A method and device for enhancing the signal of a single-beam free-precession atomic magnetometer by magneto-optical modulation

Abstract

The application discloses a light-magnetic modulation method and device for enhancing a single-beam free precession atomic magnetometer signal, and is based on synchronous light pumping technology and radio frequency magnetic field pulse control technology. Through light-magnetic modulation control, atomic spin polarization and free precession signal excitation are realized, so that more spin polarization components participate in free precession, and full-range signal enhancement of the single-beam free precession atomic magnetometer is realized. The application enhances the free precession signal and improves the signal-to-noise ratio, which is helpful to further improve the sensitivity of the single-beam free precession atomic magnetometer and is convenient for further application of the single-beam free precession atomic magnetometer in the field of quantum precise magnetic field measurement in the future.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of magnetic field measurement, in particular to a light-magnetic modulation method and device for enhancing the signal of a single-beam free precession atomic magnetometer. BACKGROUND

[0002] Optically pumped atomic magnetometers have the characteristics of small sensor size, high sensitivity and low cost, and are widely used in biological magnetic measurement, space magnetic detection and other fields. Free precession atomic magnetometers based on free precession mechanism are a typical representative of optically pumped atomic magnetometers, and have become a research and development hotspot of optically pumped atomic magnetometers in recent years due to their high accuracy, large dynamic range and simple circuit structure. Compared with the double-beam configuration, the single-beam free precession atomic magnetometer is more easily miniaturized and low-cost, but its sensitivity is lower than that of the double-beam configuration.

[0003] An important factor limiting the sensitivity of the free precession atomic magnetometer is the strength of the free precession signal generated by the magnetometer. High-strength free precession signals can achieve high signal-to-noise ratio, and thus high sensitivity of the magnetometer. In the prior art, a single-beam free precession optically pumped atomic magnetometer using all-optical modulation achieves spin polarization preparation through synchronous optical pumping, but only the transverse spin polarization component participates in the free precession, which limits the improvement of the signal strength and thus affects the sensitivity of the magnetometer. Therefore, it is a very important research problem to enhance the signal strength of the single-beam free precession atomic magnetometer. SUMMARY

[0004] The purpose of the present application is to provide a light-magnetic modulation method and device for enhancing the signal of a single-beam free precession atomic magnetometer. After synchronous optical pumping is completed, the atomic spin polarization is rotated to a plane perpendicular to the magnetic field direction by applying a transverse radio frequency magnetic field pulse, so that both the transverse component and the longitudinal component of the spin polarization can participate in the free precession, thereby achieving all-around signal enhancement of the single-beam free precession atomic magnetometer.

[0005] To achieve the above-mentioned purpose, the present application provides a light-magnetic modulation method for enhancing the signal of a single-beam free precession atomic magnetometer, comprising the following steps:

[0006] S1, turn on the laser, adjust the wavelength of the laser to the D1 line of the alkali metal atom, and form circularly polarized light through the optical path and pass through the alkali metal gas chamber;

[0007] S2, adjust the light intensity of the circularly polarized light to achieve synchronous optical pumping by using a light intensity modulator, and prepare atomic spin polarization;

[0008] S3, after completing the synchronous optical pumping, apply a transverse radio frequency magnetic field pulse to manipulate the atomic spin polarization to a plane perpendicular to the magnetic field direction;

[0009] S4. Atomic spin polarization undergoes free precession, and free precession signal detection is achieved through low-intensity laser. The data acquisition and processing system collects and processes the signal to achieve measurement of the magnetic field.

[0010] Preferably, the adjustment of the laser in step S1 specifically includes the following steps:

[0011] S11, turning on the laser and driving the light intensity modulator;

[0012] S12, adjusting the frequency of the laser to the center of the alkali metal atomic spectrum line D1;

[0013] S13. Adjust the polarization device so that the laser generates circularly polarized light after passing through the laser and passes through the alkali metal gas chamber.

