A dead zone-free laser atomic magnetism measurement method and device

Abstract

The present invention discloses a method and device for laser atomic magnetometry with zero dead zones. This device uses a high-speed polarization modulator to continuously and rapidly modulate the polarization state of the probe light. When a dead zone occurs in pump light with one polarization state, pump light with another polarization state can compensate for it, eliminating the dead zone and enabling all-around, zero-dead-zone magnetic field detection. Therefore, the present invention has strong engineering application value and practicality.

Description

Technical Field

[0001] The present invention belongs to the technical field of magnetic measurement, and in particular relates to a dead zone-free laser atomic magnetic measurement method and device. Background Art

[0002] Magnetic field detection is one of the most effective methods of geophysical exploration. Differences between the object being measured and its surroundings cause differences in the magnetic field. By measuring and analyzing the magnetic field, the depth, location, and even age of the object can be determined. Magnetic field detection has a wide range of applications, including anti-submarine warfare, geological surveys, magnetic fuses, underwater detection, unexploded ordnance detection, and humanitarian aid.

[0003] Among various types of magnetometers, atomic magnetometers use atoms as magnetically sensitive media. Their basic principle is to obtain magnetic field information by utilizing the various effects that occur after light passes through atomic gas. They have the advantages of high sensitivity, low power consumption, and small size.

[0004] Due to principle limitations, existing atomic magnetometers cannot measure magnetic fields along a specific direction, that is, a "dead zone" phenomenon will occur, which makes them have certain limitations in actual measurements. Summary of the Invention

[0005] In view of the above, the present invention provides a dead-zone-free laser atomic magnetism measurement method and device, which can realize all-round dead-zone-free magnetism measurement.

[0006] A dead-zone-free laser atomic magnetometry method and device, characterized by comprising a laser frequency stabilization subsystem, a magnetic field detection subsystem, and a signal and data processing subsystem. The laser frequency stabilization subsystem is used to output narrow-linewidth, stable-wavelength laser light, which is used for both atomic pumping and magnetic field detection. Within the magnetic field detection subsystem, the laser's polarization state is modulated, passing through an atomic gas chamber and then a photodetector. The output signal of the photodetector is demodulated and processed by the signal and data processing system to obtain a magnetic field signal. This scheme continuously and rapidly modulates the polarization state of the detection light. When a dead zone occurs in the detection light of one polarization state, the detection light of another polarization state can compensate for it, thereby eliminating the dead zone and achieving all-round, dead-zone-free magnetic field detection.

[0007] Furthermore, the laser frequency stabilization subsystem includes:

[0008] A signal source, used to generate a reference signal to perform phase modulation and demodulation on the laser;

[0009] A phase modulator, used to modulate the reference signal generated by the signal source into the optical domain;

[0010] The first atomic gas cell is used for absorption resonance calibration and coarse setting of laser frequency shift;

[0011] an optical circulator, configured to transmit reflected light from the ultra-stable optical cavity to a first photodetector;

[0012] Ultra-stable optical resonator, used to select the wavelength of the laser output from the laser frequency stabilization system and narrow the output laser linewidth;

[0013] a first photodetector, configured to convert an optical signal emitted from the optical resonant cavity into an electrical signal;

[0014] a first mixer, configured to mix the electrical signal received by the photodetector with a reference signal to generate a phase difference signal;

[0015] a first low-pass filter, configured to perform low-pass filtering on the mixed phase difference signal and output an error signal for controlling a precision current source;

[0016] Precision current source, used to control the laser input current, thereby controlling the wavelength of the laser output;

[0017] a laser for generating a detection laser;

[0018] The optical coupler is used to split the laser output into two parts with a ratio of 10:90, of which 10% of the laser is used for laser frequency stabilization and 90% of the laser is used for magnetic measurement.

[0019] Furthermore, the laser output by the laser frequency stabilization subsystem is locked to the D1 line 6S of the cesium atom. 1 / 2 , F = 4 to 6S 1 / 2 , on the F=5 spectral line, the wavelength is 895 nanometers, and the output optical power of the laser is about 10 milliwatts.

[0020] Furthermore, the resonant wavelength of the ultra-stable optical resonator is consistent with the output wavelength of the laser, both being 895 nanometers.

