CN104297702B - Measurement method and device of Bell-Bloom self-modulation three-axis magnetic field - Google Patents

Abstract

The invention relates to a Bell-Bloom self-modulating three-axis magnetic field measurement method and device. The conventional Bell-Bloom magnetometer uses a single beam to measure the scalar magnetic field. It has a simple structure and high measurement accuracy, and is widely used in the measurement of various scalar magnetic fields. The single-beam Bell-Bloom magnetometer can only measure the scalar magnetic field, but many places in scientific research and production and life require three-axis magnetic field information, that is, a vector magnetometer is required. The invention is based on a single-beam Bell-Bloom magnetometer, and utilizes the self-modulation effect of atomic precession on detection light to realize three-axis magnetic field measurement. Compared with common vector atom magnetometers, there are no radio frequency coils and magnetic shielding barrels, etc., so the optical path is simple, the structure is compact, and it is easy to miniaturize and integrate; the heating temperature is lower, and the required electronic components are less, so the system is reduced. The power consumption and operating conditions are easier to achieve; the self-modulation effect of atomic precession on the detection light is used, the noise is lower, and the detection accuracy is high.

Description

A method and device for Bell-Bloom self-modulating three-axis magnetic field measurement

technical field

The invention relates to a method and device for Bell-Bloom self-modulation three-axis magnetic field measurement, belonging to the technical field of atomic magnetometers.

Background technique

Bell-Bloom is a method of measuring scalar magnetic fields based on optical pumping technology, which uses the precession of atoms to achieve magnetic field measurement. Its general idea is to use circularly polarized light with the same frequency as the D1 line transition frequency of alkali metal atoms to pump the atoms. When the modulation frequency of the circularly polarized light is equal to the precession frequency of the atoms, optical magnetic resonance occurs. By extracting the resonance The frequency can be used to calculate the magnitude of the magnetic field. A Bell-Bloom magnetometer is a type of atomic magnetometer.

The atomic magnetometer relies on the measurement of transmitted light. After the light passes through the atomic gas cell, its optical properties will change due to the interaction between light and magnetic field. The interaction of light with atoms, and atoms with magnetic fields, significantly changes the type and mode of operation of the magnetometer. Common atomic magnetometers include SERF (Spin-exchange Relaxation-free, no spin exchange relaxation) magnetometer, Mx magnetometer and Bell-Bloom magnetometer, all of which are based on optical pumping technology to achieve magnetic measurement of. SERF magnetometers are required to operate under extremely weak magnetic fields, so either passive magnetic shielding with excellent shielding performance cannot be reduced in size, or coils are used for magnetic compensation, and the compensation accuracy and magnetic interference of the current in the coil are difficult to control. The Mx magnetometer needs to add an oscillating field to the atomic gas cell, so the synchronization of pumping and spin has a great influence, and there is also a mutual coupling error when measuring the magnetic field. For the Bell-Bloom magnetometer, it can operate under a large magnetic field or even the geomagnetic field, so the requirement for the magnetic field is very low. Compared with the SERF magnetometer, the structure is simple and easy to integrate; since the oscillating field is replaced by a modulated light source, Therefore, there is no synchronization error between pumping and spin, and there is no coupling error between the two magnetic fields, which is also a great advantage compared with the Mx magnetometer.

The conventional Bell-Bloom magnetometer uses a single beam as the pumping light and the detection light at the same time, and can only measure the scalar magnetic field. However, in many places in scientific research and production and life, more magnetic field information is needed, that is, a vector magnetometer is required. . How to use the Bell-Bloom principle to realize high-precision three-axis magnetic field measurement is a crucial research content of the atomic magnetometer, and there is no related report yet.

Contents of the invention

The technical problem of the present invention is to overcome the deficiencies of the prior art, and provide a method and device for Bell-Bloom self-modulation three-axis magnetic field measurement, on the basis of a single-beam Bell-Bloom scalar magnetometer, use double beams to realize three-axis Magnetic field measurement, compared with the common vector atom magnetometer, has a simple optical path, compact structure, low power consumption, easy to achieve operating conditions, easy miniaturization and integration, and has high detection accuracy and practical value.

