Rubidium atom magnetometer based on free induction attenuation and magnetic field measurement method

Abstract

The invention discloses a rubidium atom magnetometer based on free induction attenuation and a magnetic field measurement method. The magnetometer physical probe comprises a VCSEL laser, an optical shaping unit composed of a convex lens, a polaroid and a 1 / 4 wave plate, a rubidium atom gas chamber, a radio frequency coil assembly and a photoelectric detector. The circuit part comprises a signal conditioning circuit and an oscillation feedback circuit. The method comprises the following steps: working in a periodic pulse mode, firstly starting a laser and radio frequency coil in a period, and performing optical pumping and polarization on rubidium atoms by using left-hand circularly polarized light and a polarized magnetic field along an optical axis; then, the polarized magnetic field is rapidly closed within microsecond magnitude time, so that the atoms spin and freely precession in the external magnetic field to be measured, and an FID signal is generated; after being detected by the photoelectric detector, the signals are fed back to the radio frequency coil through amplification and phase locking processing, and self-oscillation with the frequency locked at the atomic Larmor precession frequency is formed; and measuring the oscillation frequency and calculating the external magnetic field intensity according to the gyromagnetic ratio. The invention has the advantages of high sensitivity, large bandwidth, fast response and small volume.

Description

Technical Field

[0001] This application relates to the field of high-precision magnetic field measurement technology, and in particular to a rubidium atomic magnetometer and magnetic field measurement method based on free induction decay, which is especially suitable for precision magnetic field measurement in highly dynamic environments. Background Technology

[0002] An atomic magnetometer is a highly sensitive magnetic field measurement device based on the Zeeman splitting of atomic energy levels in a magnetic field and the interaction between light and atoms. Optically pumped atomic magnetometers use circularly polarized light (pump light) to pump atoms to specific energy levels, causing the atomic system to generate a macroscopic magnetic moment. By detecting the Larmor precession frequency of this macroscopic magnetic moment in an external magnetic field, an absolute measurement of the magnetic field can be achieved (ω = γB, where γ is the gyromagnetic ratio).

[0003] To improve response speed and simplify the system, self-excited oscillating atomic magnetometers (such as SERF or Mx modes) have been widely used. Their basic principle is as follows: the detected weak optical signal reflecting atomic precession (i.e., Larmor frequency) is amplified and appropriately phase-shifted, then fed back to a coil that generates a radio frequency magnetic field, thus forming a closed-loop oscillation circuit. The stable oscillation frequency of this circuit is equal to the Larmor frequency, and the magnetic field can be directly calculated by frequency counting.

[0004] However, existing mainstream self-oscillating atomic magnetometers typically operate in continuous wave or quasi-continuous modulation modes. In these modes, the radio frequency magnetic field used to excite or maintain magnetic resonance overlaps or alternates with the optical detection used to detect changes in atomic states in time. This operating mode leads to several inherent drawbacks that limit further improvements in magnetometer performance:

[0005] (1) There are bottlenecks in signal-to-noise ratio and sensitivity. During optical detection, the continuous radio frequency magnetic field will introduce additional electromagnetic noise and may directly interfere with the sensitive photoelectric detection circuit through stray coupling. At the same time, the thermal noise of the current of the radio frequency coil will also be introduced into the detection bandwidth. These "online" noises fundamentally limit the reduction of the system's background noise and become the main obstacle to improving sensitivity.

[0006] (2) Limited measurement bandwidth: Traditional self-excited oscillation systems are based on the principle of continuous feedback, and their effective bandwidth is usually limited by the response speed of the electronic feedback loop and the relaxation time of the atomic spin itself. In order to maintain stable oscillation, the system gain and phase adjustment are usually conservative, resulting in a slow dynamic response and a narrow measurement bandwidth (usually only on the order of 20-100 Hz), making it difficult to accurately track rapidly changing magnetic fields.

[0007] (3) Signal interpretation complexity and error: When pump / probe light and radio frequency field are present at the same time, the dynamic process of atomic system is complex. The signal may contain multiple frequency components and modulation sidebands, requiring complex demodulation algorithms, which not only increases the system complexity but may also introduce additional demodulation errors.

[0008] To address the aforementioned issues, particularly overcoming the limitations of signal-to-noise ratio and bandwidth, those skilled in the art have sought new working principles. Free induction decay (FID) is a mature technique in the field of nuclear magnetic resonance (NMR), referring to the free precession process of a spin system after short-pulse excitation without interference from an excitation field. Introducing the FID concept into atomic magnetometers theoretically allows for complete temporal separation of excitation and detection, potentially yielding signals with extremely high purity within the detection window.

