Self-excited cesium optical pump atom magnetometer and magnetic field measurement method

Abstract

The invention provides a self-excited cesium optical pump atomic magnetometer and a magnetic field measurement method. The magnetometer comprises a physical probe part and a circuit part. The physical probe part comprises a cesium spectrum lamp, an optical element, a cesium atom absorption gas chamber, a radio frequency coil and a photoelectric detector; the circuit part comprises a photoelectric conversion circuit, a phase-locked amplification circuit and a radio frequency excitation circuit modulated by square waves. A macroscopic magnetic moment is generated by pumping cesium atoms through left-handed circularly polarized light, a radio frequency magnetic field induces Larmor precession, light intensity changes are detected and fed back to a radio frequency coil to form self-oscillation, and the external magnetic field intensity is calculated according to the linear relation between oscillation frequency and the magnetic field. Square wave modulation is adopted to broaden the measurement bandwidth, a magnetic field direction adjusting system is additionally arranged to automatically keep an angle of 45 degrees between an optical axis and a magnetic field so as to eliminate a blind area, the hemisphere automatic switching function is achieved, and the advantages of being high in sensitivity, fast in response, high in adaptability, small in size and low in power consumption are achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high-precision magnetic field measurement, and in particular to a self-excitation cesium optical pumping atomic magnetometer and a magnetic field measurement method. BACKGROUND

[0002] An atomic magnetometer is a high-sensitivity magnetic field measurement device based on the principle that atomic energy levels are split in a magnetic field and light interacts with atoms. An optical pumping atomic magnetometer pumps atoms to a specific energy level by circularly polarized light (pump light), so that the atomic system generates a macroscopic magnetic moment. By applying a radio frequency field resonant with the Larmor frequency, atomic magnetic resonance can be induced, and the macroscopic magnetic moment precesses, thereby modulating the absorption rate of the probe light. By detecting the change in transmitted light intensity, a magnetic resonance signal can be obtained, and the resonance frequency is proportional to the external magnetic field strength (ω=γB, Y is the gyromagnetic ratio), thereby realizing absolute measurement of the magnetic field.

[0003] Existing optical pumping magnetometers are mainly divided into self-excitation (SERF) and non-self-excitation (such as Mx magnetometer). Non-self-excitation magnetometers require an external scanning radio frequency field, and the system is relatively complex, and the dynamic response and bandwidth may be limited. The self-excitation magnetometer feeds back the detected magnetic resonance signal to the radio frequency coil, forming a self-excitation oscillation, and the oscillation frequency is directly equal to the Larmor frequency, which simplifies the system structure and improves the response speed.

[0004] However, the existing self-excitation atomic magnetometer still has some shortcomings: the heating system of the gas chamber usually generates additional magnetic field, introduces noise, and limits the further improvement of measurement sensitivity; when the optical axis is parallel or perpendicular to the magnetic field direction, the magnetometer signal will weaken or disappear, forming a measurement blind area, which limits the attitude adaptability of the device; the traditional feedback circuit based on sinusoidal modulation has a relatively narrow measurement bandwidth (about 20Hz), which is difficult to quickly track the dynamic changes of the magnetic field; the magnetic field directions of the north and south hemispheres of the earth are opposite, and different working configurations are required, and ordinary magnetometers are not convenient to use between different hemispheres.

[0005] In view of the above problems, the technical personnel in the field are committed to providing a high-performance self-excitation atomic magnetometer with high sensitivity, wide bandwidth, no blind area and adaptability to different regions around the world, and a magnetic field measurement method. SUMMARY

[0006] The purpose of the present application is to overcome the shortcomings of the prior art, and to provide a self-excitation cesium optical pumping atomic magnetometer with high sensitivity, large measurement bandwidth, small working blind area, and automatic switching between the north and south hemispheres, and a magnetic field measurement method.

[0007] To achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0008] In a first aspect, the application provides a self-excited cesium optical pumping atomic magnetometer, which comprises a physical probe part and a circuit part.

[0009] The physical probe part constitutes the sensing core of the magnetometer, which comprises:

[0010] Cesium spectral lamp: as a pumping light source, it emits a light beam containing 894 nm (D1 line).