[0014] Preferably, the synchronous optical pumping in step S2 specifically includes the following steps:

[0015] S21. Inputting a square wave pulse sequence with a period in the millisecond range and a certain duty cycle into the light intensity modulator through a signal generator, wherein the duty cycle of the square wave pulses in the sequence is 50% and the pulse repetition frequency is the Larmor precession frequency corresponding to the magnetic field;

[0016] S22. Fine-tune the repetition frequency of the square wave pulses in the sequence until the magnetometer output signal reaches a maximum at the end of the square wave pulse sequence, thereby achieving synchronous optical pumping.

[0017] Preferably, in step S3, manipulating the atomic spin polarization to a plane perpendicular to the magnetic field direction specifically comprises the following steps:

[0018] S31. At the moment of removing the input to the light intensity modulator in each cycle, a signal generator is used to input an input to the radio frequency coil to generate a transverse radio frequency magnetic field pulse perpendicular to the laser direction. The oscillation frequency of the input radio frequency magnetic field is the same as the repetition frequency of the square wave pulse in step S22, and the duration is much shorter than the free precession period.

[0019] S32. Control other parameters unchanged and adjust the size of the radio frequency magnetic field pulse so that the signal output by the magnetometer reaches a maximum. At this time, the spin polarization is controlled to a plane orthogonal to the magnetic field.

[0020] Preferably, the collection and processing of the free precession signal in step S4 specifically includes the following steps:

[0021] S41. After optical magnetic modulation, the spin polarization undergoes free precession. The signal generator inputs a low-amplitude DC signal to the optical intensity modulator to ensure a certain laser intensity, and detects the free precession signal of the spin polarization.

[0022] S42, collecting, by a data acquisition and processing system, a signal output by the magnetometer after passing through a transimpedance amplifier and a bandpass filter;

[0023] S43, processing the collected free precession signal, extracting the oscillation frequency of each periodic signal, dividing by the gyromagnetic ratio to obtain the measured magnetic field, the oscillation frequency being the Larmor precession frequency.

[0024] Preferably, in step S4, the spin polarization is free precession, and the evolution of the spin polarization in the direction of laser propagation can be expressed as:

[0025]

[0026] wherein the first term of the formula is the expression of the slow varying direct current term caused by optical pumping, and the second term is the expression of the free precession of the spin polarization, S z (t) is the spin polarization component in the direction of the laser, t is time, is the pumping rate in the free precession stage, R r1 is the longitudinal relaxation rate in the free precession stage, R r2 is the transverse relaxation rate in the free precession stage, θ is the angle between the measured magnetic field and the direction of laser propagation, γ is the gyromagnetic ratio, B0 is the magnitude of the measured magnetic field, is the signal phase, P1 is the longitudinal component of the spin polarization, P2 is the transverse component of the spin polarization, and the expressions of P1 and P2 are as follows:

[0027]

[0028] wherein t1 is the duration of the synchronous pumping, t3 is the duration of the free precession stage, R p0 is the peak optical power P p in the synchronous optical pumping stage, R1 is the longitudinal relaxation rate in the synchronous optical pumping stage, and R2 is the transverse relaxation rate in the synchronous optical pumping stage.

[0029] Preferably, in step S4, after the circularly polarized light signal passes through the photodetector, the transimpedance amplifier and the bandpass filter, the slow varying direct current term is filtered out, and the signal collected by the data acquisition and processing system is expressed as:

[0030]

[0031] wherein V0 is the amplitude of the voltage signal output by the bandpass filter.