The technical solution of the present invention: a Bell-Bloom self-modulation three-axis magnetic field measurement method, using the self-modulation effect of the detection light when the atom precesses to realize the three-axis magnetic field measurement, and adding a magnetic field B _x along the x-axis direction, there is Two beams of laser light act on the alkali metal atom gas cell at the same time, and a beam of circularly polarized light with amplitude-modulated D1 line transition frequency is used as pumping light along the z-axis direction. According to the principle of a single-beam Bell-Bloom scalar magnetometer, alkali metal atoms will precess around the magnetic field under the action of an external magnetic field. The relationship between the precession frequency of alkali metal atoms and the external magnetic field is:

ω _L ＝2πγ·B (1)

where γ is the gyromagnetic ratio of the atom.

Change the magnitude of the x-axis magnetic field. When the precession frequency and modulation frequency meet the resonance conditions, resonance occurs. The frequency point where resonance occurs can be found through the z-axis photodetector. The resonance conditions for a single-beam Bell-Bloom magnetometer are:

ω _L ＝mω _mod ,m＝1,2,3,... (2)

Among them, when m=1, that is, ω _L =ω _mod , the resonance amplitude is the largest, and the resonance phenomenon is the most obvious. According to the modulation frequency ω _mod at the maximum resonance amplitude and the applied magnetic field ΔB _x , the x-axis magnetic field can be calculated:

B _x ＝ω _mod /2πγ-ΔB _x (3)

Along the x-axis direction, there is a continuous beam of linearly polarized light with D2 line transition frequency as the detection light. The polarized atoms precess under the action of the main magnetic field, and the polarization vector has a self-modulation effect on the linearly polarized light, which is caused by B _y The phase difference between the x-axis light intensity change caused by Bz and the x-axis light intensity change caused by B _z is π/2. Taking the _{z-axis light intensity signal as a reference signal and the x-axis light intensity signal as a signal to be demodulated, By using a lock-in amplifier, By and B z can} be obtained.

Based on the above-mentioned Bell-Bloom self-modulation three-axis magnetic field measurement method, a Bell-Bloom self-modulation three-axis magnetic field measurement device is designed, including a pump light laser, a detection light laser, an alkali metal atomic gas cell, a photodetector, and a lock-in amplifier. , signal generator, driver of AOM, AOM, mirror, beam expander, polarizer, 1/4 wave plate and feedback controller. The output signals are respectively x-direction output, y-direction output and z-direction output with three-axis magnetic field information.

Magnetic field measurement in the x-axis direction: the signal generator (outputs a modulating signal to the driver of the acousto-optic modulator, and the driver outputs a radio frequency signal carrying the modulating signal to control the diffraction effect of the acousto-optic crystal in the acousto-optic modulator to the laser. Pumping light The linearly polarized light generated by the laser is injected into the acousto-optic modulator through the reflector to obtain the amplitude-modulated first-order diffracted light. The amplitude-modulated linearly polarized light becomes parallel light through the beam expander, and passes through the polarizer and 1/ After 4 wave plates, it becomes circularly polarized light. The circularly polarized light pumps the alkali metal atoms in the gas cell as pumping light. A magnetic field is added along the x-axis direction. Under the action of the external magnetic field, the alkali metal atoms revolve around the direction of the main magnetic field Precession, precession frequency ω _L =2πγ·B, where γ is the gyromagnetic ratio of the atom. Change the x-axis magnetic field size, when the precession frequency of the atom and the modulation frequency of the pumping light satisfy the condition ω _L =mω _mod , Resonance occurs when m _{=1, 2, 3, .... Compensate By and B z to} 0 point, use photodetector to detect the change of light intensity, find the modulation frequency and the applied magnetic field ΔB _x when the resonance amplitude is maximum, At this time B _x =ω _mod /2πγ-ΔB _x .