[0021] Furthermore, the magnetic field detection subsystem includes:

[0022] A polarization modulator for high-speed polarization modulation of the input laser;

[0023] The second atomic gas chamber has cesium atoms inside that produce a magneto-optical effect with the input detection light;

[0024] The second photoelectric detector is used to convert the detection light signal after passing through the atomic gas chamber into an electrical signal.

[0025] Furthermore, the polarization modulator is composed of a fiber collimator, a polarizer, a quarter-wavelength wave plate and a lead magnesium niobate-lead titanate (PMN-PT) crystal, and has the advantages of a large electro-optical effect and a low half-wave voltage. The detection light outputs a collimated light beam through the fiber collimator, becomes circularly polarized light after passing through the polarizer and the quarter-wavelength wave plate, and is input into the PMN-PT electro-optical crystal. A polarization modulation electric field is applied to the PMN-PT crystal, so that the polarization state of the input laser undergoes a periodic change from left-handed circular polarization, left-handed elliptical polarization, linear polarization, right-handed elliptical polarization, and right-handed circular polarization. The polarization modulation control signal is generated and controlled by the signal and data processing system, and its frequency ω0 corresponds to the Larmor frequency ω corresponding to the magnetic field in which the cesium atom is located. L According to the Larmor frequency ω L Relationship with magnetic field Bω L =γB to obtain the magnetic field size, where γ is the gyromagnetic ratio of the cesium atom.

[0026] Furthermore, the first and second atomic gas chambers are identical, containing cesium atomic gas. A buffer gas is added to the chambers, and an anti-relaxation film is coated on the inner walls of the chambers to suppress the atomic relaxation mechanism. High-frequency alternating current is applied to a non-magnetic heating plate to heat the second atomic gas chamber to control the saturated vapor pressure of the atoms.

[0027] Furthermore, the photodetector is a non-magnetic element that converts the light signal after passing through the atomic gas chamber into an electrical signal.

[0028] Furthermore, the signal and data processing system includes:

[0029] Microprocessor, used to detect, process electrical signal data and control the entire measuring device;

[0030] Direct digital frequency synthesizer, used to generate polarization modulation control signal, signal frequency ω0 corresponds to Larmor frequency ω L , and is also used to generate a reference signal for demodulation;

[0031] a second mixer, configured to mix the detection signal received by the photodetector with a reference signal generated by a direct digital frequency synthesizer;

[0032] A second low-pass filter, configured to perform low-pass filtering on the mixed signal;

[0033] A first electrical amplifier, configured to amplify the weak signal of the low-pass filtered signal;

[0034] Analog-to-digital converter, used to convert the low-pass filtered electrical signal into a digital signal and provide it to the microprocessor for data processing;

[0035] a bandpass filter for filtering a polarization control signal generated by a direct digital frequency synthesizer;

[0036] The second electrical amplifier is used to amplify the polarization control signal after bandpass filtering.

[0037] According to theoretical calculations, the angle between the laser transmission direction and the quantization axis is When the angle is 90 degrees, the circularly polarized pump light is insensitive to magnetic field detection and a dead zone appears. When the polarization is 54 / 126 degrees, linearly polarized pump light is insensitive to magnetic field detection, resulting in a dead zone. Therefore, the device of the present invention uses a high-speed polarization modulator to modulate the polarization state of the pump light. When a dead zone occurs in the pump light of one polarization state, the pump light of another polarization state can compensate for it, thereby eliminating the dead zone and achieving omnidirectional magnetic field detection without a dead zone. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is a system block diagram of the device of the present invention, which mainly includes a laser frequency stabilization system, a magnetic measurement system, and a signal and data processing system.

[0039] Figure 2 Schematic diagram of the specific structure of the device of the present invention.

[0040] In the figure: 1—signal source, 2—phase modulator, 3—first atomic gas chamber, 4—optical circulator, 5—ultra-stable optical resonator ORC, 6—first photodetector, 7—first mixer, 8—first low-pass filter, 9—precision current source PCS, 10—laser, 11—optical coupler OC, 12—polarization modulator, 13—second atomic gas chamber, 14—second photodetector, 15—second mixer, 16—second low-pass filter, 17—first electrical amplifier, 18—analog-to-digital converter ADC, 19—microprocessor CPU, 20—direct digital frequency synthesizer DDS, 21—bandpass filter, 22—second electrical amplifier.

[0041] Figure 3 This is a diagram of the polarization modulator used in the device of the present invention.