[0032] The application also provides a device for enhancing a single-beam free precession atomic magnetometer signal, comprising, in sequence, a laser, an optical intensity modulator, a polarization maintaining optical fiber, a collimator, a half-wave plate, a polarizer, a quarter-wave plate, an alkali metal cell, and a photodetector, an output end of the photodetector being connected to an input end of a data acquisition and processing system through a transimpedance amplifier and a band-pass filter, the alkali metal cell being wrapped by an oven and a non-magnetic electric heating coil and being heated to achieve high atomic density, a radio frequency coil and a magnetometer support shell being arranged outside the alkali metal cell, an input end of the optical intensity modulator being connected to an output end of a signal generator, and the radio frequency coil being connected to another output end of the signal generator perpendicularly to an axis of the laser direction.

[0033] Preferably, a frequency range of the band-pass filter is centered on a Larmor precession frequency corresponding to a magnetic field to be measured.

[0034] Therefore, the application has the following beneficial effects by using the above-mentioned optical magnetic modulation method and device for enhancing a single-beam free precession atomic magnetometer signal.

[0035] (1) A conventional single-beam all-optical modulation type free precession atomic magnetometer realizes a free precession signal through synchronous optical pumping, and the application adds a transverse radio frequency magnetic field pulse to control rotation of atomic spin polarization, so that more spin polarization components participate in free precession, thereby enhancing the free precession signal.

[0036] (2) The application realizes all-around enhancement of a free precession signal of a single-beam free precession atomic magnetometer and improvement of a signal-to-noise ratio, thereby realizing improvement of sensitivity of the magnetometer and facilitating further research and development of the single-beam free precession atomic magnetometer in the field of quantum precise magnetic field measurement.

[0037] (3) The application realizes all-around enhancement of a free precession signal of a magnetometer without increasing complexity of a single-beam free precession atomic magnetometer system.

[0038] The technical solutions of the application are described in further detail below with reference to the drawings and embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 A flow chart of an optical magnetic modulation method for enhancing a single-beam free precession atomic magnetometer signal according to an embodiment of the application;

[0040] Figure 2 A structural schematic diagram of a device for enhancing a single-beam free precession atomic magnetometer signal according to an embodiment of the application;

[0041] Figure 3 A signal timing diagram related to an optical magnetic modulation method according to an embodiment of the application;

[0042] Reference signs

[0043] 1, laser; 2, light intensity modulator; 3, polarization maintaining optical fiber; 4, collimator; 5, half-wave plate; 6, polarizer; 7, quarter-wave plate; 8, alkali metal cell; 9, photodetector; 10, transimpedance amplifier; 11, band-pass filter; 12, data acquisition and processing system; 13, signal generator; 14, oven; 15, non-magnetic electric heating coil; 16, radio frequency coil; 17, magnetometer support shell. DETAILED DESCRIPTION

[0044] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the following will be combined with the accompanying drawings of the embodiments of the present application to make a clear and complete description of the technical solutions in the embodiments of the present application. Obviously, the described embodiments are some of the embodiments of the present application but not all the embodiments. The components of the embodiments of the present application described and shown in the accompanying drawings can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0045] It should be noted that: similar reference numbers and letters represent similar items in the following drawings, therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0046] EMBODIMENT

[0047] As shown in Figure 1 and Figure 3 , the present application provides a light-magnetic modulation method for enhancing a single-beam free precession atomic magnetometer signal, comprising the following steps:

[0048] S1, turning on the laser 1, adjusting the wavelength of the laser 1 to the alkali metal atom D1 line, and making the laser pass through the optical path to form circularly polarized light and pass through the alkali metal cell 8.

[0049] In this embodiment, the adjustment of the laser in step S1 specifically comprises the following steps:

[0050] S11, turning on the laser 1 and driving the light intensity modulator 2 and other devices;

[0051] S12, adjusting the frequency of the laser 1 to the center of the alkali metal atom spectral line D1 line;

[0052] S13, adjusting the polarization device to make the laser produce circularly polarized light after passing through, and passing through the alkali metal cell 8.

[0053] S2, adjusting the light intensity of the circularly polarized light by the light intensity modulator 2 to realize synchronous optical pumping to prepare the atomic spin polarization.

