Constant current source current noise measurement device and method based on optically pumped atomic magnetometer

Abstract

This invention belongs to the field of atomic magnetometer technology and discloses a constant current source current noise measurement device based on an optically pumped atomic magnetometer. The device includes: a pump laser, a probe laser, and a magnetic shielding cylinder; the magnetic shielding cylinder contains an atomic gas cell, a pair of co-directional Helmholtz coils for generating a radio frequency magnetic field along the z-direction of the atomic gas cell, and a pair of momentless coils for generating a uniform magnetic field along the y-direction of the atomic gas cell under the action of the constant current source under test; the pump light, after passing through a first switching module and a first optical processor, becomes circularly polarized light and is incident along the y-direction into the atomic gas cell, causing polarization of the atoms within the atomic gas cell; the probe light, after passing through a second switching module and a second optical processing device, becomes linearly polarized light and is incident along the x-direction into the atomic gas cell, intersecting with the pump light; the probe light passing through the atomic gas cell is then processed by a polarimeter and a signal extraction and processing system for data acquisition and processing. This invention can achieve accurate measurement and characterization of constant current source current noise.

Description

Technical Field

[0001] This invention relates to the field of atomic magnetometer technology, and more specifically to a constant current source current noise measurement device and method based on an optically pumped atomic magnetometer. Background Technology

[0002] Low-noise, highly stable constant current sources have significant application value in fields such as quantum precision measurement, metrology, and fundamental physics research. They are commonly used to drive various narrow-linewidth, single-frequency continuous-wave semiconductor lasers, which are crucial in cold atom physics, cold atom microwave atomic clocks, cold atom optical frequency atomic clocks, optically pumped atomic magnetometers, cold atom gravimetric accelerometers, and Rydberg atom-based microwave electric field meters. With the rapid development of these fields, the requirements for the noise performance of constant current sources are increasing, necessitating a more precise, lower-noise, less susceptible-to-external-interference, and highly stable constant current source. Current noise characteristics are a key indicator reflecting the performance of a constant current source; lower current noise indicates a more stable output current. Therefore, establishing a method for evaluating and testing the current noise characteristics of constant current sources is essential.

[0003] Traditionally, Ohm's law can be used to indirectly characterize the noise characteristics of the output current of a constant current source. The constant current source applies a constant current to a high-precision resistor. By analyzing the voltage signal noise on the resistor over a period of time, the current noise of the constant current source can be deduced according to Ohm's law. However, the resistivity of this method is easily affected by temperature effects, and the accuracy and error of the measurement results will also deteriorate accordingly.

[0004] High-sensitivity magnetometers based on the interaction between light and matter can achieve precise measurements of magnetic fields. Optically pumped atomic magnetometers mostly operate in environments shielded by the Earth's magnetic field and generate a nearly uniform magnetic field region through a magnetic field coil system. Based on the interaction between current and magnetic field, applying atomic magnetometers to constant current source noise measurement would undoubtedly greatly improve the accuracy of constant current source current noise measurement. Summary of the Invention

[0005] The present invention overcomes the shortcomings of the prior art, and the technical problem to be solved is: to provide a constant current source current noise measurement device and method based on an optically pumped atomic magnetometer, so as to realize the accurate measurement and characterization of constant current source current noise.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a constant current source current noise measurement device based on an optically pumped atomic magnetometer, comprising: a pump laser, a probe laser, a magnetic shielding cylinder, a polarimeter, and a signal extraction and processing system;

[0007] The magnetic shielding cylinder is equipped with an atomic gas cell, a pair of co-directional Helmholtz coils, and a pair of momentless coils. The pair of co-directional Helmholtz coils is used to generate a radio frequency magnetic field along the z-direction to the atomic gas cell under energized conditions. The pair of momentless coils is used to generate a uniform magnetic field along the y-direction to the atomic gas cell under the action of the constant current source under test.

[0008] The pump light output from the pump laser is converted into circularly polarized light after passing through the first switching module and the first optical processor, and then incident on the atomic gas cell along the y-direction to polarize the atoms in the atomic gas cell; the probe light output from the probe laser is converted into linearly polarized light after passing through the second switching module and the second optical processing device, and then incident on the atomic gas cell along the x-direction and intersects with the pump light. The probe light passing through the atomic gas cell is then subjected to data acquisition and processing by the signal extraction and processing system after passing through the polarimeter.

[0009] The constant current source noise measurement device based on optically pumped atomic magnetometer further includes a radio frequency magnetic field generating device. The radio frequency magnetic field generating device includes a voltage-controlled constant current source and a signal generator, which are used to drive the co-directional Helmholtz coil pair to generate a magnetic field with a frequency equal to the Larmor precession frequency.