[0009] However, simply applying the FID pulse mode to magnetic field measuring instruments faces a core contradiction: FID signals are transient and discontinuous, while engineering applications often require continuous and stable magnetic field readings. How to construct a closed-loop measurement system that can both utilize the high purity of FID signals and achieve long-term stable, self-sustaining oscillation is a pressing technical challenge in this field.

[0010] Existing technologies lack an effective systematic solution to seamlessly embed the pulse FID detection mechanism into a high data rate, high precision self-excited oscillating magnetometer architecture. Summary of the Invention

[0011] The purpose of this invention is to overcome the technical shortcomings of existing self-excited oscillating atomic magnetometers in continuous operation mode, namely low signal-to-noise ratio and narrow bandwidth, and to provide a rubidium atomic magnetometer and measurement method based on the principle of free induction decay. This method, through innovative pulse timing control and closed-loop feedback design, resolves the contradiction between the transient FID signal and the continuous stable measurement output, achieving high sensitivity, wide bandwidth, and fast response magnetic field measurement.

[0012] To achieve the above objectives, the present invention adopts the following technical solution:

[0013] In a first aspect, the present invention provides a rubidium atomic magnetometer based on free induction decay, comprising a physical probe part and a circuit part.

[0014] The physical probe section includes: a laser emitting unit, employing a VCSEL laser, for emitting a 795nm laser beam with a wavelength resonating with the D1 line of rubidium atoms; an optical shaping unit, disposed in the optical path of the VCSEL laser, consisting of a convex lens, a polarizer, and a quarter-wave plate arranged sequentially along the optical path, for converting the laser beam into left-handed circularly polarized light; a rubidium atom gas cell, disposed on the light-emitting side of the optical shaping unit, for generating atomic spin polarization under the irradiation of the left-handed circularly polarized light; a radio frequency coil assembly, wound around the rubidium atom gas cell, for generating a magnetic field along the optical axis within the rubidium atom gas cell; and a photoelectric detection unit, disposed on the light-emitting side of the rubidium atom gas cell, for detecting the intensity of transmitted light and outputting a corresponding electrical signal.

[0015] The circuit section includes: a signal conditioning circuit connected to the photoelectric detection unit for amplifying and filtering the electrical signal; and an oscillation feedback circuit connected to the signal conditioning circuit for generating a radio frequency drive signal based on the processed electrical signal. The radio frequency drive signal is fed to the radio frequency coil assembly, thereby forming a closed-loop self-excited oscillation circuit in the magnetometer. The oscillation feedback circuit is configured to operate in a pulse-timed manner, first driving the radio frequency coil assembly to generate a polarized magnetic field for a period of time within one measurement cycle, then turning off the polarized magnetic field and entering the free-induction attenuation signal detection stage.

[0016] Preferably, the oscillation feedback circuit is configured to: generate a polarization magnetic field that lasts for a first preset duration within one cycle, and then quickly shut down the polarization magnetic field within a second preset duration, wherein the second preset duration is much shorter than the first preset duration.

[0017] More preferably, the first preset duration is on the order of hundreds of microseconds, and the second preset duration is on the order of microseconds or sub-microseconds. More specifically, the first preset duration is 500 microseconds, and the second preset duration is 1 microsecond.

[0018] Preferably, the radio frequency coil assembly is a Helmholtz coil wound on the gas chamber, and a compensating Helmholtz coil is added in the opposite direction of the gas chamber Helmholtz coil to cancel and compensate for the residual magnetic field of the gas chamber Helmholtz coil, so that there is no interference from the residual magnetic field when measuring the Larmor precession frequency.

[0019] Preferably, the magnetometer further includes a non-magnetic heating system for heating the rubidium atom gas chamber, wherein the heating coil of the non-magnetic heating system is wound with twisted-pair wire.

[0020] Preferably, the signal conditioning circuit includes a preamplifier and a lock-in amplifier.