Magnetic field measurement in the y-axis direction and the z-axis direction: the linearly polarized light generated by the detection light laser 2 is directly injected into the atomic gas cell through the mirror, and the incident direction is perpendicular to the direction of the pumping light and passes through the pumping light. When there is a small magnetic field on the y-axis or the z-axis, the precession vector of atoms around the total magnetic field is projected on the x-axis, which has a self-modulation effect on the detection light, and the light intensity of the detection light changes periodically. And the light intensity change produced by B _y has the same frequency as the light intensity change produced by B _z , and the phase difference is π/2. The light intensity change of the detected light is obtained by using a photodetector as a signal to be demodulated, and the light intensity change of the pumping light is used as a reference signal. The reference signal and the signal to be demodulated are input into the lock-in amplifier at the same time, and the in-phase and out-of-phase channels of the lock-in amplifier output the voltage value corresponding to _{By and B z respectively} . Fig. 4 is the relationship curve between the in-phase output and B _y when B _z = 0, and Fig. 5 is the relationship curve between the out-of-phase output and B _z when By _y = 0. When B _y =B _z =0, the outputs of both channels are 0, so the atomic self-modulation has a natural zero-crossing point, which is very advantageous for realizing closed-loop. The voltage values corresponding to By _y and B _z output by the lock-in amplifier are respectively input into the feedback controller, and the output currents are respectively added to the y-axis and z-axis coils to accurately compensate the y-axis and z-axis magnetic fields. When the output of both channels of the lock-in amplifier is 0, By _y and B _z are the magnitudes of the magnetic fields applied to the two-axis coils.

The advantage of the present invention compared with prior art is:

(1) Existing atomic scalar magnetometers (such as Bell-Bloom magnetometer, Mx magnetometer, etc.) realize magnetic field measurement by measuring atomic precession frequency, but can only obtain scalar magnetic field information. In contrast, the method proposed in the present invention can obtain three-axis magnetic field information, which meets the needs of scientific research activities and practical applications;

(2) Existing atomic vector magnetometers (such as SERF magnetometers) can only be used for the measurement of weak magnetic fields. In contrast, the method proposed by the present invention can realize real-time vector magnetic field measurement in a geomagnetic environment under the premise of ensuring ultra-high precision (detectable to the fT level).

(3) Compared with other atomic magnetometers that extract vector information from scalar magnetometers, the method proposed in the present invention utilizes the self-modulation effect of atomic precession on detection light, and does not require additional modulation or bias magnetic field, so The introduced noise is lower, enabling ultra-high precision measurement.

Description of drawings

Fig. 1 is the principle schematic diagram of measuring method of the present invention;

Fig. 2 is the optical path and structural representation of measuring device of the present invention;

Figure 3 is the resonance and dispersion curves corresponding to the x-axis magnetic field;

Fig. 4 is the relationship curve between the y-axis magnetic field and the in-phase output amplitude of the lock-in amplifier;

Fig. 5 is a relationship curve between the z-axis magnetic field and the out-of-phase output amplitude of the lock-in amplifier.

detailed description

As shown in FIG. 1 , the Bell-Bloom self-modulation three-axis magnetic field measurement method proposed by the present invention utilizes the self-modulation effect of atomic precession on the detection light, and uses dual-beam detection to realize three-axis magnetic field measurement.

As shown in Figure 2, the specific implementation steps of using the measuring device of the present invention to realize the three-axis magnetic field measurement are as follows:

Step 1: Acquisition of amplitude-modulated pump light:

The signal generator 9 is used to output a modulating signal to the driver 10 of the acousto-optic modulator, which is generally a sine wave or a square wave. The linearly polarized light generated by the pumping laser 1 enters the acousto-optic modulator 11 through the reflector 12 to obtain first-order diffracted light with amplitude modulation. The amplitude-modulated linearly polarized light becomes parallel light after passing through the beam expander 13, and becomes circularly polarized light after passing through the polarizer 14 and 1/4 wave plate 15 to obtain the amplitude-modulated pumping light.

Step 2: Magnetic field measurement of the x-axis:

Apply a large magnetic field along the x-axis direction, requiring Make the main magnetic field approximately along the x-axis direction. Under the action of an external magnetic field, the alkali metal atoms precess around the direction of the main magnetic field, and the precession frequency ω _L =2πγ·B, where γ is the gyromagnetic ratio of the atom. Changing the magnitude of the x-axis magnetic field, resonance occurs when the precession frequency of the atoms and the modulation frequency of the pumping light satisfy the condition ω _L =mω _mod , m=1,2,3,.... Use the photodetector 4 to detect the change of light intensity, find the external magnetic field ΔB _x (resonance point as shown in Figure 3) when the resonance amplitude is the largest and the dispersion curve crosses 0 point (resonance point as shown in Figure 3), at this time ω _L = ω _mod , you can get the x-axis Magnetic field size, B _x =ω _mod /2πγ-ΔB _x .