[0010] The first optical processor includes a first polarizer and a quarter-wave plate, the first polarizer and the quarter-wave plate being used to convert the pump light into circularly polarized light;

[0011] The second optical processor includes a second polarizer and a second light guide device. Multiple second light guide devices are used to adjust the propagation direction of the probe light so that it is incident on the atomic gas cell and then incident on the polarizer after passing through the atomic gas cell.

[0012] The constant current source noise measurement device based on optically pumped atomic magnetometer further includes an optical power feedback control module, which is used to suppress the intensity noise of the probe light output by the probe laser through power feedback.

[0013] The signal extraction and processing system includes a balanced differential photodetector, a data acquisition card, and a computer; the polarimeter includes a half-wave plate and a Wollaston prism.

[0014] The constant current source noise measurement device based on optically pumped atomic magnetometer further includes a non-magnetic heating system for heating the atomic gas chamber.

[0015] The atomic chamber contains rubidium atoms, and the pump laser frequency is locked at the 52s D1 transition line of rubidium-87 atoms. 1 / 2 (F=2)-52P 1 / 2 (F=1), the probe laser frequency is locked at the 52S D2 transition line of the rubidium 87 atom. 1 / 2 (F=1)-52P3 / 2 (F=2) Blue detuning 6GHz.

[0016] The constant current source noise measurement device based on optically pumped atomic magnetometer further includes a timing control module. The timing control module is used to control the first switching module, the radio frequency magnetic field, and the second switching module so that the pump light, the radio frequency magnetic field, and the probe light are turned on independently in sequence in each cycle.

[0017] Furthermore, this invention also provides a method for measuring the noise of a constant current source based on an optically pumped atomic magnetometer, implemented using the aforementioned device, comprising the following steps:

[0018] S1. Within one cycle, the first switch module, the radio frequency magnetic field, and the second switching module are sequentially turned on and off;

[0019] S2. Obtain a free-induction attenuated signal within one cycle through the signal extraction and processing system;

[0020] S3. Perform FFT transformation on the obtained free induction attenuation signal to obtain the magnetic resonance signal in the frequency domain.

[0021] S4. Lorentz fitting is performed on the magnetic resonance spectral lines in the frequency domain to obtain the peak frequency of the peak as the Larmor precession frequency.

[0022] S5. Calculate the magnetic field strength of the uniform magnetic field based on the Larmor precession frequency and the gyromagnetic ratio.

[0023] S6. Repeat steps S1 to S5 to obtain the magnetic field strength of the uniform magnetic field in several periods, and calculate the magnetic field noise power spectral density based on the series of magnetic field strengths obtained.

[0024] S7. Change the output current of the constant current source under test, and repeat steps S1 to S5 to obtain the magnetic field strength of the uniform magnetic field under different output current values; obtain the coil coefficients of the momentless coil pair through linear fitting.

[0025] S8. Obtain the current noise power spectral density of the constant current source under test by using the magnetic field noise power spectral density and the coil coefficient.

[0026] The method for measuring the current noise of a constant current source based on an optically pumped atomic magnetometer further includes the following steps: adjusting the driving frequency of the same-direction Helmholtz coil pair according to the Larmor precession frequency obtained in step S4, thereby changing the frequency of the radio frequency field so that its angular frequency is equal to the Larmor precession frequency.

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] 1. This invention provides a device and method for measuring the current noise of a constant current source based on an optically pumped atomic magnetometer. It utilizes a radio frequency magnetic field with an angular frequency equal to the Larmor frequency to cause spin-polarized atomic ensembles to transition back and forth between adjacent Zeeman sublevels, thereby obtaining a narrow-linewidth, high signal-to-noise ratio magnetic resonance spectrum. Based on the atomic magnetometer, the magnetic field generated by the driving magnetic field coil system of the constant current source under test is accurately measured. The power spectral density of the magnetic field noise measured by the atomic magnetometer is converted into the current noise of the constant current source using the coil coefficient, thus realizing the measurement and characterization of the constant current source current noise. Its principle is simple, easy to operate, and has high measurement accuracy.

[0029] 2. This invention utilizes timing control to separate the pump light, radio frequency magnetic field, and probe light in the time domain, effectively avoiding the impact of crosstalk between the three on the measurement signal and sensitivity. It further avoids the influence on the characterization of constant current source current noise. Moreover, the timing control system can change the measurement period of the atomic magnetometer. By measuring the magnetic field strength within multiple measurement periods, the noise power spectral density at different analysis frequencies can be calculated, enabling the characterization of sensitivity at different analysis frequencies, and further enabling the characterization of constant current source current noise at different analysis frequencies.

[0030] 3. This invention uses a torque-free coil with a highly uniform internal magnetic field to provide a magnetic field along the y-direction, which reduces the mutual influence between the magnetic field coil and the shielding layer, eliminates the influence of the magnetic field gradient, and effectively improves the accuracy of magnetic field measurement and current noise characterization.