[0054] In this embodiment, the synchronous optical pumping in step S2 specifically includes the following steps:

[0055] S21, inputting a square wave pulse sequence with a period of millisecond order and a certain duty cycle to the light intensity modulator 2 by the signal generator 13, wherein the duty cycle of the square wave pulse in the sequence is 50%, and the pulse repetition frequency is about the Larmor precession frequency corresponding to the magnetic field;

[0056] S22, fine-tuning the repetition frequency of the square wave pulse in the sequence to the maximum of the output signal of the magnetometer at the end of the square wave pulse sequence to realize synchronous optical pumping.

[0057] S3, applying a transverse radio frequency magnetic field pulse to manipulate the atomic spin polarization to a plane perpendicular to the direction of the magnetic field after completing the synchronous optical pumping.

[0058] In this embodiment, the manipulation of the atomic spin polarization to a plane perpendicular to the direction of the magnetic field in step S3 specifically includes the following steps:

[0059] S31, at the moment when the input to the light intensity modulator 2 is removed every period, inputting a transverse radio frequency magnetic field pulse perpendicular to the direction of the laser by the signal generator 13 to the radio frequency coil 16, the oscillation frequency of the input radio frequency magnetic field is the same as the repetition frequency of the square wave pulse in step S22, and the duration is much smaller than the free precession period;

[0060] S32, adjusting the size of the radio frequency magnetic field pulse signal while keeping other parameters unchanged to make the output signal of the magnetometer reach the maximum, at this time, the spin polarization is manipulated to a plane orthogonal to the magnetic field, and the free precession signal reaches the maximum.

[0061] S4, the atomic spin polarization undergoes free precession, the free precession signal is detected by low light intensity laser, and the signal is collected and processed by the data acquisition and processing system 12 to realize the measurement of the magnetic field.

[0062] In this embodiment, the collection and processing of the free precession signal in step S4 specifically includes the following steps:

[0063] S41, after the optical magnetic modulation, the spin polarization undergoes free precession, the signal generator 13 inputs a low-amplitude direct current signal to the light intensity modulator 2 to ensure a certain light intensity of the laser, and the free precession signal of the spin polarization is detected;

[0064] S42, collecting the signal output by the magnetometer after the transimpedance amplifier 10 and the band-pass filter 11 by the data acquisition and processing system 12;

[0065] S43, processing the collected free precession signal, extracting the oscillation frequency of each periodic signal, i.e. the Larmor precession frequency, dividing by the gyromagnetic ratio to obtain the measured magnetic field.

[0066] In step S4 of the embodiment, the spin polarization is free precession, and the evolution of the spin polarization in the laser propagation direction can be expressed by the following formula:

[0067]

[0068] wherein the first term of the formula is the expression of the slow varying direct current term caused by optical pumping, and the second term is the expression of the free precession of the spin polarization, S z (t) is the spin polarization component in the laser direction, t is time, is the pumping rate in the free precession stage, R r1 is the longitudinal relaxation rate in the free precession stage, R r2 is the transverse relaxation rate in the free precession stage, θ is the angle between the measured magnetic field and the laser propagation direction, γ is the gyromagnetic ratio, B0 is the measured magnetic field size, is the signal phase, P1 is the longitudinal component of the spin polarization, and P2 is the transverse component of the spin polarization, and the expressions of P1 and P2 are as follows:

[0069]

[0070] wherein t1 is the duration of the synchronous pumping, t2 is the duration of the radio frequency pulse, t3 is the duration of the free precession stage, R p0 is the peak optical power P p in the synchronous optical pumping stage, R1 is the longitudinal relaxation rate in the synchronous optical pumping stage, and R2 is the transverse relaxation rate in the synchronous optical pumping stage.

[0071] As can be seen from the expression of the free precession, in the optical magnetic modulation method of the embodiment, the transverse component and the longitudinal component of the atomic spin polarization are both involved in the free precession after being manipulated by the radio frequency pulse magnetic field, thus realizing the signal enhancement of the single-beam free precession atomic magnetometer.