Step 3: Magnetic field measurement of y-axis and z-axis:

The linearly polarized light generated by the detection light laser 2 directly enters the atomic gas cell 3 through the mirror 12, and the incident direction is perpendicular to the direction of the pumping light and passes through the pumping light. When there is a small magnetic field not higher than the main magnetic field on the y-axis or z-axis, the precession vector of atoms around the total magnetic field is projected on the x-axis, which has a self-modulation effect on the detection light, and the light intensity of the detection light changes periodically. And the light intensity change produced by B _y has the same frequency as the light intensity change produced by B _z , and the phase difference is π/2. The light intensity change of the detected light is obtained by using the photodetector 4 as a signal to be demodulated, and the light intensity change of the pumping light is used as a reference signal. The reference signal and the signal to be demodulated are input in the lock-in amplifier 5 simultaneously, and the in-phase channel (in-phase) and the π/2 phase difference channel (out-of-phase) of the lock-in amplifier output respectively the corresponding voltage value of B _y and B The voltage value corresponding to _z . Fig. 4 is the relationship curve between the in-phase output and B _y when B _z = 0, and Fig. 5 is the relationship curve between the out-of-phase output and B _z when By _y = 0. When B _y =B _z =0, the outputs of both channels are 0, so the atomic self-modulation has a natural zero-crossing point, which is very advantageous for realizing closed-loop. The voltage values corresponding to B _y and B _z output by the lock-in amplifier 5 are respectively input into the feedback controller 16, and the output currents are respectively added to the y-axis and z-axis coils to accurately compensate the y-axis and z-axis magnetic fields . When the output of both channels of the lock-in amplifier is 0, By _y and B _z are the magnitudes of the magnetic fields applied to the two-axis coils.

The above embodiments are provided only for the purpose of describing the present invention, not to limit the scope of the present invention. The scope of the invention is defined by the appended claims. Various equivalent replacements and modifications made without departing from the spirit and principle of the present invention shall fall within the scope of the present invention.

Claims (7)

1. a kind of method of Bell-Bloom automodulations three-axle magnetic field measurement, it is characterised in that：An additional magnetic field along the x-axis direction B_x, there are two beam laser while acting in alkali metal atom air chamber (3), there is a branch of pumping light along the z-axis direction, using photodetection Device (4) detection light intensity change, while polarized atom, B is calculated according to x to output (6)_x；There is a branch of inspection along the x-axis direction There is precession in light-metering, the atom of polarization, there is automodulation effect to line polarized light, using photodetector under main field effect (4) detect light intensity change, using z-axis light intensity signal as reference signal, x-axis light intensity signal as signal to be demodulated, using lock phase Amplifier (5) obtains y to output (7) and z to output (8), so as to be calculated B_yAnd B_z。

2. the method for Bell-Bloom automodulations three-axle magnetic field measurement according to claim 1, it is characterised in that：It is described to take out Fortune light is the circularly polarized light of amplitude modulation, and wavelength is alkali metal atom D1 line jump frequency respective wavelengths.

3. the method for Bell-Bloom automodulations three-axle magnetic field measurement according to claim 1, it is characterised in that：The inspection Light-metering is continuous line polarized light, and wavelength is alkali metal atom D2 lines or D1 line jump frequency respective wavelengths.

4. the method for Bell-Bloom automodulations three-axle magnetic field measurement according to claim 1, it is characterised in that：It is described to take out Fortune light is orthogonal with detection light and at grade.

5. the method for Bell-Bloom automodulations three-axle magnetic field measurement according to claim 1, it is characterised in that：It is describedMain field is approximate along the x-axis direction；According to the principle of single beam Bell-Bloom scalar magnetometers, alkali gold Category atom can be around magnetic field precession under external magnetic field, and the precession frequency of alkali metal atom is with the relation of external magnetic field：

ω_L=2 π γ B (1)

Wherein, γ is the gyromagnetic ratio of the atom；

Change x-axis magnetic field size, when precession frequency and modulating frequency meet resonance condition, resonate, by the photoelectricity of z-axis Detector can find the Frequency point that resonance occurs；The resonance condition of single beam Bell-Bloom gaussmeters is：

ω_L=m ω_mod, m=1,2,3 ... (2)