[0031] 4. This invention places the atomic gas chamber within a four-layer permalloy magnetic shielding cylinder to suppress ambient magnetic field noise. The magnetic shielding cylinder has a large size and a high shielding coefficient, and employs an "AC demagnetization method" for demagnetization. This involves applying a 50Hz AC current to the magnetic field coil through external demagnetization, gradually reducing the AC magnetic field until it reaches zero, thus providing the atomic magnetometer with a near-zero magnetic environment and further improving measurement accuracy.

[0032] 5. This invention effectively suppresses the intensity noise of the probe light through the feedback loop, thereby reducing the electronic noise of the signal extraction and processing system, further improving the magnetic field measurement sensitivity of the optically pumped rubidium atom FID magnetometer, and enabling the measurement and characterization of the current noise of the constant current source with a lower current noise level. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of a constant current source current noise measurement device based on an optically pumped atomic magnetometer, provided in an embodiment of the present invention.

[0034] Figure 2 This is a schematic diagram of timing control used in an embodiment of the present invention;

[0035] Figure 3This is a schematic diagram of a typical free-induction decay signal (a) and the signal amplitude after FFT processing obtained in this embodiment (b);

[0036] Figure 4 This is a schematic diagram of the experimental data and linear fitting of the magnetic field and constant current source current obtained in this invention; the slope in the figure is the coil coefficient of the torqueless coil, and the coil coefficient and the magnitude of the residual magnetic field in the magnetic shielding cylinder can be obtained through linear fitting.

[0037] Figure 5 Six different constant current sources were subjected to the same 100mA current along the y-direction. The magnetic field noise power spectral density (a) and the corresponding current noise power spectral density (b) were analyzed under the frequency range of 1-100Hz. The constant current sources corresponding to the curves from top to bottom in the figure are: KeySight B2961A, ThorLabs LDC205C, SRS LDC501 and CS580, a self-made current source, and GWInstek 2303S.

[0038] In the figure, 1 is the first laser, 2 is the first switch module, 3 is the telescope system, 4 is the first reflector, 5 is the first polarizer, 6 is the quarter-wave plate, 7 is the first garbage dump, 8 is the same-direction Helmholtz coil pair, 9 is the torque-free coil pair, 10 is the second laser, 11 is the optical power feedback control system, 12 is the second switch module, 13 is the second reflector, 14 is the second polarizer, 15 is the first right-angle prism, 16 is the atomic gas cell, 17 is the non-magnetic heating device, 18 is the second right-angle prism, 19 is the third reflector, 20 is the polarimeter, 21 is the signal extraction and processing system, 22 is the first constant current source, 23 is the radio frequency magnetic field generating device, and 24 is the magnetic shielding cylinder. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] Example 1

[0041] like Figure 1 As shown, Embodiment 1 of the present invention provides a constant current source current noise measurement device based on an optically pumped atomic magnetometer, including: a pump laser 1, a probe laser 10, a magnetic shielding cylinder 24, a polarimeter 20, and a signal extraction and processing system 21;

[0042] The magnetic shielding cylinder 24 is equipped with an atomic gas chamber 16, a pair of Helmholtz coils 8 in the same direction, and a pair of inverted coils 9. The pair of Helmholtz coils 8 in the same direction is used to generate a radio frequency magnetic field along the z direction for the atomic gas chamber 16 under the condition of being energized. The pair of inverted coils 9 is used to generate a uniform magnetic field along the y direction for the atomic gas chamber 16 under the action of the constant current source 22 to be tested.

[0043] The pump light output from the pump laser 1 is converted into circularly polarized light after passing through the first switch module 2 and the first optical processing device, and then incident on the atomic gas cell 16 along the y-direction to polarize the atoms in the atomic gas cell 16; the probe light output from the probe laser 10 is converted into linearly polarized light after passing through the second switch module 11 and the second optical processing device, and then incident on the atomic gas cell 16 along the x-direction and intersects with the pump light. The probe light passing through the atomic gas cell 16 is then subjected to data acquisition and processing by the signal extraction and processing system 21 after passing through the polarimeter 20.

[0044] Specifically, the constant current source noise measurement device based on optically pumped atomic magnetometer in this embodiment also includes a radio frequency magnetic field generating device 23. The radio frequency magnetic field generating device 23 includes a voltage-controlled constant current source and a signal generator, which is used to drive the same-direction Helmholtz coil pair 8 to generate a radio frequency magnetic field with an angular frequency equal to the Larmor precession frequency.

[0045] In this embodiment, both the co-directional Helmholtz coil pair 8 and the momentless coil pair 9 include two coils. The two circular coils of the co-directional Helmholtz coil pair 8 are arranged above and below the atomic gas chamber 16 along the Z-axis direction. Due to the angle, Figure 1 Only one Helmholtz coil can be displayed. The two saddle-shaped coils in the pitchless coil pair 9 are positioned in front of and behind the atomic gas chamber 16 along the Y direction.

