A SERF magnetic field weak measurement detection method based on laser spatial mode

Abstract

A method for measuring and detecting weak SERF magnetic fields based on laser spatial modes avoids signal drift caused by fluctuations in laser power by directly measuring the spatial displacement of the detection laser beam. The method includes: Step 1, establishing a Sagnac interference loop using the transmitted and reflected light formed by the first polarizing beam splitter on the laser emission path of the alkali metal gas cell; Step 2, after mutual interference, the transmitted and reflected light return to the first polarizing beam splitter, combine, and exit the Sagnac interference loop, then pass sequentially through a second λ / 2 waveplate and a second polarizing beam splitter to complete a selection process; Step 3, using a quadrant detector to identify the change in detection beam displacement caused by the linear polarization rotation angle of the detection laser after the selection process; Step 4, determining the linear polarization rotation angle by the change in detection beam displacement, and then determining the x-direction component of the electron spin polarization vector using the linear polarization rotation angle, thereby achieving the measurement of weak SERF magnetic fields.

Description

Technical Field

[0001] This invention relates to the field of spin magnetic field measurement technology for spin-free exchange-relaxed atoms, and particularly to a method for measuring and detecting weak SERF magnetic fields based on laser spatial modes. Background Technology

[0002] Spin-Exchange Relaxation Free (SERF) magnetic field measurement devices possess extremely high sensitivity for theoretical magnetic field measurements. Their subsystems include a pump optics system, a detection optics system, a magnetic shielding and magnetic compensation system, and an electronic control and acquisition system. The detection optics system, based on the optical rotation effect of linearly polarized light, detects the rotation angle of the detection laser after it rotates through the SERF-state alkali metal gas cell. Since the rotation angle is related to the magnetic field being measured, this method enables the measurement of weak magnetic fields. Therefore, fluctuations in the detection laser power and noise in the detection system significantly affect the sensitivity of SERF atomic spin magnetic field measurements.

[0003] In SERF atomic spin magnetic field measurements, the magnetic field induces optical rotation in linearly polarized light passing through the alkali metal gas cell. The magnitude of the rotation angle is directly proportional to the output detection laser power. Therefore, detection laser errors and noise caused by power fluctuations are directly introduced into the measured signal, thus affecting the accuracy and sensitivity of the magnetic field measurement. Therefore, a novel detection method is needed to avoid the influence of detection laser power fluctuations on the measured signal. Summary of the Invention

[0004] To address the impact of laser power fluctuations on the measured signal in SERF atomic spin magnetic field measurements, this invention proposes a weak SERF magnetic field measurement detection method based on laser spatial patterns. Based on weak measurement theory, and using the spatial pattern of the detection laser as a measurement pointer, a weak SERF magnetic field measurement detection model based on the detection laser spatial pattern is established. This model directly measures the lateral displacement of the detection laser caused by the magnetic field, thereby achieving magnetic field sensitivity measurement. This method suppresses the influence of laser power fluctuations on the measured signal in SERF magnetic field measurements.

[0005] The technical solution of the present invention is as follows:

[0006] A method for measuring and detecting weak SERF magnetic fields based on laser spatial modes, characterized by comprising the following steps:

[0007] Step 1: Establish a Sagnac interference loop using the transmitted and reflected light formed by the first polarizing beam splitter on the output side of the detection laser in the alkali metal gas cell.

[0008] Step 2: The transmitted and reflected light after interfering with each other on the Sagnac interference loop return to the first polarizing beam splitter, are combined, and exit the Sagnac interference loop. After passing through the second λ / 2 waveplate and the second polarizing beam splitter in sequence, the selection process is completed.

[0009] Step 3: Use a quadrant detector to identify the change in the displacement of the detection beam caused by the selected process after the completion of the detection laser linear polarization rotation angle.

[0010] Step 4: Determine the linear polarization rotation angle by detecting the change in beam displacement, and determine the x-direction component of the electron spin polarization vector by the linear polarization rotation angle, thereby realizing the weak measurement of the SERF magnetic field.

[0011] The weak coupling effect is achieved by a weak coupling module based on the Sagnac interference loop. The weak coupling module includes a second reflecting mirror connected to the transmission side of the first polarizing beam splitter, and a first reflecting mirror connected to the reflection side of the first polarizing beam splitter and the second reflecting mirror, respectively. The transmitted light formed by the first polarizing beam splitter propagates clockwise in the Sagnac interference loop, and the reflected light formed by the first polarizing beam splitter propagates counterclockwise in the Sagnac interference loop.