[0011] Optical shaping unit: including convex lens, optical filter, polarizer and 1 / 4 wave plate. The convex lens is used for collimating the light beam; the optical filter is used for screening out the D1 line; the polarizer is used for generating linearly polarized light; and the 1 / 4 wave plate is used for converting the linearly polarized light into left-handed circularly polarized light.

[0012] Cesium atom absorption cell: filled with cesium atom vapor, which is the place where optical pumping and magnetic resonance occur.

[0013] Radio frequency coil: wrapped around the cell, used to generate a radio frequency magnetic field to excite magnetic resonance and receive feedback signals to maintain self-excited oscillation.

[0014] Photodetector: located behind the cell, used to detect changes in transmitted light intensity and convert them into electrical signals.

[0015] Magnetic-free heating system: used for heating the cesium atom cell, the heating coil of which is twisted into a double braid, so that the magnetic field generated by it is canceled out, greatly reducing the thermal noise.

[0016] The circuit part is responsible for signal processing and closed-loop control of the system, which comprises:

[0017] Optical-to-electrical conversion and amplification circuit: pre-amplifies the weak signal output by the photodetector.

[0018] Phase-locked amplification circuit: used to extract specific frequency signals related to magnetic resonance from noise and amplify them with high gain.

[0019] Radio frequency excitation circuit: generates a radio frequency power signal to drive the radio frequency coil according to the amplified signal. This circuit uses square wave modulation technology to increase the system measurement bandwidth from the traditional 20 Hz to 1 kHz.

[0020] Magnetic bias angle measurement and adjustment circuit: used to identify and switch working modes to adapt to different environmental magnetic field directions, ensuring the normal operation of the magnetometer worldwide.

[0021] Hemisphere conversion circuit: used to automatically switch the working phase according to the geographical location or magnetic field direction to adapt to the different magnetic field directions in the northern and southern hemispheres.

[0022] The optical path of the magnetometer of the application adopts a 45-degree angle design, that is, the light propagation direction and the direction of the magnetic field to be measured form a 45-degree angle, and an optical beam splitter is added in the optical path, which effectively improves the light pumping efficiency and eliminates the working blind area in the traditional design. At the same time, the method of square wave modulation greatly increases the measurement bandwidth of the magnetometer, making the magnetometer more sensitive. The combination of square wave modulation and 45-degree angle non-blind optical path design provides a self-excited atomic magnetometer with higher sensitivity, higher bandwidth and efficient measurement on the basis of conventional technology, which meets the needs of high-sensitivity measurement scenarios.

[0023] Further, the radio frequency excitation circuit adopts a square wave modulation technology with adjustable duty cycle.

[0024] In a second aspect, the application provides a method for measuring a magnetic field using the above-mentioned magnetometer, comprising the steps of:

[0025] The probe orientation is automatically adjusted by the magnetic field direction adjustment system so that the light propagation direction and the direction of the magnetic field to be measured form a 45-degree angle.

[0026] The cesium atoms are pumped by left-handed circularly polarized light to prepare quantum states.

[0027] The Larmor precession is induced by the radio frequency magnetic field.

[0028] The periodic change in transmitted light intensity caused by precession is detected and converted into an electrical signal.

[0029] After the electrical signal is amplified by a phase-locked amplifier, the radio frequency coil is driven by a square wave modulation circuit to form a self-excited oscillation.

[0030] The frequency of the self-excited oscillation is accurately measured, and the external magnetic field strength is calculated according to the formula B0 = f / 3.5 (nT / Hz).

[0031] The application has the following advantages:

[0032] (1) High sensitivity and low noise: The non-magnetic heating system effectively suppresses thermal noise, making the sensitivity reach the sub-pico Tesla level.

[0033] (2) Wide bandwidth and fast response: The square wave modulation with adjustable duty cycle replaces the traditional sine wave modulation, significantly increasing the magnetometer measurement bandwidth to more than 1 kHz, enabling fast response to dynamic magnetic field changes.