[0046] In this embodiment, the angular frequency of the radio frequency magnetic field is set to be equal to the Larmor precession frequency, which is calculated from the magnitude of the static magnetic field and the gyromagnetic ratio of the rubidium atom's ground state. Under the influence of the radio frequency magnetic field, the spin-polarized atomic ensemble transitions back and forth between adjacent Zeeman sublevels, that is, the spin-polarized atomic ensemble completes one transition in the quantization axis direction (y-axis) of the external magnetic field B, i.e., a magnetic resonance transition, and the spin-polarized atomic ensemble precesses with a π pulse. In this embodiment, the spin-polarized atomic ensemble is set to precess with a π / 2 pulse, that is, the spin-polarized atomic ensemble precesses from the y-axis to the xoz plane.

[0047] Specifically, in this embodiment, the first optical processor includes a first polarizer 5 and a quarter-wave plate 6, which are used to convert the pump light into circularly polarized light. In addition, the first optical processor may also include a first reflector 4, which is disposed between the first switching module and the first polarizer 5, for the purpose of allowing the pump light to be incident on the atomic gas cell 16.

[0048] Specifically, in this embodiment, as Figure 1 As shown, the pump light is set to the y-direction, and the probe light is set to the x-direction. The static magnetic field is set to the y-direction, and the radio frequency magnetic field is set to the z-direction. The directions of the pump light, the radio frequency magnetic field, and the probe light are perpendicular to each other.

[0049] Furthermore, in this embodiment, the first optical processor also includes a telescope system 3, which is disposed at the output end of the first switching module 2 and is used to expand the pump light beam. In this embodiment, the telescope system 3 consists of two lenses, one in front and one behind, with focal lengths of f = 30 mm and f = 150 mm respectively. The diameter of the expanded pump light beam is approximately 10 mm. This ensures that the pump light interacts fully with the atomic ensemble, ensuring complete atomic polarization.

[0050] Specifically, in this embodiment, the second optical processor includes a second polarizer 14 and a second light guide device. Multiple second light guide devices are used to adjust the propagation direction of the probe light so that it enters the atomic gas cell 16 and then enters the polarizer 20 after passing through the atomic gas cell 16. Specifically, the second light guide device includes a first right-angle prism 15 and a second right-angle prism 18, which are located on opposite sides of the atomic gas cell 16. These prisms not only adjust the direction of the light but also maintain the polarization state of the probe light before and after entering the atomic gas cell 16. Furthermore, the probe light path may also include a first reflecting mirror 13 and a second reflecting mirror 19, used to direct the probe light into the first right-angle prism 15 inside the magnetic shielding cylinder 24, and the probe light output from the magnetic shielding cylinder 24 via the second right-angle prism 18, which then directs the probe light into the polarizer 20 and the signal extraction and processing system 21. Those skilled in the art will understand that if the second laser 10 and the polarizer 20 are rotated to a suitable position, the first reflecting mirror 13 and the second reflecting mirror 19 may not be necessary.

[0051] Specifically, in this embodiment, the signal extraction and processing system 21 includes a balanced differential photodetector, a data acquisition card, and a computer; the polarizer 20 includes a half-wave plate and a Wollaston prism. In this embodiment, when there is no magnetic field, the linearly polarized probe light remains unchanged after passing through the atomic gas cell 16, and then is incident on the polarizer 20, where it is split into two beams of linearly polarized light with the same intensity, which are detected by the two detection ports of the balanced differential photodetector. At this time, the output of the balanced differential photodetector is zero. When there is a magnetic field, the spin polarization vector of the atom is projected onto the direction of the probe light, so the polarization state of the linearly polarized probe light changes after passing through the atomic gas cell 16, and the polarization plane of the probe light rotates. After passing through the polarizer 20, the output of the balanced differential photodetector is no longer zero. The greater the polarization signal, the greater the deflection angle, and the greater the output of the balanced differential photodetector.

[0052] Specifically, in this embodiment, the common-mode noise rejection ratio of the balanced differential detector is 50dB. The differential detection method can effectively suppress the common-mode noise of the measurement system. The data acquisition card used is DAQ (NI-USB6363), which has high-speed data acquisition capabilities and ultra-low noise.

[0053] Furthermore, the constant current source noise measurement device based on an optically pumped atomic magnetometer in this embodiment also includes an optical power feedback control module 11. This module is used to control the intensity noise of the probe light output by the probe laser 10 through power feedback. Specifically, the optical power feedback control module can be a commonly used power feedback control module in the art.

