] Furthermore, the high-precision constant current source mainly outputs the sinusoidal high-frequency AC signal from the aforementioned DDS reference signal. The VI conversion is performed and output to the magnetic field coil to maintain the resonance state of alkali metal atoms, thereby realizing the closed-loop control of the rubidium optical pump magnetometer.

[0043] Furthermore, the initial operating frequency of the aforementioned DDS reference signal Frequency difference Atomic resonance frequency The excitation phase of the high-precision constant current source is set and generated digitally, with each of the three frequency control words being 32-bit. In the formula f This represents the frequency value. If the DDS clock is 10MHz, then the frequency resolution is... This means that the magnitude of the magnetic field that the hardware circuit can distinguish in real time at the excitation end is... This resolution is already far smaller than the nT level of current rubidium optical pump magnetometers used in geomagnetic measurements, thus fully meeting design and usage requirements, eliminating the need for higher-bit control words. Furthermore, this frequency control word accumulation control method has another major advantage: it allows direct use of the frequency compensation control word output by the PID controller. The magnitude of the magnetic field in the environment where the magnetometer is located can be calculated. In the formula, 10M represents the clock frequency of the DDS reference signal as 10MHz. If other clock frequency values ​​are used, their values ​​can be changed accordingly. The frequency difference is represented in FPGA using 32-bit digital format, due to the initial operating frequency value of the atom. It was obtained when the frequency scan started, and The PID controller provides real-time digital output, allowing for the calculation of the current ambient magnetic field magnitude using the formula described above. The excitation phase control uses a 16-bit control word with a theoretical resolution of [missing information]. This is also far less than the signal phase detection resolution of 0.1 of currently available rubidium optically pumped magnetometers. The scale is large enough to support high-precision closed-loop control of rubidium optical pump magnetometers for geomagnetic measurements.

[0044] Example: like Figure 1As shown, an embodiment of the present invention provides a control method for a rubidium optically pumped magnetometer for geomagnetic measurement. This control method includes a 795nm pump laser 1, and sequentially arranged along the beam propagation direction of a 780nm probe laser 2, comprising an alkali metal atom gas cell 3, a photodetector and signal amplification and conditioning 6, and a phase error extraction module 9. A non-magnetic heating module 4 and a magnetic field coil 5 are arranged outside the alkali metal atom gas cell 3. An amplitude demodulation module 7 and a high-speed frequency discrimination module 8 are arranged in the photodetector and signal amplification and conditioning module 6. The photodetector and signal amplification and conditioning 6 transmits the emitted light through a rubidium optically pumped magnetometer to the target area. The optical signal after the alkali metal atom gas chamber 3 is converted into an electrical signal. The electrical signal is then amplified and conditioned for amplitude adjustment and high-speed frequency discrimination. At the same time, the phase error extraction module 9 mixes the amplified and conditioned signal with the DDS reference signal 11 and extracts the phase difference of the magnetometer's electrical signal through a three-stage low-pass filter. The PID controller 10 calculates the frequency difference and sends it to the DDS reference signal module 11. Based on this DDS reference signal, a fixed excitation phase value is superimposed. The high-precision constant current source 12 outputs a current to maintain the resonance of the rubidium atom signal to the magnetic field coil, realizing the closed-loop stable control of the rubidium optical pump magnetometer.

[0045] This configuration provides a control method for a rubidium optically pumped magnetometer used in geomagnetic measurements. When the magnetic field of the environment around the magnetometer changes, the rubidium atomic resonance frequency changes by a fixed coefficient. At this time, the frequency of the detected magnetometer electrical signal differs from the frequency of the DDS reference signal from the previous moment. The phase error between the two signals is extracted, and the PID controller calculates this phase difference, outputting the calculated frequency difference to the DDS reference signal. The DDS reference signal can then track the alkali metal atomic resonance frequency in real time. A high-precision constant current source is applied to the magnetic field coil to maintain atomic resonance, thereby achieving real-time closed-loop control of the optically pumped magnetometer. All signal processing and control of the system are digitally completed in an FPGA, with a very simple structure, strong module reusability, and ease of operation, meeting the miniaturization and high-precision requirements of rubidium optically pumped magnetometers for geomagnetic measurements.

[0046] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A control system for a rubidium optical pump magnetometer for geomagnetic measurement, characterized in that, include: The system includes a 795nm pump laser, a 780nm detector laser, an alkali metal atom gas cell, a non-magnetic heating module, a magnetic field coil, a photodetector and signal amplification and conditioning module, an amplitude demodulation module, a high-speed frequency discriminator module, a phase error extraction module, a PID controller, a DDS reference signal, and a high-precision constant current source. A non-magnetic heating module is installed outside the alkali metal atom gas chamber, and a magnetic field coil is installed outside the non-magnetic heating module. The light output from the 795nm pump laser and the 780nm probe laser is orthogonally incident into the alkali metal atom gas chamber. The light emitted from the alkali metal atom gas chamber enters the photodetector and signal amplification and conditioning module. The output of the photodetector and signal conditioning and amplification module is electrically connected to the input of the amplitude demodulation module, the input of the high-speed frequency discriminator module, and one input of the phase error extraction module, respectively. The output of the phase error extraction module is connected to the input of the PID controller. The output of the PID controller is connected to the input of the DDS reference signal module. The first output of the DDS reference signal module is connected to the other input of the phase error extraction module. The second output of the phase error extraction module is connected to the input of the high-precision constant current source. The output of the high-precision constant current source is connected to the magnetic field coil.

2. The control system for the rubidium optical pump magnetometer for geomagnetic measurement according to claim 1, characterized in that, The alkali metal atom gas chamber is filled with Rb alkali metal atoms, buffer gas N2, and anti-relaxation gas H2. H2 reacts with Rb atoms to form an Rb-H anti-relaxation film, which increases the lifetime of Rb atoms.