Wherein, m=1, i.e. ω are worked as_L=ω_modWhen, resonant amplitude is maximum, and covibration is most obvious, according to resonant amplitude maximum Modulating frequency ω_modWith added magnetic field △ B_x, that is, calculate x-axis magnetic field：

B_x=ω_mod/2πγ-△B_x (3)。

6. the method for Bell-Bloom automodulations three-axle magnetic field measurement according to claim 1, it is characterised in that：By B_yDraw The x-axis light intensity for rising changes and by B_zThe x-axis light intensity variation phase difference for causing for pi/2, with x-axis light intensity signal with frequency z-axis light Strong signal obtains B as reference using lock-in amplifier demodulation_yAnd B_z。

7. a kind of Bell-Bloom automodulations three-axle magnetic field measurement apparatus are designed based on claim 1 methods described, its feature exists In：Mutually put including pumping laser device (1), detection light laser (2), alkali metal atom air chamber (3), photodetector (4), lock Big device (5), signal generator (9), the driver (10) of acousto-optic modulator, acousto-optic modulator (11), reflecting mirror (12), beam expanding lens (13), the polarizer (14), quarter wave plate (15) and feedback controller (16)；The output signal respectively x with three-axle magnetic field information To output (6), y to output (7) and z to output (8)；

The magnetic-field measurement in x-axis direction is：Signal generator (9) is exported to (10) modulation letters of driver of acousto-optic modulator Number, driver (10) output of acousto-optic modulator is loaded with the interior acousto-optic crystalline substance of radiofrequency signal control acousto-optic modulator (11) of modulated signal Diffracting effect of the body to laser；The line polarized light that pumping laser device (1) is produced injects acousto-optic modulator through reflecting mirror (12) (11) in, 1 order diffraction light of amplitude modulation is obtained, the line polarized light of amplitude modulation becomes directional light, Jing through beam expanding lens (13) Crossing becomes circularly polarized light after the polarizer (14) and quarter wave plate (15), circularly polarized light is to the alkali metal atom reality in atomic air chamber (3) Existing optical pumping, as pumping light；Along the x-axis direction plus a magnetic field, it is desirable toMain field is made approximately along x-axis Direction, under external magnetic field, alkali metal atom is around the precession of main field direction, precession frequency ω_L=2 π γ B, wherein γ are The gyromagnetic ratio of the atom, changes x-axis magnetic field size, when the precession frequency of atom and the modulating frequency of pumping light meet condition ω_L =m ω_mod, m=1,2,3 ... when resonate；By B_yAnd B_zCompensate to 0 point, the change of light intensity is detected using photodetector (4) Change, find resonant amplitude it is maximum when modulating frequency and added magnetic field △ B_x, now B_x=ω_mod/2πγ-△B_x；

The magnetic-field measurement in y-axis direction and z-axis direction is：The line polarized light that detection light laser (2) is produced is through reflecting mirror (12) It is emitted directly toward alkali metal atom air chamber (3), incident direction is vertical with pumping light direction and through pumping light, when y-axis or z-axis are present During one small magnetic field for being not higher than main field, there is projection in precession vector of the atom around total magnetic field in x-axis, have to detection light certainly Modulating action, the change of detection light light intensity generating period, and B_yThe light intensity change of generation and B_zThe light intensity change frequency phase of generation Together, phase contrast is pi/2；The light intensity change of detection light is obtained using photodetector (4), as signal to be demodulated, and pumping light Light intensity change is turned to reference signal, and reference signal is input in lock-in amplifier (5) simultaneously with signal to be demodulated, lock-in amplifier (5) homophase passage (in-phase) and out-phase passage (out-of-phase) exports respectively B_yCorresponding magnitude of voltage and B_zCorrespondence Magnitude of voltage, work as B_y=B_zWhen=0, the output of two passages is all 0, so the natural zero crossing of atom automodulation, for realizing closed loop B very advantageous, that lock-in amplifier (5) is exported_yCorresponding magnitude of voltage and B_zCorresponding magnitude of voltage difference input feedback control In device (16), the electric current of output is added separately in y-axis and z-axis line circle, accurate to compensate y-axis and z-axis magnetic field, works as lock-in amplifier When (5) two passage outputs are all 0, B_yAnd B_zAdded magnetic field size as on two axial lines circle.