[0054] Furthermore, the constant current source noise measurement device based on an optically pumped atomic magnetometer in this embodiment also includes a non-magnetic heating system 17, which is used to heat the atomic gas chamber 16. Specifically, the non-magnetic heating system 17 includes a boron nitride ceramic furnace body with good thermal conductivity and a flexible thin-film electric heating element; the heated atomic gas chamber 16 is placed inside the boron nitride ceramic furnace body, and the flexible thin-film electric heating element is placed on the furnace wall of the boron nitride ceramic furnace body to uniformly heat it, ensuring that the atomic gas chamber is heated evenly. In this embodiment, the heating temperature is 85℃. In addition, the electric heating element uses a non-magnetic PT100 thermistor as a temperature sensor to avoid interference caused by the magnetic field of the temperature sensor itself. In this embodiment, the flexible thin-film electric heating element is driven by 477kHz AC power, which is much higher than the measurement bandwidth and Larmor frequency, to ensure that the heating system does not interfere with the magnetic field measurement.

[0055] Specifically, in this embodiment, the atomic gas chamber 16 is a square bubble with a shape of 15×15×15mm. It is filled with 100 Torr nitrogen gas and isotopically purified rubidium-87 atoms. Nitrogen gas, as a fluorescence quenching gas, can protect the spin polarization of ground-state atoms to the greatest extent; nitrogen gas can also act as a buffer gas, which can greatly suppress the decrease in spin relaxation rate caused by spin exchange collisions between alkali metal atoms and spin-destructive collisions between atoms and the inner wall of the gas chamber.

[0056] In this embodiment, the pump light frequency is relative to the D1 transition line 5 of the rubidium 87 atom. 2 S 1 / 2 (F=2)-5 2 P 1 / 2 (F=1) Resonance, specifically, in this embodiment, the pump light source frequency is locked at the D1 transition line of rubidium 87 atoms. 2 S 1 / 2 (F=2)-5 2 P 1 / 2 (F=1). The probe light frequency should be relative to the D2 transition line of the rubidium 87 atom. 2 S 1 / 2(F=1)-5 2 P 3 / 2 (F=2) Detuning, specifically, can be blue detuning or red detuning, with a detuning amount ranging from several hundred MHz to tens of GHz. Specifically, in this embodiment, the probe light frequency is locked at the rubidium 87 atom D2 transition line 5. 2 S 1 / 2 (F=1)-5 2 P 3 / 2 (F=2) Blue detuning at 6GHz. Furthermore, in this embodiment of the invention, the atoms within the atomic gas chamber 16 can also be other alkali metal atoms, such as cesium atoms. In this case, the pump light frequency and the probe light frequency should resonate or detune with the corresponding atomic transition lines, respectively.

[0057] Specifically, in this embodiment, the magnetic shielding cylinder 24 is made of a four-layer permalloy material with high magnetic permeability, which has a high shielding coefficient and can suppress environmental magnetic field noise and geomagnetic field noise. It is a closed structure with only a few light-transmitting holes.

[0058] Specifically, in this embodiment, the atomic gas chamber, the co-directional Helmholtz coil pair, the boron nitride furnace body, and the non-magnetic thin-film electric heating element are all placed at the center of the four-layer permalloy magnetic shielding cylinder. Demagnetization is performed using an "AC demagnetization method," which involves applying a 50Hz AC current to the magnetic field coil through external demagnetization, gradually reducing the AC magnetic field until it reaches zero, thus providing a near-zero magnetic environment for magnetic field measurement and characterization of constant current source noise.

[0059] Specifically, in this embodiment, the magnetic field coil along the y-direction is selected from a pair of torqueless coils 9, which are less affected by temperature and frequency effects. The magnetic field coil along the z-direction is wound from a pair of Helmholtz coils in the same direction. This generates a highly uniform magnetic field at the center of the atomic gas chamber. The length of the uniform magnetic field region at the center of the magnetic shielding cylinder is 20 mm, and the uniformity of the generated magnetic field is >99.95%. This effectively improves the accuracy of magnetic field measurement and the accuracy of constant current source current noise characterization.

[0060] In this embodiment, the magnitude of the static magnetic field generated by the torque-free coil 9 is... 87 The relationship between the frequencies of Larmor precession of the macroscopic magnetic moment of Rb atoms in an external magnetic field can be expressed as:

[0061] B = ω / γ; (1)

[0062] Where ω is the frequency of Larmor precession, B is the static magnetic field strength, and γ is the gyromagnetic ratio of the ground-state rubidium-87 atom. Therefore, the magnitude of the magnetic field can be calculated using the Larmor frequency of the rubidium-87 atom ensemble and the gyromagnetic ratio of the ground-state rubidium-87 atom.

[0063] Specifically, the constant current source noise measurement device based on an optically pumped atomic magnetometer in this embodiment further includes a timing control module. This timing control module controls the first switching module, the radio frequency magnetic field, and the second switching module, so that the pump light, the radio frequency magnetic field, and the probe light are sequentially and independently activated in each cycle. For example... Figure 2 The diagram shown is a timing diagram of the activation of the first switch module, the radio frequency magnetic field, and the second switch module in this embodiment.