[0034] (3) The optical axis and the magnetic field form a 45-degree angle through the magnetic field direction adjustment system, eliminating the working blind area in the traditional design and improving the attitude adaptability and measurement accuracy of the device.

[0035] (4) Global adaptability: It has the functions of automatic recognition and switching of half spheres, and can work normally in different regions of the world without manual intervention.

[0036] (5) Excellent performance: wide measurement range (10,000-100,000 nT), covering the range of the geomagnetic field; fast transient response; compact structure, low power consumption, high reliability, and very suitable for applications such as airborne magnetic survey. BRIEF DESCRIPTION OF DRAWINGS

[0037] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the application and, together with the description, serve to explain the principles of the application.

[0038] Figure 1 The structure principle diagram of the physical probe part of the self-excited cesium atomic magnetometer of the application;

[0039] Figure 2 The system composition block diagram of the circuit part of the self-excited cesium atomic magnetometer of the application;

[0040] Figure 3 The linear relationship diagram of the self-excited oscillation frequency (Larmor frequency) and the external magnetic field strength;

[0041] Figure 4 The diagram of the sensitivity (noise level) measured by the magnetometer changing with the magnetic field;

[0042] Figure 5 The diagram of the light field and the magnetic field direction;

[0043] Figure 6 The principle diagram of the magnetic field direction adjustment system;

[0044] Figure 7 The principle diagram of the sine wave to square wave;

[0045] Figure 8 The winding diagram of the coil of the non-magnetic heating system;

[0046] Figure 9 The principle diagram of measuring the magnetic declination based on the magnetic sensing measurement chip.

[0047] Component description: 1, cesium spectral lamp (i.e. cesium spectral lamp); 2, convex lens; 3, optical filter; 4, polarizer; 5, 1 / 4 wave plate; 6, cesium atomic absorption cell (cesium absorption bubble); 7, radio frequency coil; 8, photodetector.

[0048] Through the above drawings, the specific embodiments of the application have been shown, and will be described in more detail in the following. These drawings and textual descriptions are not intended to limit the scope of the concept of the application in any way, but to illustrate the concept of the application to those skilled in the art by referring to specific embodiments. DETAILED DESCRIPTION

[0049] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application. Rather, they are merely examples of apparatuses and methods consistent with some aspects of this application as detailed in the appended claims.

[0050] Example 1

[0051] like Figure 1 and Figure 2 As shown, the self-excited cesium optical pump atomic magnetometer of this embodiment consists of a physical probe and a circuit box.

[0052] The physical probe is internally sealed, and its optomechanical structure is as follows: After the cesium spectral lamp 1 is ignited, the emitted light is converted into parallel light by the convex lens 2. After passing through the filter 3, only the D1 line at 894nm remains. This monochromatic light is converted into linearly polarized light by the polarizer 4. The linearly polarized light then passes through a quarter-wave plate 5, whose fast and slow axes are precisely adjusted to make the outgoing light left-handed circularly polarized light. This left-handed circularly polarized light is incident on the cesium atom absorption gas chamber 6. The gas chamber is heated by a non-magnetic heating system (such as... Figure 8 The light is maintained at the operating temperature (e.g., 50-60°C). An RF coil 7, wound around the air chamber, carries an RF current, generating an oscillating magnetic field perpendicular to the optical axis. Light transmitted through the air chamber is focused by a convex lens at the other end onto a photodetector 8 (such as a photodiode).

[0053] The circuit box integrates all circuits: the output of photodetector 8 is connected to photoelectric conversion and amplification circuit 9 for pre-amplification. The amplified signal is sent to lock-in amplifier circuit 10 for frequency selection and further amplification. The reference signal of the lock-in amplifier comes from its internal voltage-controlled oscillator (VCO). The processed signal is sent to the radio frequency excitation circuit. This circuit uses a square wave modulation strategy (e.g., ...). Figure 7 This generates sufficient power to drive the RF coil 7. The circuit then flows from coil 7 to detector 8, then to the circuit, and finally back to coil 7, forming a complete positive feedback closed loop. When the loop gain is greater than 1 and the phase shift is an integer multiple of 2π, the system generates stable self-oscillation at the Larmor frequency.