[0021] Secondly, the present invention provides a method for measuring atomic magnetic fields based on free induction decay, implemented using the aforementioned rubidium atomic magnetometer, the method comprising the following steps performed periodically:

[0022] S1. Polarization Preparation Stage: A left-handed circularly polarized pump laser and a polarization magnetic field along the laser propagation direction are applied to the rubidium atom gas cell, polarizing the rubidium atom spins in the gas cell to align with the direction of the polarization magnetic field. S2. Free-Induction Decay Excitation Stage: After the polarization magnetic field has been applied for a preset time, it is rapidly removed within a time period much shorter than the preset time. S3. Optical Detection Stage: After the polarization magnetic field is removed, the intensity of transmitted light passing through the rubidium atom gas cell is detected. The transmitted light intensity contains an AC signal component due to the modulation effect of the freely precessing rubidium atom spins on the light, and the intensity change is converted into a first electrical signal. S4. Signal Processing and Feedback Stage: The first electrical signal is amplified and frequency-selectively processed to extract the signal corresponding to the atomic Larmor precession frequency. The processed signal is used to generate a radio frequency drive signal for the next cycle, S1 stage. S5. Frequency Calculation Stage: The stable oscillation frequency f of the closed-loop self-excited oscillation formed in the signal processing and feedback stage is measured, and the frequency is calculated using the formula B0 = f / The strength B0 of the external magnetic field to be measured is calculated by γ, where γ is the gyromagnetic ratio of rubidium atoms; wherein, the radio frequency driving signal generated in steps S1 and S4 is the source of driving the generation of the polarization magnetic field, and steps S1 to S4 form a closed-loop self-excited oscillation circuit.

[0023] Preferably, the application time of the polarizing magnetic field is a first preset time, and the time for removing the polarizing magnetic field is a second preset time; wherein the first preset time is on the order of hundreds of microseconds, and the second preset time is on the order of microseconds or sub-microseconds. More preferably, the first preset time is 500 microseconds, the second preset time is 1 microsecond, and the period of the periodic execution is 1 millisecond.

[0024] Preferably, the gyromagnetic ratio γ is 6.998 Hz / nT.

[0025] Preferably, in the signal processing and feedback stage, a lock-in amplifier is used to process the first electrical signal to extract the signal corresponding to the Larmor precession frequency.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] (1) Ultra-high signal-to-noise ratio and sensitivity: The pulse free induction attenuation working mode is adopted, and the radio frequency field is completely shut off during the optical detection stage, eliminating the electromagnetic noise and optical interference brought by the radio frequency field itself, and obtaining a detection signal with extremely high purity. Combined with the non-magnetic heating system, the system has extremely low background noise, and the measured sensitivity can reach the order of 0.6 pT / √Hz.

[0028] (2) Wide measurement bandwidth and fast instantaneous response: Pulse timing control and fast closed-loop feedback design enable the system to operate at a high sampling rate (e.g., 1 kHz) and respond quickly to magnetic field changes in each cycle. The effective measurement bandwidth of the system can be extended to more than 500 Hz, far exceeding that of traditional continuous wave self-excited oscillating magnetometers (about 20 Hz), and can accurately track dynamically changing magnetic fields.

[0029] (3) The contradiction of FID mode engineering application was resolved: Through ingenious timing control (polarization -> fast shutdown -> detection) and closed-loop feedback loop design, the transient and discontinuous FID signal was successfully converted into a long-term stable and continuous output self-excited oscillation frequency, realizing the engineering application of high-performance pulsed atomic magnetometer.

[0030] (4) Compact structure and high reliability: The system uses the same laser beam and the same set of coils, and time-division multiplexing is achieved through timing control, which simplifies the optical path and structure. By using devices such as VCSEL and specific optical shaping units, the entire probe is small in size, low in power consumption, and highly adaptable to the environment, making it suitable for applications such as airborne magnetic measurement and mobile platform mounting.

[0031] (5) Absolute measurement and wide range: Based on the absolute frequency measurement of atomic gyromagnetic ratio, it has high accuracy and requires no external calibration. The measurement range is wide (10,000 to 150,000 nT), which can fully cover the global geomagnetic field range and changes. Attached Figure Description

[0032] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0033] Figure 1 This is a system block diagram of a rubidium atomic magnetometer based on free induction decay according to an embodiment of the present invention; Figure 2 This is a timing diagram of the operation of the magnetometer of the present invention and a schematic diagram of the free induction attenuation signal; Figure 3 A schematic diagram of the Larmor precession frequency of a free-induction decay atomic magnetometer; Figure 4 This is a schematic diagram of the magnetometer sensitivity index measured in the embodiment.

[0034] The accompanying drawings illustrate specific embodiments of this application, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the concept in any way, but rather to illustrate the concept of this application to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0035] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0036] Example 1

[0037] Reference Figure 1 The magnetometer in this embodiment consists of two main parts: a physical probe and a circuit box.