[0012] The first reflector is mounted on a dual-axis displacement stage. The dual-axis displacement stage adjusts the displacement g of the first reflector so that the coupling strength of the detection beam in space is g after weak coupling in space.

[0013] The second λ / 2 waveplate and the second polarizing beam splitter constitute a post-selection module. The right side of the post-selection module is connected to the quadrant detector, and the left side of the post-selection module is connected to the first polarizing beam splitter via a third reflecting mirror.

[0014] The detection laser incident side of the alkali metal gas chamber is connected to a narrow linewidth laser via a λ / 4 waveplate, a photoelastic modulator, a first λ / 2 waveplate, and a Glan-Taylor prism.

[0015] The alkali metal gas chamber is located inside the triaxial magnetic compensation coil, which is located inside the passive magnetic shielding layer.

[0016] The laser wavelength emitted by the narrow linewidth laser is detuned to the D2 line resonance peak of the alkali metal atoms.

[0017] Step 4 includes the following formula:

[0018]

[0019] Among them U outputK1 represents the scale coefficient between the displacement and the final output voltage signal, θ is the product of the detector's photoelectric conversion coefficient and current amplification factor, σ is the linear polarization rotation angle, σ is the waist of the Gaussian beam, and g is the displacement of the detection beam in space.

[0020] The technical effects of this invention are as follows: This invention provides a SERF weak magnetic field measurement and detection method based on laser spatial mode, which avoids the drift of detection signal caused by the fluctuation of detection laser power by measuring the magnetic field of the spatial displacement of the detection laser beam.

[0021] The advantages of this invention compared to the prior art are as follows:

[0022] (1) Based on the general process of weak measurement detection method, this invention establishes a weak measurement magnetic field detection model based on the detection laser spatial mode, and realizes quantum precision measurement of magnetic field.

[0023] (2) The present invention directly measures the spatial displacement of the detection laser beam, avoiding the drift of the detection signal caused by the fluctuation of the detection laser power. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the device structure involved in implementing the SERF weak magnetic field measurement and detection method based on the detection of laser spatial mode according to the present invention.

[0025] The reference numerals in the attached figures are explained as follows: 1-Narrow linewidth laser; 2-Glan Taylor prism; 3-First λ / 2 waveplate; 4-Photo-elastic modulator (PEM); 5-λ / 4 waveplate; 6-First polarization beam splitter; 7-First mirror; 8-Second mirror; 9-Third mirror; 10-Dual-axis displacement stage; 11-Second λ / 2 waveplate; 12-Second polarization beam splitter; 13-Quadrant detector; 14-Alkali metal gas cell; 15-Triaxial magnetic compensation coil; 16-Passive magnetic shielding layer; 17-Weakly coupled module; 18-Back selection module. Detailed Implementation

[0026] The following is in conjunction with the attached diagram ( Figure 1 The invention will be described in the following sections and examples.

[0027] Figure 1 This is a schematic diagram of the device structure involved in implementing the SERF weak magnetic field measurement and detection method based on laser spatial mode according to the present invention. (Reference) Figure 1As shown, a method for measuring and detecting weak SERF magnetic fields based on laser spatial mode includes the following steps: Step 1, establishing a Sagnac interference loop using the transmitted and reflected light formed by the first polarization beam splitter on the output side of the detection laser in the alkali metal gas cell; Step 2, the transmitted and reflected light after interfering with each other on the Sagnac interference loop return to the first polarization beam splitter, are combined, and exit the Sagnac interference loop, then pass through the second λ / 2 waveplate and the second polarization beam splitter in sequence to complete the selection process; Step 3, using a quadrant detector to identify the change in detection beam displacement caused by the linear polarization rotation angle of the detection laser after the completion of the selection process; Step 4, determining the linear polarization rotation angle by detecting the change in beam displacement, determining the x-direction component of the electron spin polarization vector by the linear polarization rotation angle, and thus realizing the measurement of weak SERF magnetic fields.

[0028] The Sagnac interference loop is implemented through a weak coupling module 17. The weak coupling module 17 includes a second reflecting mirror 8 connected to the transmission side of the first polarizing beam splitter 6, and a first reflecting mirror 7 connected to the reflection side of the first polarizing beam splitter 6 and the second reflecting mirror 8, respectively. The transmitted light formed by the first polarizing beam splitter 6 propagates clockwise in the Sagnac interference loop, and the reflected light formed by the first polarizing beam splitter 6 propagates counterclockwise in the Sagnac interference loop. The first reflecting mirror 7 is mounted on a biaxial displacement stage 10. The biaxial displacement stage 10 adjusts the displacement g of the first reflecting mirror to achieve a weak coupling with a spatial coupling strength g for the detection beam.