[0054] Magnetic field direction adjustment system (e.g.) Figure 6 Before operation: Start the magnetic sensing module (e.g., QMC5883L, see principle). Figure 9 To measure magnetic north, a direction-oriented module (such as a dual-antenna GPS) acquires true north. The processing unit calculates magnetic declination and then controls a servo mechanism (such as a stepper motor and worm gear) to adjust the probe's orientation, ensuring the light propagation direction forms a 45-degree angle with the geomagnetic field direction (e.g., ...). Figure 5 ).

[0055] The oscillation frequency is measured by a high-precision frequency meter. Ultimately, the magnetic field value is calculated and displayed by the embedded computer according to the formula B0 = f / 3.5. The entire system is automatically adapted to the working area by the semi-sphere conversion circuit.

[0056] Embodiment 2

[0057] On the basis of embodiment 1, the magnetic field direction adjustment system can realize the conversion of the magnetometer in different test environments, such as Figure 6 As shown in the magnetic deflection angle measurement circuit (the principle diagram for measuring the magnetic deflection angle based on the QMC5883L magnetic sensor measurement chip is shown in Figure 9 The deflection angle measurement circuit calculates the deflection angle adjustment data, and the non-magnetic rotating platform driver drives the non-magnetic rotating platform to rotate through the non-magnetic rotating platform driving rod under the control of the servo control system. The magnetometer placed on the non-magnetic platform rotates 360 degrees. The non-magnetic rotating platform is supported by a rotating bearing, the rotating mechanism adopts a worm gear structure, and the control system adopts a 57-step motor. The control accuracy reaches 0.01 degrees. During the rotation process, the direction of the magnetometer is adjusted by the servo mechanism to make the magnetic field direction and the magnetometer form a 45-degree angle, thereby improving the magnetic field measurement accuracy.

[0058] Embodiment 3

[0059] The magnetometer described in embodiment 1 is used for magnetic field measurement, including the following steps:

[0060] Start-up and initialization: power on the system, start the non-magnetic heating system, and heat the cesium atom chamber to the predetermined working temperature. Turn on the cesium spectrum lamp.

[0061] Automatic orientation: the magnetic field direction adjustment system is started, the magnetic deflection angle is measured, and the probe orientation is automatically adjusted so that the left-handed circularly polarized light propagation direction and the to-be-measured magnetic field direction form a 45-degree angle.

[0062] Optical pumping: the light emitted by the cesium lamp is shaped into left-handed circularly polarized light to pump the cesium atoms in the gas chamber. After a certain time of optical pumping, the cesium atoms are prepared to a specific energy level, generating a macroscopic magnetization along the magnetic field direction.

[0063] Induced precession and detection: the component with the same frequency as the Larmor frequency in the wideband radio frequency noise applied by the radio frequency coil induces the precession of the atomic magnetic moment. The precession modulates the transmitted light intensity, which is detected by the photodetector and converted into a weak electrical signal with a frequency of the Larmor frequency.

[0064] Establish self-oscillation: the weak electrical signal is amplified and filtered by the circuit system. The phase-locked amplifier accurately controls the phase of the signal to ensure that the signal fed back to the radio frequency coil forms a positive feedback. The system quickly establishes a stable self-oscillation at this frequency.

[0065] Measurement and calculation: The frequency meter accurately measures the oscillation frequency f at this time. The microprocessor calculates and outputs the magnetic field value B0 in real time according to the pre-stored calibration formula B0 = f / 3.5 (nT / Hz).

[0066] Dynamic tracking: When the external magnetic field changes, the Larmor frequency changes accordingly. The self-excited oscillation circuit automatically tracks this change through rapid phase and frequency adjustment, and stabilizes the oscillation at the new frequency, thereby realizing continuous, rapid and high-precision measurement of the magnetic field. If the hemisphere is crossed, the hemisphere conversion circuit automatically acts to ensure the correctness of the feedback phase and maintain the oscillation.