[0064] Specifically, the timing control module uses a digital delay signal generator from SRS Corporation. The first and second switching modules can be acousto-optic modulators, and the digital delay signal generator controls the on / off state of the probe and pump light by controlling the acousto-optic modulators. In this embodiment, the pump light, radio frequency magnetic field, and probe light are sequentially separated. First, the pump light is turned on, and the spin-polarized state of the rubidium-87 atom ensemble is prepared from t0 to t1. The macroscopic magnetic moment of the polarized rubidium-87 atom is along the y-direction at time t1. Then, the pump light is turned off, and a radio frequency magnetic field with an angular frequency equal to the Larmor precession frequency is applied during the time interval t1 to t2. After a π / 2 pulse time, the macroscopic magnetic moment of the atom precesses to the xoz plane. Finally, the radio frequency magnetic field is turned off, and the probe light is turned on during the time interval t2 to t3. The macroscopic magnetic moment of the atom freely evolves at the Larmor frequency until it reaches thermal equilibrium. At this point, the freely induced decay signal can be obtained through the polarimeter 20 and the signal extraction and processing system 21. In this embodiment, the pump laser, radio frequency magnetic field, and probe laser are separated from the time domain by timing control, thus avoiding crosstalk to the measurement signal and sensitivity.

[0065] like Figure 3 The image shows typical time-domain and frequency-domain free induction attenuation signals measured by the signal extraction and processing system 21 in this embodiment of the invention. In this embodiment, the pump light power is set to 5mW and the probe light power is set to 30μW. A 50mA current is applied along the y-direction by the first constant current source, corresponding to a static magnetic field of approximately 6.3μT. The period T is set to 50ms, and the pump light on-time T is... pump The on-time of the radio frequency magnetic field is 20ms. RF The detection light on time is 0.2 ms. probe It takes 29.8ms. Figure 3 In the middle (a), the signal is a free-induction decayed signal within one period in the time domain, and the inset is a local magnification of the free-induction decayed signal. Figure 3(b) is the magnetic resonance signal in the frequency domain obtained by FFT transformation of the free induction decay signal in the time domain. The magnetic resonance spectrum is fitted by Lorentz, and the full width at half maximum (FWHM) of the magnetic resonance spectrum is 292.4 ± 2.9 Hz. The peak frequency, i.e. the Larmor precession frequency, is 44471.2 Hz. Dividing the Larmor frequency by the gyromagnetic ratio of the ground state F = 2 of the rubidium 87 atom (γ is 6.99583 Hz / nT), the magnitude of the magnetic field is 6.356788 μT.

[0066] Example 2

[0067] Embodiment 2 of the present invention provides a method for measuring the current noise of a constant current source based on an optically pumped atomic magnetometer, which is implemented based on the device described in Embodiment 1, and includes the following steps:

[0068] S1. Keep the output current of the constant current source under test constant, and sequentially turn on and off the first switch module 2, the radio frequency magnetic field, and the second switching module 3 within one cycle;

[0069] S2. Obtain a free-induction attenuated signal within one cycle through the signal extraction and processing system 21;

[0070] S3. Perform FFT transformation on the obtained free induction attenuation signal to obtain the magnetic resonance signal in the frequency domain.

[0071] S4. Lorentz fitting is performed on the magnetic resonance spectral lines in the frequency domain to obtain the peak frequency of the peak as the Larmor precession frequency.

[0072] S5. Calculate the magnetic field strength of the uniform magnetic field based on the Larmor precession frequency and gyromagnetic ratio.

[0073] S6. Repeat steps S1 to S5 to obtain the magnetic field strength of the uniform magnetic field in several periods, and calculate the magnetic field noise power spectral density based on the series of magnetic field strengths obtained.

[0074] S7. Change the output current of the constant current source to be tested, and repeat steps S1 to S5 to obtain the magnetic field strength of the uniform magnetic field under different output current values; obtain the coil coefficient of the momentless coil pair (9) by linear fitting.

[0075] S8. Obtain the current noise power spectral density of the constant current source under test by using the magnetic field noise power spectral density and the coil coefficient.

[0076] Furthermore, the measurement method of the present invention further includes the following steps:

[0077] The magnetic shielding cylinder is demagnetized using the "AC demagnetization method," also known as external demagnetization, to provide a near-zero magnetic environment.

[0078] The first laser frequency is locked at the atomic transition line to prepare the atomic polarization state, and the second laser frequency is locked at the frequency that is detuned relative to the atomic transition line to realize the Faraday rotation effect of the optical field.