[0038] The physical probe section constitutes the sensing core, specifically including: a laser emitting unit: employing a vertical-cavity surface-emitting laser (VCSEL) with an output wavelength of 795nm, its wavelength locked to the D1 absorption line of rubidium atoms by an external control system; an optical shaping unit: arranged sequentially along the laser beam path. First, a convex lens is used to collimate the diverging light emitted by the VCSEL laser. Next, a polarizer is used to generate high-purity linearly polarized light. Finally, a quarter-wave plate, whose optical axis is at a 45-degree angle to the direction of linearly polarized light, is used to convert the linearly polarized light into left-handed circularly polarized light; and a rubidium atom gas chamber: a glass bulb filled with rubidium atom vapor. It is externally encased in a non-magnetic heating system, which uses twisted-pair resistance wire as a heating coil. The induced magnetic fields generated during heating cancel each other out, thus greatly reducing additional magnetic noise while heating the gas chamber to the operating temperature (approximately 80°C). Radio Frequency Coil Assembly: A Helmholtz coil wound around the periphery of the rubidium atom gas chamber is used to generate a polarized magnetic field along the laser propagation direction (optical axis) inside the gas chamber. To achieve a cleaner measurement environment, an additional compensating Helmholtz coil is added in the opposite direction to the gas chamber Helmholtz coil. This compensating coil is used to accurately cancel and compensate for any residual magnetic field that may be generated after the gas chamber Helmholtz coil is turned off, ensuring that within the detection window for Larmor precession frequency measurement, the atoms are only affected by the external magnetic field to be measured (such as the Earth's magnetic field) and are not interfered with by the residual magnetic field of the coil itself. Photoelectric Detection Unit: Located behind the gas chamber, this unit uses a silicon photodiode to receive the light signal transmitted through the gas chamber and convert its intensity into a voltage signal.

[0039] The circuit section is responsible for signal processing and system control, specifically including: Signal conditioning circuit: connected to photodiode. First, a low-noise transimpedance amplifier (preamplifier) ​​is used to convert the weak current signal of the photodiode into a voltage signal and preamplify it. Then the signal enters a lock-in amplifier, whose reference frequency is provided by the system's own oscillation signal. The lock-in amplifier is used to extract the AC component that is strictly synchronized with the atomic Larmor precession frequency from the noisy voltage signal and amplify it with high gain, while greatly suppressing out-of-band noise. Oscillation feedback circuit: This is the core control unit of the invention. It receives the sinusoidal signal output from the lock-in amplifier. The circuit contains a high-precision frequency / phase detection module, a microprocessor or field-programmable gate array timing controller, and a power drive module. When working, it performs the following key functions: (1) accurately calculates the starting phase of the RF drive signal required for the next cycle according to the frequency and phase of the input signal. (2) According to the preset pulse timing (such as Figure 2 (3) The power module is driven to generate a large current pulse output to the Helmholtz coil.

[0040] Example 2

[0041] Figure 2 This diagram illustrates a pulsed rubidium optically pumped magnetometer (OPM), implemented based on the free-inducible attenuation (FID) optical detection scheme provided by this invention. Its underlying physics and operation are described below. First, light emitted from a 795 nm VCSEL laser is circularly polarized using a quarter-wave plate. Next, the circularly polarized light passes through a transparent rubidium vapor cell. After passing through the vapor cell, the light is captured by a photodetector. The vapor cell is electrically heated to approximately 80 degrees Celsius to increase the vapor pressure of rubidium atoms, causing the laser wavelength to be electron-locked to the rubidium D1 optical transition. The sensor operates in two phases, repeated every 1 ms (period time). In the initial 500 µs, a strong polarization magnetic field (Bp) parallel to the light beam is activated using a set of Helmholtz coils surrounding the vapor cell. The combination of the light beam and the longitudinal polarization field causes the rubidium atoms to spin-polarize (align with the polarization field). Next, the polarization field is rapidly deactivated in less than 1 µs. This rapid deactivation of the polarization field causes the rubidium atoms to precess (oscillate) around the Earth's magnetic field. The precession frequency of rubidium atoms is proportional to the background magnetic field. Precessing rubidium atoms modulate light passing through the atomic gas cell, thus enabling real-time measurement of the precession, for example, by amplifying the output of a photodiode inserted into an oscilloscope [Figure (right)]. In this scheme, the measurement lasts approximately 500 µs, constituting a periodic measurement phase. During the measurement phase, the raw electrical output of the photodiode is amplified and sent to a built-in high-performance frequency counter. The frequency counter measures the rubidium precession frequency (e.g., ...). Figure 3The precise background magnetic field value is deduced based on the fixed relationship between the background magnetic field and the precession frequency given by 6.998 Hz / nT. For example, if the precession frequency is found to be 350 kHz, the background field is determined to be 50014.289 nT. Magnetic field measurements are performed every 1 ms period.