[0067] Example 4

[0068] Based on Example 1, this embodiment uses a duty cycle adjustable square wave modulation technology. By adjusting the duty cycle of the square wave, the output power of the pulse width modulation (PWM) signal can be optimized, signal attenuation and interference can be reduced, and the signal-to-noise ratio can be improved. The duty cycle adjustment can accurately control the average value of the output voltage or current, thereby optimizing the energy consumption and efficiency of the magnetometer.

[0069] The specific implementation steps are:

[0070] (1) Design a square wave generation circuit with adjustable duty cycle, consisting of:

[0071] Comparator circuit: converts the sine signal output by the phase-locked amplifier into a square wave.

[0072] PWM controller: such as 555 timer or MCU (such as STM32) PWM output module.

[0073] Analog switch or MOSFET driver: used to control the on-off of the radio frequency coil.

[0074] The implementation is: sine signal → comparator (such as LM2903) → fixed duty cycle square wave → PWM control chip (such as 555 or MCU) → adjustable duty cycle square wave → radio frequency power amplifier → radio frequency coil.

[0075] (2) Duty cycle control strategy.

[0076] Read the current oscillation frequency (i.e. Larmor frequency) or external magnetic field strength estimate value through the MCU. Dynamically adjust the square wave duty cycle according to the frequency range, for example:

[0077] Low frequency band (<10 kHz): duty cycle 50%;

[0078] Medium frequency band (10-50 kHz): duty cycle 40%;

[0079] High frequency band (>50 kHz): duty cycle 30%.

[0080] (3) Step 3: System integration and debugging.

[0081] Integrate the adjustable duty cycle square wave module into the radio frequency excitation circuit.

[0082] Calibrate the system bandwidth and sensitivity at different duty cycles through experiments.

[0083] Write a control algorithm to realize adaptive adjustment of the duty cycle.

[0084] Through Example 4, a self-excited atomic magnetometer with adjustable duty cycle is obtained. By adjusting the duty cycle of the square wave, the distribution of the radio frequency magnetic field energy can be optimized, the resonance conditions under different magnetic field strengths can be adapted, and the dynamic range and anti-interference ability of the system can be further improved.

[0085] The above describes only the preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

[0086] In the above-described embodiments 1-4, the description of each embodiment has its own focus, and the parts not described in detail in a certain embodiment can be referred to the relevant description of other embodiments. Each technical feature of the above-described embodiments can be combined arbitrarily. In order to make the description concise, all possible combinations of each technical feature in the above-described embodiments are not described, however, as long as the combination of these technical features does not exist contradictory, it should be considered as the scope disclosed in the specification.

[0087] Other embodiments of the application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. The application is intended to cover any variations, uses or adaptations of the application following, in general, the principles of the application and including such departures from the present disclosure as come within known or customary practice in the art to which the application pertains or can relate. The specification and examples are to be regarded as exemplary only, and the true scope and spirit of the application are indicated by the following claims.

[0088] It should be understood that the application is not limited to the precise construction that has been described above and shown in the accompanying drawings, and that various modifications and changes can be effected therein by those skilled in the art without departing from the scope of the application. The scope of the application is to be limited only by the appended claims.

Claims

1. A self-excited cesium optical-pumping atomic magnetometer comprising a physical probe portion and an electrical circuit portion; the physical probe portion comprising: Cesium spectral lamp (1); A convex lens (2), a filter (3), a polarizer (4), and a quarter-wave plate (5) are arranged sequentially along the optical path; Cesium atomic absorption chamber (6); radio frequency coil (7); photodetector (8); The circuit section includes: a photoelectric conversion and amplification circuit; a lock-in amplification circuit; and a radio frequency excitation circuit; characterized in that: The radio frequency excitation circuit is configured to drive the radio frequency coil (7) using square wave modulation, so that the measurement bandwidth of the magnetometer is significantly higher than that of a traditional magnetometer using sine wave modulation; and the physical probe part also includes a magnetic field direction adjustment system, which includes: a magnetic sensing module for measuring the direction of the ambient magnetic field and outputting magnetic north direction data; The orientation module is used to acquire true north direction data; The processing unit is configured to calculate the magnetic declination based on the magnetic north direction data and the true north direction data; the servo control mechanism is configured to adjust the orientation of the physical probe part according to the magnetic declination data output by the processing unit, so that the propagation direction of the left-hand circularly polarized light forms a preset 45-degree angle with the direction of the magnetic field to be measured.

