Device and method for improving electronic polarization uniformity and stability of a serf magnetic field measurement device

Abstract

A device and method for improving the electronic polarization uniformity and stability of a SERF magnetic field measurement device are proposed. This method employs a laser with a spectral linewidth on the same order of magnitude as the gas cell pressure broadening as the pump light to polarize atoms. The spectral linewidth of the light source is broadened by applying voltage modulation to an electro-optic phase modulator, causing external phase modulation of the narrow-linewidth laser light passing through the modulator. Gaussian white noise is used as the modulation signal, and the output laser spectral linewidth is controlled by adjusting the noise bandwidth. Compared to SERF magnetic field measurement devices using a single broadband light source, using an electro-optic phase modulator to broaden the pump light linewidth not only possesses the ability of a wide-linewidth light source to improve the uniformity of electronic polarization in the gas cell, but also retains the advantages of a narrow-linewidth light source, such as center wavelength stability and relatively low intensity noise. This results in superior calibration coefficient stability and lower random noise in the SERF magnetic field measurement device, which is beneficial for improving the gradient differential sensitivity limit of the SERF magnetic field measurement device.

Description

Technical Field

[0001] This invention relates to SERF atomic magnetic field measurement technology, specifically to a device and method for improving the uniformity and stability of electronic polarization in SERF magnetic field measurement devices. Background Technology

[0002] Benefiting from the development of high-performance narrow-linewidth semiconductor lasers, optically pumped spin-exchange relaxation-free (SERF) magnetic field measurement devices have rapidly developed and are widely used in fundamental physics research, biomedicine, and other fields. SERF magnetic field measurement devices typically operate at 160℃-200℃ to maintain a high alkali metal density in the gas cell. However, this high alkali metal density causes absorption of the pump light intensity, leading to non-uniform electron polarization distribution. Furthermore, unavoidable fluctuations in laser power and frequency cause variations in pump efficiency, resulting in unstable calibration coefficients and consequently reducing the sensitivity of magnetic field measurements. Therefore, a device and method are needed to improve the uniformity and stability of electron polarization distribution.

[0003] To address the large electronic polarizability gradients at high densities, researchers have proposed various schemes, including counter-pumping, hybrid pumping, and atomic diffusion. For polarizability stability, narrow-linewidth lasers are typically used, with high-precision frequency and power stabilization. These narrow-linewidth lasers are usually set to operate at the alkali metal resonance peak. However, in high-density gas cells, the laser attenuates rapidly, causing a large electronic polarizability gradient. Furthermore, near the resonance frequency, fluctuations in laser frequency and power cause significant changes in pump rate and optical frequency shift, placing high demands on the frequency stability of the laser used. The inventors believe that using a broadband laser can, on the one hand, reduce the laser attenuation rate in the gas cell, improve polarizability uniformity, and eliminate the need for complex polarizability gradient suppression configurations; on the other hand, the smaller average absorption cross-section of the broadband laser's interaction with the alkali metal reduces the system's sensitivity to laser frequency and power fluctuations. Specifically, setting the center frequency of the broadband laser at the alkali metal atomic resonance peak allows the optical frequency shifts generated on both sides of the spectral line to cancel each other out. Summary of the Invention

[0004] The purpose of this invention is to provide a device and method for improving the uniformity and stability of electronic polarization in a SERF magnetic field measurement device. The invention employs an electro-optic phase modulator to broaden the narrow linewidth of a laser spectral to the level of gas cell pressure broadening, using the laser as pump light to polarize atoms. By setting the center frequency of the broadband laser at the resonance peak of alkali metal atoms, the electronic polarizability gradient can be effectively reduced and the stability of electronic polarizability can be improved.

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

[0006] A device for improving the uniformity and stability of electronic polarization in a SERF magnetic field measurement device is characterized by comprising a pump laser for generating a narrow-linewidth light source, an electro-optic phase modulator for inducing optical field fluctuations, a power- and bandwidth-adjustable Gaussian noise source applied to the electro-optic phase modulator, an alkali metal gas cell, and a detection and magnetic field control system. The narrow-linewidth laser generated by the pump laser, after passing through the electro-optic phase modulator, outputs a broadband laser as the pump laser to polarize the alkali metal gas cell. The spectral linewidth of the broadband laser is adjusted by controlling the driving power and bandwidth applied to the electro-optic phase modulator to broaden the narrow-linewidth laser spectral linewidth to the order of gas cell pressure broadening. The power- and bandwidth-adjustable Gaussian noise source includes a Gaussian white noise source, a low-pass filter bank, a broadband RF power amplifier module, and / or an adjustable power attenuator.

[0007] The center frequency of the broadband laser is the same as the resonance peak frequency of the alkali metal atoms in the alkali metal gas chamber.

[0008] The output signal of the Gaussian white noise source is connected to a low-pass filter bank via a first-channel line connection switch. The output signal of the low-pass filter bank is connected to a broadband RF power amplifier module via a second multi-channel line connection switch. After passing through an adjustable power attenuator, it is input to the electrical port of the electro-optic phase modulator to apply a driving effect. The laser output from the pump laser passes sequentially through the optical port of the electro-optic phase modulator, the first polarizer, and the first quarter-wave plate before being used as pump light to polarize alkali metal atoms in an alkali metal gas cell. A non-magnetic electric heating system, a triaxial magnetic compensation coil, and a magnetic shielding barrel are arranged sequentially from the inside to the outside around the alkali metal gas cell. The detection light output from the detection laser passes sequentially through a second polarizer, a photoelastic modulator, a second quarter-wave plate, the alkali metal gas cell, and an analyzer before being incident on a photodetector. The photodetector converts the optical signal into an electrical signal and transmits it to a lock-in amplifier via a coaxial cable. The lock-in amplifier is connected to a modulation controller and a host computer. The modulation controller is connected to the photoelastic modulator. The host computer is connected to the triaxial magnetic compensation coil via a function generator.