[0079] Furthermore, the constant current source noise measurement method based on an optically pumped atomic magnetometer in this embodiment further includes the following steps: adjusting the driving frequency of the co-directional Helmholtz coil pair 8 according to the Larmor precession frequency obtained in step S4, thereby changing the frequency of the radio frequency magnetic field so that its angular frequency is equal to the Larmor precession frequency. In this embodiment, by repeatedly measuring the free-induction attenuated signal, performing FFT transformation and Lorentz fitting, and comparing the fitted Larmor precession frequency with the set radio frequency magnetic field angular frequency, if the difference between the two is less than a threshold, then the angular frequency of the radio frequency magnetic field is considered equal to the Larmor precession frequency, and the frequency of the radio frequency magnetic field can be kept constant in subsequent measurements.

[0080] like Figure 4 The figure shows a magnetic field-current curve obtained by changing the output current of the constant current source under test and measuring the corresponding magnetic field magnitude in an embodiment of the present invention. This curve can be used to calibrate the coil coefficient of the torqueless coil pair 9. Using a KeySight B2961A as the constant current source under test, current is applied to the torqueless coil pair 9. When the period is 50ms, the pump light on-time T... pump The on-time of the radio frequency magnetic field is 20ms. RF The detection light on time is 0.2 ms. probe The duration was 29.8 ms. A free-induction decaying signal was recorded for 240 seconds (4800 cycles) using a signal extraction and processing system. The Larmor frequency of each cycle's free-induction decaying signal could be obtained through FFT transformation, thus yielding the magnetic field value. In this embodiment, the average of a series of calculated magnetic field values ​​corresponding to several cycles could also be used as the magnetic field value under the corresponding current. A constant current power supply B2961A was gradually applied in the range of I = 2-250 mA, the magnetic field was measured, and a straight line was fitted. The linear fitting expression was: B = 126.956I - 4.914. The calibrated coil constant was 126.956 ± 0.076 nT / mA, and the residual magnetic field was approximately 4.914 nT.

[0081] like Figure 5 The image shows a specific embodiment of the present invention, which characterizes current noise based on an optically pumped rubidium atomic magnetometer. Figure 5In (a), six typical commercial constant current sources (KeySight B2961A, ThorLabs LDC205C, SRS LDC501, SRS CS580, a self-made constant current source, and GWInstek 2303S) apply the same 100mA current (corresponding to a static magnetic field of approximately 12.6μT) along the y-direction to nine pairs of torque-free coils. The magnetic field noise power spectral density (sensitivity) is obtained from the DC magnetic field values ​​obtained by measuring the free-induction decay type signal over 6000 cycles, within the analysis frequency range of 1–100Hz. Dividing the obtained magnetic field noise power spectral density by the corresponding coil coefficient yields... Figure 5 (b) Current noise power spectral density, enabling accurate measurement of noise from the current source.

[0082] Table 1 shows the characterization of current noise of different constant current sources based on an optically pumped atomic magnetometer in this embodiment of the invention. Dividing the sensitivity of the atomic magnetometer by the coil coefficient yields the current noise of the constant current source at the corresponding analysis frequency.

[0083] Table 1. Characterization of current noise from different constant current sources based on optically pumped atomic magnetometers.

[0084] B2961A (KeySight) 17.0 133.905±0.080 LDC205C (ThorLabs) 27.2 214.247±0.128 LDC501(SRS) 34.6 272.535±0.163 CS580(SRS) 41.0 322.947±0.193 CCS (Homemade) 69.8 549.797±0.329 2303S (GWInstek) 87.9 692.366±0.414

[0085] The above data demonstrates that this invention, based on an optically pumped atomic magnetometer, achieves the characterization and measurement of the current noise of a constant current source. It should be noted that the sensitivity of the optically pumped atomic magnetometer is limited by various factors, such as photon shot noise, spin projection noise, fluctuations in the remanent magnetic field, intensity noise of the probe light, and electronic noise from the balanced differential photodetector and DAQ data acquisition system. All of these noise types are included in the acquired free-induction attenuated signal and in the calculated sensitivity and constant current source current noise measurement results. Therefore, the measurement results of this invention are essentially the upper limit of the constant current source current noise.

[0086] Specifically, in this embodiment, by using a timing control system to change the measurement period of the free-induction decay magnetometer, the current noise can be characterized within different analysis frequency ranges. By appropriately increasing the intensity of the pump beam, under the same level of rubidium atom spin polarization, the duration of the pump beam on can be significantly shortened, thereby shortening the measurement period and enabling the characterization of constant current source noise over a wider analysis frequency range.