[0072] In step S4 of the embodiment, after the circularly polarized light signal is detected by the photodetector 9, transimpedance amplified by the transimpedance amplifier 10, and filtered by the bandpass filter 11, the slow varying direct current term is filtered out, and the signal collected by the data acquisition and processing system 12 can be expressed as:

[0073]

[0074] wherein V0 is the voltage signal amplitude output by the bandpass filter 11. The data acquisition and processing system 12 analyzes the collected signal, extracts the oscillation component in the signal to obtain the Larmor precession frequency, and thus the information of the measured magnetic field can be obtained.

[0075] Reference Figure 2 , the xyz is the three-axis (x-axis, y-axis, z-axis) of the Cartesian coordinate system established with the laboratory pumping light direction as the z-axis. The application also provides a device for enhancing the signal of a single-beam free precession atomic magnetometer, comprising a laser 1, an optical intensity modulator 2, a polarization maintaining optical fiber 3, a collimator 4, a half-wave plate 5, a polarizer 6, a quarter-wave plate 7, an alkali metal cell 8 and a photodetector 9 connected in sequence. The output end of the photodetector 9 is connected to the input end of a data acquisition and processing system 12 through a transimpedance amplifier 10 and a band-pass filter 11. The frequency range of the band-pass filter 11 is centered on the Larmor precession frequency corresponding to the magnetic field to be measured. The half-wave plate 5, the polarizer 6 and the quarter-wave plate 7 form circularly polarized light and have maximum optical power before the pumping light passes through the alkali metal cell 8. The alkali metal cell 8 is wrapped by an oven 14 and a non-magnetic electric heating coil 15 and is heated to achieve high atomic density. An RF coil 16 and a magnetometer support shell 17 are arranged outside the alkali metal cell 8. The input end of the optical intensity modulator 2 is connected to the output end of a signal generator 13, and the RF coil 16 is connected to another output end of the signal generator 13 perpendicular to the laser direction.

[0076] Therefore, the application adopts the above-mentioned optical magnetic modulation method and device for enhancing the signal of a single-beam free precession atomic magnetometer. First, atomic spin polarization is prepared by synchronous optical pumping technology, then a transverse RF magnetic field pulse is applied to control the atomic spin polarization to a plane perpendicular to the magnetic field direction, the pumping light is kept at a low light intensity during the free precession stage to realize the detection of the free precession signal, and the magnetic field information can be obtained by collecting and processing the signal through the data acquisition and processing system.

[0077] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the application and not to limit them. Although the application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the application can still be modified or replaced by equivalents, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the spirit and scope of the technical solutions of the application.