[0042] Figure 4 In the figure, (a) and (b) are schematic diagrams of the principle of high-sensitivity magnetic field signal tracking and detection, and (c) is a diagram of the implementation method of high-sensitivity magnetic field signal tracking and detection. DETAILED DESCRIPTION

[0043] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0044] like Figure 1As shown, the device of the present invention is mainly composed of a laser frequency stabilization subsystem, a magnetic measurement subsystem and a signal and data processing subsystem.

[0045] like Figure 2 As shown, a dead-zone-free laser atomic magnetometry method and device is provided. The entire device includes a signal source 1, a phase modulator 2, a first atomic gas chamber 3, an optical circulator 4, an ultra-stable optical resonator 5, a first photodetector 6, a first mixer 7, a first low-pass filter 8, a precision current source 9, a laser 10, an optical coupler 11, a polarization modulator 12, an atomic gas chamber 13, a second photodetector 14, a second mixer 15, a second low-pass filter 16, a first electrical amplifier 17, an analog-to-digital converter 18, a microprocessor 19, a direct digital frequency synthesizer 20, a bandpass filter 21, and a second electrical amplifier 22.

[0046] The principle of the present invention to achieve magnetic measurement is as follows: the laser is polarized and modulated, and the modulated laser passes through the atomic gas chamber and is received by the photodetector after being affected by the atomic Larmor precession, and then the received signal is demodulated. L When , the amplitude of the demodulated signal will reach an extreme value. By sweeping the frequency of the polarization modulation signal (i.e., resonant scanning mode), the extreme point of the demodulated signal can be obtained, and then the Larmor frequency ω can be obtained. L , according to the Larmor frequency ω L The relationship between the polarization of the laser and the magnetic field can be used to obtain the magnitude of the magnetic field. In the present invention, the polarization of the laser can be continuously and rapidly modulated over time, thereby achieving all-round and dead-zone-free magnetic field measurement.

[0047] In this embodiment, the laser light output by the laser 10 is phase modulated by the phase modulator 2, and the modulation signal is generated by the signal source 1. The modulated laser signal first enters the first atomic gas chamber 3, and through coarse adjustment, the laser wavelength output by the laser 10 falls within the absorption peak range of the cesium atomic gas. The laser light then enters port 1 of the optical circulator 4, enters the ultra-stable optical resonator 5 through port 2 of the optical circulator 4, resonates with the ultra-stable optical resonator 5, and its reflected light is input to the first photodetector 6 through port 3 of the optical circulator 4 to obtain a reflected light signal. The reflected light signal is mixed with the reference signal generated by the signal source 1 through the first mixer 7. The mixed signal is filtered by the first low-pass filter 8 to obtain an error signal. The error signal is fed back to the precision current source 9 through the PID system to control the current input to the laser 10, ultimately achieving the goal of locking the laser output to the resonant wavelength of the ultra-stable optical resonator 5.

[0048] In this embodiment, the output laser of the laser 10 is locked to the D1 line 6S of the cesium atom. 1 / 2 , F = 4 to 6S 1 / 2The output laser of the laser 10 is split by the optical coupler 11, with 10% of the laser beam entering the laser frequency stabilization subsystem and 90% of the laser beam entering the magnetic measurement subsystem.

[0049] In this embodiment, the laser beam for magnetic force measurement after being split by the optical coupler 11 is connected to the high-speed polarization modulator 12 through the polarization-maintaining optical fiber. Figure 3 As shown, it is mainly composed of a polarization-maintaining fiber collimator, a polarizer, a 1 / 4 wavelength wave plate and a PMN-PT electro-optic crystal, and has the advantages of a large electro-optic effect and a low half-wave voltage.

[0050] In this embodiment, the polarization of the laser used for magnetic measurement is modulated after passing through a high-speed polarization modulator 12. The polarization state undergoes a periodic change from left-handed circular polarization, left-handed elliptical polarization, linear polarization, right-handed elliptical polarization, and right-handed circular polarization. A polarization modulation control signal is generated by a direct digital frequency synthesizer 20 controlled by a microprocessor 19. This control signal is filtered by a bandpass filter 21 and amplified by a second amplifier 22 before being applied to the modulation electrodes of the high-speed polarization modulator 12.