[0042] Example 3

[0043] Based on the magnetometer implemented in Embodiment 1 or 2 above, the core workflow and measurement method of this embodiment are as follows. This process is repeated within each measurement cycle (set to 1 millisecond in this embodiment):

[0044] Reference Figure 2 Each period T (1 ms) is clearly divided into two stages: the polarization stage (0 ~ 500 μs): During this stage, the oscillating feedback circuit drives the Helmholtz coil to generate a strong polarization magnetic field B_p along the optical axis (e.g., much larger than the Earth's magnetic field). Simultaneously, a left-handed circularly polarized laser with a wavelength of 795 nm continuously irradiates the gas chamber. Under the combined action of optical pumping and the polarization magnetic field, the rubidium atom spins are effectively polarized, and their macroscopic magnetic moment direction aligns with the optical axis (i.e., the B_p direction). The free-induction decay and detection stage (501 ~ 1000 μs): At 500 μs, the oscillating feedback circuit controls the power module to reduce the coil current to zero within a very short time (Δt ≈ 1 μs), thereby causing the polarization magnetic field B_p to disappear rapidly. Due to the use of a compensated Helmholtz coil to cancel out the remaining magnetic field, the magnetic field in the gas chamber region rapidly and purely transitions to the external magnetic field environment B0 to be measured. Subsequently, the polarized atomic spins begin Larmor precession under the influence of the Earth's magnetic field and the external magnetic field B0, with a precession frequency f_L = γ * B0. This precession periodically changes the absorptivity of the atoms to the probe light, causing the intensity of the transmitted light received by the photodiode to oscillate sinusoidally at a stable frequency f_L, which is the free-induction decay signal.

[0045] The FID signal generated by the photodiode is extremely weak. It is first amplified by a low-noise amplifier in the signal conditioning circuit, and then sent to a lock-in amplifier. The lock-in amplifier uses the current oscillation frequency f of the system as a reference frequency to perform phase-sensitive detection on the signal, and outputs a DC error voltage that is proportional to the signal amplitude and has extremely low noise.

[0046] The key innovation of this embodiment lies in the feedback logic: the oscillation feedback circuit uses the phase information of the FID signal detected in the current cycle to accurately calculate the starting phase of the RF pulse in the "polarization phase" of the next cycle. Thus, although only half the time (the detection phase) in each cycle generates an effective frequency signal, through this cross-cycle phase locking and pulse synchronization, the entire system macroscopically forms an extremely stable closed-loop self-excited oscillator. After the system stabilizes, the frequency of the RF drive signal output by the oscillation feedback circuit is equal to the Larmor precession frequency f_L of an atom.

[0047] Magnetic field calculation: The system has a built-in high-precision frequency counter that measures the self-excited oscillation frequency f in real time. Based on the gyromagnetic ratio γ of rubidium-87 atoms (internationally recognized value is 6.998 Hz / nT), the strength of the magnetic field to be measured can be directly calculated: B0 = f / 6.998. Units: B0 is nanotesla (nT), and f is hertz (Hz).

[0048] Example 4

[0049] Performance tests were conducted on the aforementioned Examples 1, 2, and 3: the fabricated magnetometer probe and the magnetic field measurement results based on the magnetometer were tested, with the sensitivity index results as follows: Figure 4 Illustration. Sensitivity: The noise spectral density of the system output was measured under magnetic shielding conditions. At 1 Hz, the noise equivalent magnetic field strength is approximately 0.6 pT / √Hz, exhibiting extremely high sensitivity. Bandwidth and Sampling Rate: The system's duty cycle is 1 ms, therefore the output sampling rate for the magnetic field readings is 1 kHz. By applying alternating test magnetic fields of different frequencies to the system, the effective measurement bandwidth of -3 dB was measured to exceed 500 Hz. Measurement Range: By adjusting the system parameters, this magnetometer can operate stably in a magnetic field range of 10,000 nT to 150,000 nT.