[0087] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A constant current source noise measurement device based on an optically pumped atomic magnetometer, characterized in that, include: Pump laser (1), probe laser (10), magnetic shielding cylinder (24), polarimeter (20) and signal extraction and processing system (21); The magnetic shielding cylinder (24) is equipped with an atomic gas chamber (16), a pair of Helmholtz coils (8) in the same direction, and a pair of inverted coils (9). The pair of Helmholtz coils (8) in the same direction is used to generate a radio frequency magnetic field along the z direction to the atomic gas chamber (16) under the condition of being energized. The pair of inverted coils (9) is used to generate a uniform magnetic field along the y direction to the atomic gas chamber (16) under the action of the constant current source (22) to be measured. The pump light output from the pump laser (1) is converted into circularly polarized light after passing through the first switch module (2) and the first optical processor, and then incident on the atomic gas cell (16) along the y direction to polarize the atoms in the atomic gas cell (16); the probe light output from the probe laser (10) is converted into linearly polarized light after passing through the second switch module (11) and the second optical processing device, and then incident on the atomic gas cell (16) along the x direction and intersects with the pump light. The probe light passing through the atomic gas cell (16) is then processed by the signal extraction and processing system (21) after passing through the polarimeter (20). It also includes a timing control module, which controls the first switching module, the radio frequency magnetic field, and the second switching module so that the pump light, the radio frequency magnetic field, and the probe light are turned on independently in sequence during each cycle.

2. The constant current source current noise measurement device based on an optically pumped atomic magnetometer according to claim 1, characterized in that, It also includes a radio frequency magnetic field generating device (23), which includes a voltage-controlled constant current source and a signal generator for driving the same-direction Helmholtz coil pair (8) to generate a magnetic field with a frequency equal to the Larmor precession frequency.

3. The constant current source current noise measurement device based on an optically pumped atomic magnetometer according to claim 1, characterized in that, The first optical processor includes a first polarizer (5) and a quarter-wave plate (6), which are used to convert the pump light into circularly polarized light. The second optical processor includes a second polarizer (14) and a second light guide device. The second light guide device is used to adjust the direction of probe light propagation so that it enters the atomic gas cell (16) and enters the polarizer (20) after passing through the atomic gas cell (16).

4. The constant current source current noise measurement device based on an optically pumped atomic magnetometer according to claim 1, characterized in that, It also includes an optical power feedback control module (11), which is used to suppress the intensity noise of the probe light output by the probe laser (10) through power feedback.

5. The constant current source current noise measurement device based on an optically pumped atomic magnetometer according to claim 1, characterized in that, The signal extraction and processing system (21) includes a balanced differential photodetector, a data acquisition card and a computer; the polarimeter (20) includes a half-wave plate and a Wollaston prism.

6. The constant current source current noise measurement device based on an optically pumped atomic magnetometer according to claim 1, characterized in that, It also includes a non-magnetic heating system (17) for heating the atomic gas chamber (16).

7. The constant current source current noise measurement device based on an optically pumped atomic magnetometer according to claim 1, characterized in that, The atomic gas chamber (16) contains rubidium atoms, and the pump laser (1) is frequency-locked at the 52 s D1 transition line of the rubidium 87 atom. 1 / 2 (F=2)-52P 1 / 2 (F=1), the frequency of the probe laser (10) is locked at the 52S transition line of the D2 transition line of the rubidium 87 atom. 1 / 2 (F=1)-52P 3 / 2 (F=2) Blue detuning 6 GHz.

8. A method for measuring the noise of a constant current source based on an optically pumped atomic magnetometer, implemented using the device described in claim 1, characterized in that... Includes the following steps: S1. Within one cycle, the first switch module (2), the radio frequency magnetic field, and the second switching module (3) are sequentially turned on and off. S2. Obtain a free-induction attenuated signal within one cycle through the signal extraction and processing system (21); S3. Perform FFT transformation on the obtained free induction attenuation signal to obtain the magnetic resonance signal in the frequency domain. S4. Lorentz fitting is performed on the magnetic resonance spectral lines in the frequency domain to obtain the peak frequency of the peak as the Larmor precession frequency. S5. Calculate the magnetic field strength of the uniform magnetic field based on the Larmor precession frequency and the gyromagnetic ratio. S6. Repeat steps S1 to S5 to obtain the magnetic field strength of the uniform magnetic field in several periods, and calculate the magnetic field noise power spectral density based on the series of magnetic field strengths obtained. S7. Change the output current of the constant current source to be tested, and repeat steps S1~S5 to obtain the magnetic field strength of the uniform magnetic field under different output current values; obtain the coil coefficient of the momentless coil pair (9) by linear fitting. S8. Obtain the current noise power spectral density of the constant current source under test by using the magnetic field noise power spectral density and the coil coefficient.

9. The method for measuring constant current source noise based on an optically pumped atomic magnetometer according to claim 8, characterized in that, It also includes the following steps: Based on the Larmor precession frequency obtained in step S4, the driving frequency of the co-directional Helmholtz coil pair (8) is adjusted, thereby changing the frequency of the radio frequency magnetic field so that its angular frequency is equal to the Larmor precession frequency.

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