[0029] The second λ / 2 waveplate 11 and the second polarizing beam splitter 12 constitute a post-selection module 18. The right side of the post-selection module 18 is connected to the quadrant detector 13, and the left side of the post-selection module 18 is connected to the first polarizing beam splitter 6 via a third reflecting mirror 9. The detection laser incident side of the alkali metal gas chamber 14 is connected to the narrow linewidth laser 1 in sequence via a λ / 4 waveplate 5, a photoelastic modulator 4, a first λ / 2 waveplate 3, and a Glan-Taylor prism 2. The alkali metal gas chamber 14 is located inside a triaxial magnetic compensation coil 15, which is located inside a passive magnetic shielding layer 16. The laser wavelength emitted by the narrow linewidth laser 1 is far from the detuned alkali metal atom D2 line resonance peak.

[0030] Step 4 includes the following formula:

[0031]

[0032] Where K1 represents the calibration coefficient between the displacement and the final output voltage signal, which is the product of the detector's photoelectric conversion coefficient and current amplification factor, U output It is the signal detected by the quadrant detector, and θ is the linear polarization rotation angle.

[0033] This invention discloses a weak measurement and detection method for SERF magnetic fields based on laser spatial modes, comprising a narrow-linewidth laser 1, a Glan Taylor prism 2, a first λ / 2 waveplate 3, a photoelastic modulator 4, a λ / 4 waveplate 5, a first polarizing beam splitter 6, a first reflecting mirror 7, a second reflecting mirror 8, a third reflecting mirror 9, a biaxial displacement stage 10, a second λ / 2 waveplate 11, a second polarizing beam splitter 12, a quadrant detector 13, an alkali metal gas cell 14, a triaxial magnetic compensation coil 15, and a passive magnetic shielding layer 16, as well as a weak coupling module 17 and a post-selection module 18. This invention utilizes weak measurement and detection theory to detect the rotation angle of linearly polarized light in a SERF atomic spin precession magnetic field measurement system, achieving ultra-high sensitivity measurement of the magnetic field. Furthermore, this invention uses the beam spatial mode as the measurement pointer, directly measuring the lateral displacement of the Gaussian beam in the measurement space caused by the rotation angle of the linearly polarized light, thus avoiding the influence of laser power drift on the magnetic field measurement signal and improving the sensitivity of SERF magnetic field measurement.

[0034] The laser emitted from a narrow-linewidth semiconductor laser is converted into horizontally polarized light by a Glan Taylor prism. After passing through a first λ / 2 waveplate, the horizontally polarized light becomes linearly polarized at a 45° angle to the horizontal. This light is then modulated by a combination of a photoelastic modulator and a λ / 4 waveplate before entering an alkali metal cell, where it exhibits optical rotation. The detection laser exiting the alkali metal cell carries information about the optical rotation angle, which is proportional to the magnetic field being measured. The detection laser exiting the alkali metal cell passes through a first polarizing beam splitter. The transmitted light enters the Sagnac interference loop and propagates clockwise, while the reflected light enters the Sagnac interference loop and propagates counterclockwise. The two laser beams are then combined by the first polarizing beam splitter and exit the Sagnac interference loop. Finally, the selection process is completed by a combination of a second λ / 2 waveplate and a second polarizing beam splitter.

[0035] The laser is a narrow linewidth laser, and the emitted laser wavelength is detuned and resonates with the D2 line resonance peak of alkali metal atoms.

[0036] The alkali metal gas chamber contains one or two alkali metals, namely potassium, rubidium, and cesium, as an atomic system for the sensitive magnetic field, and the alkali metal atoms in the gas chamber are in the SERF state.

[0037] The quadrant detector described above can detect the spatial pattern of the beam and has the ability to measure beam displacement at high resolution.

[0038] The weak measurement and detection process includes the preparation of the pre-selected state, the weak coupling process, and the post-selection process. The polarization rotation angle of the detection light caused by the SERF system acts on the pre-selected state preparation process, and the weak coupling process and the post-selection process are realized by optical devices in the weak coupling module and the post-selection module.

[0039] The weak measurement process is described in which the measuring pointer detects the spatial mode of the laser, and the direct measurement quantity is the displacement of the beam at the dark port.

[0040] The weak coupling module is implemented through a Sagnac interference loop, which detects the interference between the transmitted and reflected light of the laser after passing through the first polarization beam splitter in the loop, and forms a dark port and a bright port at the transmission and reflection exits of the second polarization beam splitter, respectively.