[0051] In this embodiment, in order to determine the exact magnitude of the magnetic field, it is necessary to determine the approximate range of the magnetic field strength before measurement, and then set the frequency sweep range of the output signal of the direct frequency synthesizer 18 according to the Larmor frequency range corresponding to the magnetic field range.

[0052] In this embodiment, the polarization modulated laser passes through the second atomic gas chamber 13, and a large number of gas electrons in the second atomic gas chamber 13 are fully excited to the spin excited state by the input laser. When the modulation frequency ω0 of the laser polarization is equal to the Larmor frequency ω of the magnetic field in which the atom is located, L , the dipole moment and quadrupole moment of the atom precess simultaneously.

[0053] In this embodiment, the laser output by the second atomic gas chamber 13 is converted into an electrical signal after passing through the second photodetector 14. The signal is input to the mixer 15, and the other input signal in the mixer 15 is the reference signal output by the direct frequency synthesizer 20, with a frequency of ω0 (or its double frequency 2ω0). The mixed signal is filtered by the low-pass filter 16 and amplified by the first electrical amplifier 17 (i.e., phase-locked amplification) to obtain a demodulated signal. The demodulated signal is sampled and converted into a digital signal by the analog-to-digital converter 18, and then the digital signal is transmitted to the microprocessor 19 for data processing and analysis. It can be seen from the measurement principle that the demodulated signal is a type of absorption line type, and the frequency corresponding to its extreme point is the Larmor frequency ω corresponding to the magnetic field in which the cesium atom is located. L , according to the Larmor frequency ω L The relationship between the magnetic field and the magnetic field can be used to obtain the magnitude of the magnetic field.

[0054] As an optional technical solution, the present invention can realize the tracking detection of high-sensitivity magnetic field signals. The principle is as follows: an additional frequency modulation signal with a fixed frequency of ω1 is modulated in the polarization modulation signal output by the direct frequency synthesizer. When the center frequency ω0 of the polarization modulation is equal to the Larmor frequency ω corresponding to the magnitude of the magnetic field in which the cesium atom is located, the frequency modulation signal is detected by the following method: L When the polarization modulation center frequency is inconsistent with the Larmor frequency ω1, the photodetector will generate an error signal with a frequency of ω1. L When the two signals are consistent, the photodetector will generate an error signal with a frequency of 2ω1. The principle is as follows Figure 4 (a) shows the output signal of the photodetector being input to a mixer. The other input of the mixer is a reference signal with a frequency of ω0. After mixing, low-pass filtering, and signal amplification (i.e., phase-locked amplification), the output signal is mixed and low-pass filtered with a reference signal with a frequency of ω1. The resulting error signal curve is a type of dispersion curve. The frequency corresponding to the zero-crossing point of the curve is the Larmor frequency ω. L ,like Figure 4 (b) is shown. The specific implementation is: the output frequency of the direct frequency synthesizer is a polarization modulated signal of ω0, which is at the Larmor frequency ω L Nearby, and modulate an additional FM signal with a fixed frequency of ω1 on the signal. The output signal of the photodetector is input to the mixer, and the other input of the mixer is a reference signal with a frequency of ω0. After mixing, low-pass filtering and signal amplification (i.e. after phase-locked amplification), the output signal is mixed and low-pass filtered with the reference signal with a frequency of ω1 to obtain an error signal. The error signal is digitally sampled and then compared with the 0 voltage value (corresponding to the Larmor frequency ω L ) is compared to obtain a deviation from resonance state signal, which is input into the digital incremental PID system controlled by the microcontroller to obtain a tracking control signal. The control word of the center frequency ω0 is calculated by tracking the control signal and fed back to the direct frequency synthesizer to maintain the error signal at 0 voltage value, thereby tracking the Larmor frequency ω of the detection magnetic field. L ,like Figure 4 (c) shown.

[0055] The above description of the embodiments is intended to facilitate understanding and application of the present invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention should fall within the scope of protection of the present invention.