3. The control system for the rubidium optical pump magnetometer for geomagnetic measurement according to claim 1, characterized in that, The size of the Rb alkali metal atom gas chamber is .

4. The control system for the rubidium optical pump magnetometer for geomagnetic measurement according to claim 1, characterized in that, The non-magnetic heating module consists of two multi-directional symmetrically wound resistance wires. The two resistance wires are symmetrically installed on both sides of the atomic gas chamber to achieve a near-non-magnetic heating effect where the magnetic fields cancel each other out when the current is applied to the two heating resistance wires. At the same time, the driving force adopts an AC current of up to 1~2MHz to reduce low-frequency noise and the sensitivity of the magnetometer to the heating driving signal.

5. A control method for a rubidium optical pump magnetometer for geomagnetic measurement, characterized in that, The method is executed by means of a rubidium optical pump magnetometer control system according to any one of claims 1 to 4, the method comprising: The reference operating frequency of the time-pumped magnetometer was obtained in its initial state by frequency sweeping. : The reference operating frequency of the optical pump magnetometer The initial frequency used as the DDS reference signal; The reference working phase of the optical pump magnetometer is obtained by phase scanning. ; The reference working phase of the optical pump magnetometer As the excitation phase of a high-precision constant current source; The photodetector and signal amplification and conditioning module convert the optical signal after passing through the alkali metal atom gas cell into an electrical signal, and then amplify and condition the electrical signal. The amplitude demodulation module performs amplitude demodulation on the amplified and conditioned signal to obtain the signal. ; The high-speed frequency discrimination module performs high-speed frequency discrimination on the amplified and conditioned signal to obtain the frequency. ; The phase error extraction module mixes the amplified and conditioned signal with the DDS reference signal, and then extracts the phase difference of the magnetometer's electrical signal through a three-stage low-pass filter. ; PID controller based on phase difference Calculate the frequency difference It is then sent to the DDS reference signal module; The high-precision constant current source uses the DDS reference signal as a reference and superimposes the excitation phase. The current that maintains the resonance of iodine metal atom signals is supplied to the magnetic field coil to achieve closed-loop stable control of the rubidium optical pump magnetometer.

6. The method according to claim 5, characterized in that, The amplitude demodulation module is specifically used to: perform bandpass filtering on the converted digital electrical signal from the magnetometer, with the bandpass set near the measurement frequency range of the optically pumped magnetometer; then perform digital inversion processing, which flips the less-than-zero portion of the magnetometer's sinusoidal high-frequency AC signal to above zero; and finally pass it through a low-pass filter with a cutoff frequency as low as Hz to extract the magnetometer amplitude. .

7. The method according to claim 5, characterized in that, The high-speed frequency discrimination module discriminates the converted magnetometer electrical signal. First, it passes the signal through a low-pass filter with a cutoff frequency in the MHz range to remove quantization errors caused by high-speed sampling of the A / D device. Then, it uses a clock signal multiplied by a PLL to reach up to 300MHz for gated frequency discrimination to obtain the signal frequency. Because the gyromagnetic ratio of Rb atoms is 6998 At this point, the magnitude of the measured magnetic field can be directly calculated. B , can be represented as: 。 8. The method according to claim 5, characterized in that, The phase error extraction module performs correlation demodulation and phase detection on the converted magnetometer electrical signal, multiplies and mixes the magnetometer electrical signal with the DDS reference signal, and then performs a low-pass filter operation to obtain the phase error of the magnetometer electrical signal. This invention uses a three-stage low-pass filter. The filter settings comprehensively consider the frequency range and control bandwidth of the magnetometer signal, and adopt a stepwise reduction method from high sampling rate and high cutoff frequency to lower sampling rate and low cutoff frequency. In particular, the lower sampling rate here is on the order of MHz, which is to ensure high-precision sampling of the phase error signal and to improve the response speed, thereby achieving high-precision and high-bandwidth phase error extraction. The multiplication and mixing operation can be mathematically expressed as: In the formula After passing through a three-stage low-pass filter, the second harmonic and other high-frequency components can be filtered out, such as the gas chamber heating drive signal sensed by the magnetometer, resulting in... Within a certain range of difference Infinitely close to the phase difference between the magnetometer signal and the DDS reference signal .

9. The method according to claim 5, characterized in that, The frequency difference of the PID control output satisfies: ,in, For real time i Output frequency difference value at any time. for i Time phase difference, for i -1 represents the phase difference at the previous moment. The phase difference between the two previous time points. K , , , All coefficients are constants; Utilizing the frequency difference output by the PID controller The magnitude of the magnetic field in the environment where the magnetometer is located can be calculated. In the formula, 10M represents the clock frequency of the DDS reference signal as 10MHz. If other clock frequency values ​​are used, their values ​​can be changed accordingly. The frequency difference is represented in FPGA using a 32-bit digital format.

10. The method according to claim 5, characterized in that, The reference operating frequency of the time-pumped magnetometer was obtained in its initial state by frequency sweeping. With reference working phase Specifically: Set an initial excitation phase value for the high-precision constant current source. constant; By changing the frequency of the DDS reference signal, the amplitude of the magnetometer signal at that frequency can be obtained. Compare signal amplitudes Obtain the extreme value of amplitude The corresponding operating frequency ; The excitation phase of a high-precision constant current source is obtained by phase scanning. Specifically: Set the DDS reference signal frequency to the operating frequency. By continuously changing the initial value of the excitation phase of the high-precision constant current source, the amplitude of the magnetometer signal at this phase is obtained. Compare signal amplitudes Obtain the extreme value of amplitude The corresponding excitation phase .

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