[0041] In the weak coupling module, a weak coupling displacement g of the detection beam in the spatial mode is introduced by adjusting the displacement of the first reflecting mirror 7.

[0042] Laser light emitted from a narrow-linewidth semiconductor laser is polarized after passing through a GlanTeller prism. Adjusting the angle of the GlanTeller prism produces linearly polarized light with a horizontal polarization direction. This light then passes through a first λ / 2 waveplate to adjust the polarization plane, resulting in an initial state where the initial laser polarization plane makes a 45° angle with the horizontal. Using the horizontally linearly polarized light |H> and the vertically linearly polarized light |V> as a substrate, the quantum state |ψ| of the initial laser state is represented. i0 >For:

[0043]

[0044] After passing through a combination of a photoelastic modulator and a λ / 4 waveplate, modulation occurs. The modulation amplitude and frequency can be controlled by the controller of the photoelastic modulator. When linearly polarized light passes through an alkali metal gas cell in the SERF state, it undergoes optical rotation, producing an optical rotation angle of θ. This optical rotation angle contains information about the magnetic field being measured. In SERF atomic spin magnetic field measurement, the relationship between the optical rotation angle θ and the magnetic field being measured can be expressed as:

[0045]

[0046] Where, n alkali Here, c is the density of the alkali metal, and r is the speed of light. e It is the classical radius of the electron, l represents the optical path length, and in the formula it is equal to the length of the gas cell, v D1 It is the resonant frequency of the D1 line in the alkali metal cell, f. D1 It is the resonance intensity on line D1, ν D2 It is the resonant frequency of the D2 line in the alkali metal cell, f. D2 It is the resonance intensity on line D2. The component of the electron spin polarization vector in the x-direction is proportional to the magnetic field being measured. Therefore, the relationship between the optical rotation angle and the magnetic field is established, and the measurement of the magnetic field can be transformed into the measurement of the optical rotation angle of linearly polarized light after passing through SERF-state alkali metal atoms. D1 ) and D(vv D2The following figures represent the absorption under the D1 and D2 lines for alkali metal gases, respectively:

[0047]

[0048] Γ L Pressure broadening for detecting laser beams.

[0049] After passing through the alkali metal gas cell in the SERF state, the polarization plane of the detection laser rotates. The quantum state of the detection laser that undergoes optical rotation is called the pre-selective state, denoted as:

[0050] |ψ i >=cos(π / 4-θ)|H>+sin(π / 4-θ)|V>

[0051] After the pre-selected laser beam passes through the first polarization beam splitter in the coupling module, it generates transmitted and reflected light. The transmitted light rotates clockwise, and the reflected light rotates counterclockwise. The two beams then converge and interfere after passing through the first polarization beam splitter. The displacement of the first reflecting mirror is adjusted by g, resulting in a weak coupling with a strength of g in the coupling module. After passing through the post-selection module, the spatial mode of the detected laser at the dark port can be observed. The post-selected state is:

[0052]

[0053] When θ is very small, slightly larger than the coupling parameter g, the detection beam at the dark port exhibits a double-peak configuration. When the magnetic field causes a change in the linear polarization rotation angle θ, the detection beam shows varying heights in the spatial double peaks. The displacement of the detection beam at this time can be expressed as:

[0054]

[0055] Here, σ represents the beam waist of the Gaussian beam. A quadrant detector can be used to identify displacement changes caused by the linear polarization rotation angle θ, thereby enabling magnetic field measurement. Based on this method, the beam is acquired at the first harmonic of the quadrant detector, and power spectral density analysis is performed to detect changes in magnetic field sensitivity. Furthermore, this method does not depend on changes in the detection laser power, thus avoiding the influence of detection laser fluctuations on the detection signal.

[0056] Preferably, the laser is a narrow linewidth laser, and the emitted laser wavelength is far detuned to the D2 line resonance peak of alkali metal atoms.

[0057] Preferably, the alkali metal gas chamber contains one or two alkali metals, namely potassium, rubidium, and cesium, as an atomic system for the sensitive magnetic field, and the alkali metal atoms in the gas chamber are in the SERF state.

[0058] Preferably, the quadrant detector can detect the spatial pattern of the beam and has the ability to measure beam displacement at high resolution.

[0059] Preferably, the weak measurement detection process includes the preparation of a pre-selected state, a weak coupling process, and a post-selection of the joint state, wherein the polarization rotation angle of the detection light caused by the SERF system acts on the pre-selected state preparation process.