2. The self-excited cesium optically pumped atomic magnetometer according to claim 1, characterized in that, The radio frequency excitation circuit (11) includes a square wave generation unit. The square wave generation unit converts the sinusoidal signal output by the lock-in amplifier circuit (10) into a square wave signal based on a comparator circuit, thereby driving and controlling the on / off state of the radio frequency coil (7).

3. The self-excited cesium optically pumped atomic magnetometer according to claim 1, characterized in that, The orientation module uses the BeiDou satellite navigation system or dual-antenna GPS orientation technology to obtain true north direction data.

4. The self-excited cesium optically pumped atomic magnetometer according to claim 1, characterized in that, The servo control mechanism includes a stepper motor and a worm gear transmission mechanism, with a control accuracy better than 0.1 degrees.

5. The self-excited cesium optically pumped atomic magnetometer according to claim 1, characterized in that, The circuit section also includes a hemispherical conversion circuit. The processing unit is further configured to control the hemispherical conversion circuit to switch the working phase of the self-excited oscillation circuit according to geographical location information or magnetic field direction characteristics, so as to adapt to the different magnetic field directions in the northern and southern hemispheres.

6. The self-excited cesium optically pumped atomic magnetometer according to claim 2, characterized in that, The square wave signal output by the comparator is converted into a square wave with an adjustable duty cycle by a PWM controller, and then the on and off of the radio frequency coil (7) is controlled by an analog switch or MOSFET.

7. The self-excited cesium optically pumped atomic magnetometer according to claim 6, characterized in that, The PWM controller is a 555 timer or an MCU.

8. The self-excited cesium optically pumped atomic magnetometer according to claim 7, characterized in that, The MCU reads the current oscillation frequency or the estimated value of the external magnetic field strength, and dynamically adjusts the square wave duty cycle according to the frequency range.

9. The self-excited cesium optically pumped atomic magnetometer according to claim 8, characterized in that, The strategy for dynamically modulating the duty cycle of the square wave based on the oscillation frequency range is as follows: a duty cycle of 50% is used in the low-frequency band below 10kHz; a duty cycle of 40% is used in the mid-frequency band of 10-50kHz; and a duty cycle of 30% is used in the high-frequency band above 50kHz.

10. A method for measuring the magnetic field based on the self-excited cesium optically pumped atomic magnetometer described in claims 1-9, characterized in that, Includes the following steps: Optical pumping steps: Excite the cesium spectral lamp (1) so that the light beam emitted by it is shaped into left-handed circularly polarized light by optical elements and irradiates the cesium atom absorption gas cell (6) so that the cesium atoms are prepared on specific Zeeman sublevels and generate macroscopic magnetic moments; Magnetic resonance steps: A radio frequency magnetic field is applied through a radio frequency coil (7) to cause the macroscopic magnetic moment to precess around the direction of the external magnetic field; Optical detection steps: The intensity of transmitted light that changes periodically due to Larmor precession is detected by photodetector (8) and converted into an alternating current signal; Signal processing and feedback steps: The AC signal is amplified and processed by photoelectric conversion and amplification circuit and lock-in amplifier circuit, and the RF coil (7) is driven by RF excitation circuit using a square wave modulation method with adjustable duty cycle to form self-excited oscillation; Frequency measurement and magnetic field calculation steps: Measure the oscillation frequency of the self-excited oscillation circuit when it is stable, and calculate the intensity B0 of the external magnetic field to be measured according to the formula B0=f / 3-5(nT / Hz), where f is the measured oscillation frequency; Prior to the optical pumping step, an automatic orientation step is also included: The magnetic north direction is measured and obtained through the magnetic sensing module; Obtain true north using the orientation module; Calculate the magnetic declination based on the magnetic north direction and the true north direction; Based on the magnetic declination, a servo mechanism is controlled to adjust the orientation of the magnetometer so that the propagation direction of the left-hand circularly polarized light forms a 45-degree angle with the direction of the magnetic field to be measured.

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