[0050] In summary, embodiments 1-3 of this invention creatively combine pulsed free-sensing attenuation detection with a closed-loop self-excited oscillation system, and employ specific technical means such as VCSEL, a specific optical shaping unit, and a compensation coil, to provide a rubidium atomic magnetometer solution that combines high sensitivity, wide bandwidth, high precision, and fast response capability, which has significant practical value.

[0051] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

[0052] It should be understood that this application is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of this application is limited only by the appended claims.

Claims

1. A free-induction decay based rubidium atomic magnetometer, characterized in that, The physical probe part comprises a VCSEL laser for emitting a laser beam with a wavelength resonant with the D1 line of rubidium atoms, an optical shaping unit arranged in the light path of the VCSEL laser and composed of a convex lens, a polarizer and a 1 / 4 wave plate for converting the laser beam into left-handed circularly polarized light, a rubidium atom cell arranged on the light exit side of the optical shaping unit, and a radio frequency coil assembly wound around the rubidium atom cell for generating a magnetic field along the optical axis direction in the rubidium atom cell; and the optical detector is arranged on the light exit side of the rubidium atom cell for detecting the transmitted light intensity and outputting an electrical signal; the circuit part comprises a signal conditioning circuit connected with the optical detector for amplifying and filtering the electrical signal, and an oscillation feedback circuit connected with the signal conditioning circuit for generating a radio frequency driving signal according to the processed electrical signal; wherein the radio frequency driving signal is fed to the radio frequency coil assembly to form a closed-loop self-excited oscillation circuit in the magnetometer; and the oscillation feedback circuit is configured to work in a pulse timing manner, and in one measurement period, the radio frequency coil assembly is first driven to generate a polarized magnetic field for a period of time, and then the polarized magnetic field is turned off to enter a free induction decay signal detection stage.

2. The free-induction decay-based rubidium atomic magnetometer of claim 1, wherein, The oscillation feedback circuit is configured to generate a polarized magnetic field for a first preset time T1 in one period, and then rapidly turn off the polarized magnetic field for a second preset time T2.

3. The free-induction decay-based rubidium atomic magnetometer of claim 2, wherein, The first preset time is on the order of hundreds of microseconds, and the second preset time is on the order of microseconds or sub-microseconds.

4. The free-induction decay-based rubidium atomic magnetometer of claim 3, wherein, The first preset time is 500 microseconds, and the second preset time is 1 microsecond.

5. The free-induction decay-based rubidium atomic magnetometer of claim 1, wherein, The radio frequency coil assembly comprises a Helmholtz coil wound around the cell and a compensation type Helmholtz coil arranged in the opposite direction of the Helmholtz coil.

6. The free-induction decay-based rubidium atomic magnetometer of claim 1, wherein, It further comprises a non-magnetic heating system for heating the rubidium atom cell.

7. The free-induction decay-based rubidium atomic magnetometer of claim 1, wherein, The signal conditioning circuit comprises a preamplifier and a lock-in amplifier.

8. A method of free-induction decay based rubidium atom magnetic field measurement, implemented using the rubidium atom magnetometer of any one of claims 1 to 7, characterized in that, The method comprises the following steps periodically performed: S1, polarization preparation stage: applying left-handed circularly polarized pump light generated by a VCSEL laser and shaped by a convex lens, a polarizer and a 1 / 4 wave plate to a rubidium atom cell, and applying a polarized magnetic field along the direction of laser propagation generated by a radio frequency coil assembly to polarize the spin of rubidium atoms; S2, free induction decay excitation stage: rapidly turning off the polarized magnetic field for a second preset time after the polarized magnetic field acts for a first preset time, wherein the second preset time is less than the first preset time; S3, optical detection stage: detecting the transmitted light intensity through the rubidium atom cell after turning off the polarized magnetic field and converting it into a first electrical signal; and S4, signal processing and feedback stage: amplifying and frequency-selectively processing the first electrical signal, extracting a signal corresponding to the atomic Larmor precession frequency, and generating a radio frequency driving signal for the next cycle S1 stage according to the processed signal. ​ S5, frequency resolving stage: measuring the stable oscillation frequency f of the closed loop self-excited oscillation formed in the signal processing and feedback stage, and calculating the strength B0 of the external magnetic field to be measured according to the formula B0 = f / γ, wherein γ is the gyromagnetic ratio of rubidium atom.

9. The magnetic field measuring method according to claim 8, characterized in that, The first preset time is 500 microseconds, the second preset time is 1 microsecond, and the period of the periodic execution is 1 millisecond.

10. The magnetic field measurement method according to claim 8, characterized by, The gyromagnetic ratio γ is 6.998 Hz / nT.

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