Claims

1. A dead zone-free laser atomic magnetometry device, characterized in that: It includes a laser frequency stabilization subsystem, a magnetic field detection subsystem, and a signal and data processing subsystem. The laser frequency stabilization subsystem is used to output a narrow-linewidth, stable-wavelength laser, which is used for both atomic pumping and magnetic field detection. In the magnetic field detection subsystem, the polarization state of the laser is modulated, passes through the atomic gas chamber, and then passes through a photodetector. The output signal of the photodetector is demodulated and processed by the signal and data processing system to obtain a magnetic field signal. The laser frequency stabilization subsystem includes: A signal source, used to generate a reference signal to perform phase modulation and demodulation on the laser; A phase modulator, used to modulate the reference signal generated by the signal source into the optical domain; The first atomic gas cell is used for absorption resonance calibration and coarse setting of laser frequency shift; an optical circulator, configured to transmit reflected light from the ultra-stable optical cavity to a first photodetector; Ultra-stable optical resonator, used to select the wavelength of the laser output from the laser frequency stabilization system and narrow the output laser linewidth; a first photodetector, configured to convert an optical signal emitted from the optical resonant cavity into an electrical signal; a first mixer, configured to mix the electrical signal received by the photodetector with a reference signal to generate a phase difference signal; a first low-pass filter, configured to perform low-pass filtering on the mixed phase difference signal and output an error signal for controlling a precision current source; Precision current source, used to control the laser input current, thereby controlling the wavelength of the laser output; Laser, used to generate laser light; Optical coupler, used to split the laser output into two parts with a ratio of 10:90, 10% of the laser is used for laser frequency stabilization and 90% of the laser is used for magnetic measurement; The laser output of the laser frequency stabilization subsystem is locked to the D1 line 6S of the cesium atom 1 / 2 , F=4 to 6S 1 / 2 , on the F=5 spectral line, the wavelength is 895 nanometers, and the output optical power of the laser is about 10 milliwatts; The magnetic field detection subsystem includes: A polarization modulator for high-speed polarization modulation of the input laser; The second atomic gas chamber has cesium atoms inside that produce a magneto-optical effect with the input detection light; a second photodetector, configured to convert the detection light signal after passing through the second atomic gas chamber into an electrical signal; The polarization modulator is composed of a fiber collimator, a polarizer, a quarter-wavelength wave plate and a PMN-PT crystal; the detection light outputs a collimated beam through the fiber collimator, becomes circularly polarized light after passing through the polarizer and the quarter-wavelength wave plate, and is input into the PMN-PT electro-optical crystal; a polarization modulation electric field is applied to the PMN-PT crystal to cause the polarization state of the input laser to undergo a periodic change from left-handed circular polarization, left-handed elliptical polarization, linear polarization, right-handed elliptical polarization, and right-handed circular polarization; the polarization modulation control signal is generated and controlled by the signal and data processing system, and its frequency ω0 corresponds to the Larmor frequency ω corresponding to the magnetic field in which the cesium atom is located L ; According to the Larmor frequency ω L Relationship with magnetic field B The magnitude of the magnetic field is obtained, where is the gyromagnetic ratio of cesium atoms.

2. The measuring device according to claim 1, characterized in that: The resonant wavelength of the ultra-stable optical resonator is consistent with the output wavelength of the laser, both of which are 895 nanometers.

3. The measuring device according to claim 1, wherein: The first atomic gas chamber and the second atomic gas chamber are exactly the same. The gas chamber is filled with cesium atomic gas, and a buffer gas is added to the gas chamber and an anti-relaxation film is coated on the inner wall of the gas chamber to suppress the relaxation mechanism of the atoms. High-frequency alternating current is used to load the non-magnetic heating plate to heat the second atomic gas chamber to control the saturated vapor pressure of the atoms.

4. The measuring device according to claim 1, wherein: The photoelectric detector is a non-magnetic element that converts the light signal after passing through the atomic gas chamber into an electrical signal.

5. The measuring device according to claim 1, characterized in that: The signal and data processing system includes: Microprocessor, used to detect, process electrical signal data and control the entire measuring device; Direct digital frequency synthesizer, used to generate polarization modulation control signal, signal frequency ω0 corresponds to Larmor frequency ω L , and is also used to generate a reference signal for demodulation; a second mixer, configured to mix the detection signal received by the photodetector with a reference signal generated by a direct digital frequency synthesizer; A second low-pass filter, configured to perform low-pass filtering on the mixed signal; A first electrical amplifier, configured to amplify the weak signal of the low-pass filtered signal; Analog-to-digital converter, used to convert the low-pass filtered electrical signal into a digital signal and provide it to the microprocessor for data processing; a bandpass filter for filtering a polarization control signal generated by a direct digital frequency synthesizer; The second electrical amplifier is used to amplify the polarization control signal after bandpass filtering.

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