[0060] Preferably, the weak measurement process uses a spatial mode for detecting the laser, and the direct measurement quantity is the displacement of the detection beam at the dark port.

[0061] Preferably, the Sagnac interference loop has a high interference ratio, and the transmitted and reflected light passing through the first polarizing beam splitter interferes in the loop, forming a dark port and a bright port at the transmission and reflection exits of the second polarizing beam splitter, respectively.

[0062] The implementation steps of this invention are as follows:

[0063] First, ensure that the alkali metal atoms in the alkali metal gas chamber are in the SERF state, enabling them to be sensitive to changes in the external magnetic field. A narrow-linewidth laser with a laser frequency far detuned to the D2 line resonance peak of the alkali metal atoms is used as the detection light source. After completing the triaxial magnetic compensation process, the detection laser exiting the alkali metal gas chamber is coupled into a weak measurement module with a coupling strength of g. Finally, after selection, the successfully selected detection laser spatial mode is observed. A quadrant detector is used to detect the displacement of the detection laser, and the quadrant detector signal is acquired and the demodulated first harmonic signal is recorded for power spectral density analysis.

[0064] Contents not described in detail in this specification are prior art known to those skilled in the art. It is hereby indicated that the above description is intended to help those skilled in the art understand this invention, but does not limit the scope of protection of this invention. Any equivalent substitutions, modifications, improvements, and / or simplifications of the above descriptions that do not depart from the essential content of this invention fall within the scope of protection of this invention.

Claims

1. A method for measuring and detecting weak SERF magnetic fields based on laser spatial modes, characterized in that, Includes the following steps: Step 1: Establish the Sagnac interference loop using the transmitted and reflected light formed by the first polarizing beam splitter on the output side of the detection laser in the alkali metal gas cell. Step 2: The transmitted and reflected light after interfering with each other on the Sagnac interference loop return to the first polarizing beam splitter, are combined, and exit the Sagnac interference loop. After passing through the second λ / 2 waveplate and the second polarizing beam splitter in sequence, the selection process is completed. Step 3: Use a quadrant detector to identify the change in the displacement of the detection beam caused by the selected process after the completion of the detection laser linear polarization rotation angle. Step 4: Determine the linear polarization rotation angle by detecting the change in beam displacement, and determine the x-direction component of the electron spin polarization vector by the linear polarization rotation angle, thereby realizing the weak measurement of the SERF magnetic field; The Sagnac interference loop is implemented through a weak coupling module, which includes a second reflecting mirror connected to the transmission side of the first polarizing beam splitter, and a first reflecting mirror connected to the reflection side of the first polarizing beam splitter and the second reflecting mirror respectively. The transmitted light formed by the first polarizing beam splitter propagates in the Sagnac interference loop in a clockwise direction, and the reflected light formed by the first polarizing beam splitter propagates in the Sagnac interference loop in a counterclockwise direction. The first reflector is set on a biaxial displacement stage. The biaxial displacement stage adjusts the position of the first reflector so that the detection beam produces a small displacement g in space, which is a weak coupling with a coupling strength of g. The second λ / 2 waveplate and the second polarizing beam splitter constitute a post-selection module. The right side of the post-selection module is connected to the quadrant detector, and the left side of the post-selection module is connected to the first polarizing beam splitter through a third reflecting mirror. The detection laser incident side of the alkali metal gas chamber is connected to a narrow linewidth laser via a λ / 4 waveplate, a photoelastic modulator, a first λ / 2 waveplate, and a Glan-Taylor prism.

2. The method for measuring and detecting weak SERF magnetic fields based on laser spatial modes according to claim 1, characterized in that, The alkali metal gas chamber is located inside the triaxial magnetic compensation coil, which is located inside the passive magnetic shielding layer.

3. The method for measuring and detecting weak SERF magnetic fields based on laser spatial modes according to claim 1, characterized in that, The laser emitted by the narrow linewidth laser has a wavelength far exceeding the D2 line resonance peak of the detuned alkali metal atoms.

4. The method for measuring and detecting weak SERF magnetic fields based on laser spatial modes according to claim 1, characterized in that, Step 4 includes the following formula: ， Among them U output This refers to the quadrant detector's detection signal. K1 represents the scale coefficient between the displacement and the final output voltage signal, which is the product of the detector's photoelectric conversion coefficient and current amplification factor. θ It is the linear polarization optical rotation angle. Let g be the waist of the Gaussian beam, and g be the displacement of the beam in space.

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