Claims

1. A method for enhancing the optical magnetic modulation signal of a single-beam free precession atomic magnetometer, characterized in that: The following steps are involved: S1. Turn on the laser and adjust the laser light wavelength to the alkali metal atom D1 line. The laser forms circularly polarized light through the optical path and passes through the alkali metal gas chamber. S2. Using a light intensity modulator to adjust the intensity of circularly polarized light to achieve synchronous optical pumping and prepare atomic spin polarization; S3, after completing synchronous optical pumping, applying a transverse radio frequency magnetic field pulse to manipulate the atomic spin polarization to a plane perpendicular to the magnetic field direction; S4. Atomic spin polarization undergoes free precession, and the free precession signal is detected by low-intensity laser. The data acquisition and processing system collects and processes the signal to measure the magnetic field. The synchronous optical pumping in step S2 specifically includes the following steps: S21. Inputting a square wave pulse sequence with a period in the millisecond range and a certain duty cycle into the light intensity modulator through a signal generator, wherein the duty cycle of the square wave pulses in the sequence is 50% and the pulse repetition frequency is the Larmor precession frequency corresponding to the magnetic field; S22, fine-tuning the repetition frequency of the square wave pulses in the sequence until the magnetometer output signal reaches a maximum at the end of the square wave pulse sequence, thereby achieving synchronous optical pumping; In step S3, the manipulation of the atomic spin polarization to a plane perpendicular to the magnetic field direction specifically includes the following steps: S31. At the moment of removing the input to the light intensity modulator in each cycle, a signal generator is used to input an input to the radio frequency coil to generate a transverse radio frequency magnetic field pulse perpendicular to the laser direction. The oscillation frequency of the input radio frequency magnetic field is the same as the repetition frequency of the square wave pulse in step S22, and the duration is much shorter than the free precession period. S32, controlling other parameters unchanged, adjusting the magnitude of the radio frequency magnetic field pulse so that the magnetometer output signal reaches a maximum, at which point the spin polarization is manipulated to a plane orthogonal to the magnetic field; In step S4, the spin polarization undergoes free precession, and the evolution of the spin polarization in the laser propagation direction can be expressed as: ； Among them, the first term of the formula is the expression of the slowly varying DC term caused by optical pumping, and the second term is the expression of the free precession of spin polarization. is the spin polarization component in the laser direction, For time, is the pumping rate during the free precession phase, is the longitudinal relaxation rate during the free precession phase, is the transverse relaxation rate during the free precession phase, is the angle between the magnetic field to be measured and the laser propagation direction, is the gyromagnetic ratio, is the magnitude of the magnetic field to be measured, is the signal phase, is the spin-polarized longitudinal component, is the spin-polarized transverse component, and The expression is as follows: ； ； in, is the duration of synchronous pumping, is the duration of the free precession phase, is the peak optical power in the synchronous optical pumping stage The corresponding pumping rate, is the longitudinal relaxation rate during synchronous optical pumping, is the transverse relaxation rate during the synchronous optical pumping stage.

2. The optical magnetic modulation method for enhancing the signal of a single-beam free precession atomic magnetometer according to claim 1, characterized in that: The adjustment of the laser in step S1 specifically includes the following steps: S11, turn on the laser and drive the light intensity modulator; S12, adjusting the frequency of the laser to the center of the alkali metal atomic spectrum line D1; S13. Adjust the polarization device so that the laser generates circularly polarized light after passing through the laser and passes through the alkali metal gas chamber.

3. The optical magnetic modulation method for enhancing the signal of a single-beam free precession atomic magnetometer according to claim 2, characterized in that: In step S4, after the circularly polarized light signal passes through the photodetector, transimpedance amplifier, and bandpass filter, the slowly varying DC term is filtered out. The signal collected by the data acquisition and processing system is expressed as: ； in, is the voltage signal amplitude output by the bandpass filter.

4. A device for enhancing the signal of a single-beam free precession atomic magnetometer, applied to the optical magnetic modulation method for enhancing the signal of a single-beam free precession atomic magnetometer as claimed in any one of claims 1 to 3, characterized in that: The invention comprises a laser, a light intensity modulator, a polarization-maintaining optical fiber, a collimator, a half-wave plate, a polarizer, a quarter-wave plate, an alkali metal gas chamber and a photodetector connected in sequence. The output end of the photodetector is connected to the input end of a data acquisition and processing system through a transimpedance amplifier and a bandpass filter. The alkali metal gas chamber is wrapped and heated by an oven and a non-magnetic electric heating coil to achieve a high atomic density. A radio frequency coil and a magnetometer support shell are provided on the outside of the alkali metal gas chamber. The input end of the light intensity modulator is connected to the output end of a signal generator. The radio frequency coil is connected to the other output end of the signal generator with one axis perpendicular to the laser direction.

5. The device for enhancing the signal of a single-beam free precession atomic magnetometer according to claim 4, characterized in that: The frequency range of the bandpass filter is centered on the Larmor precession frequency corresponding to the magnetic field to be measured.

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