[0009] The photoelastic modulator is driven by a modulation controller. The electrical signal and the reference signal of the modulation controller are demodulated by a lock-in amplifier to obtain the DC component, first harmonic component and second harmonic component of the inspection electrical signal, and then fed back to the host computer.

[0010] The low-pass filter bank includes discrete filters with different cutoff frequencies, allowing coarse adjustment of the bandwidth of the Gaussian white noise source output signal, thereby achieving coarse adjustment of the laser linewidth; the adjustable power attenuator can achieve power attenuation from 0 to 90 dB in 0.5 dB increments, thereby achieving fine adjustment of the laser linewidth; the output signal of the adjustable power attenuator is input to the electrical port of the electro-optic phase modulator.

[0011] A method for improving the uniformity and stability of electronic polarization in a SERF magnetic field measuring device, characterized by utilizing the aforementioned device for improving the uniformity and stability of electronic polarization in a SERF magnetic field measuring device.

[0012] Includes the following steps:

[0013] Step 1: Build the optical path according to the above instructions and connect the circuit. Set the temperature of the alkali metal gas chamber to 160-200℃, turn off the Gaussian white noise source, and adjust the optical path and the triaxial magnetic compensation coil to make the device work in SERF mode.

[0014] Step 2: Connect the first filter in the low-pass filter bank through the first multi-channel line connection switch, turn on the Gaussian white noise source, and set the power attenuation to 90dB through the adjustable power attenuator 6.

[0015] Step 3: Use three-dimensional in-situ magnetic compensation technology to compensate for the residual magnetism of the system. Use a function generator to add a calibration magnetic field on the sensitive axis Y, adjust the pump optical power to make the first harmonic component of the lock-in amplifier output the maximum, that is, the magnetic field response signal is the strongest, and record the pump optical power and the first harmonic signal strength at this time.

[0016] Step 4: Adjust the adjustable power attenuator in 0.5 dB increments within the range of 90-0 dB, and repeat step 3 for each power attenuation value;

[0017] Step 5: Connect the remaining filters in the low-pass filter bank in sequence through the first multi-channel line connection switch, and repeat step 4 for each filter.

[0018] Step 6: The electronic polarization uniformity and stability corresponding to the maximum output magnetic field response signal are optimal.

[0019] The technical effects of this invention are as follows: This invention improves the uniformity and stability of electronic polarization in a SERF magnetic field measurement device by using a laser with a spectral linewidth on the same order of magnitude as the gas cell pressure broadening as the pump light to polarize atoms. The spectral linewidth of the light source is broadened by applying voltage modulation to an electro-optic phase modulator, which externally modulates the narrow-linewidth laser light passing through the modulator. The modulation signal uses Gaussian white noise, and the output laser spectral linewidth is controlled by adjusting the noise bandwidth. Compared to a SERF magnetic field measurement device using a single broadband light source, using an electro-optic phase modulator to broaden the pump light not only possesses the ability of a wide-linewidth light source to improve the uniformity of electronic polarization in the gas cell, but also takes into account the advantages of the center wavelength stability and relatively low intensity noise of a narrow-linewidth light source. This results in superior calibration coefficient stability and lower random noise in the SERF magnetic field measurement device, which is beneficial for improving the gradient differential sensitivity limit of the SERF magnetic field measurement device.

[0020] The advantages of this invention compared with the prior art are as follows: (1) Using broadband laser can significantly reduce the attenuation of laser intensity along the pump direction caused by alkali metal atom absorption, thereby improving the uniformity of alkali metal electronic polarization; (2) The symmetrical distribution of the spectrum after the narrow linewidth laser is broadened by the electro-optic phase modulator, by setting the center wavelength at the alkali metal resonance absorption peak, can achieve the cancellation of optical frequency shift caused by red shift and blue shift components in the spectrum, thereby reducing optical frequency shift noise; (3) Using the electro-optic phase modulator to broaden the linewidth of the pump light not only has the ability of a wide linewidth light source to improve the uniformity of gas cell electronic polarization, but also takes into account the advantages of the stability of the center wavelength and relatively low intensity noise of a narrow linewidth light source, thereby giving the SERF atomic magnetometer superior calibration coefficient stability and low random noise; (4) The laser linewidth before entering the electro-optic modulator is narrow, which can be used for efficient frequency stabilization. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of the device for improving the uniformity and stability of electronic polarization in the SERF magnetic field measurement device according to the present invention.

[0022] Figure 2 include Figure 2 (a) and Figure 2 (b) Figure 2 (a) is the electronic polarizability P of alkali metal atoms in the alkali metal cell. x (z) Distribution curve along the pumping direction. Figure 2 (b) is the curve of the magnetic field response signal Signal as a function of the radius r of the detection spot. Figure 2 A solid line indicates a pump beamwidth of 10MHz; a dashed line indicates a pump beamwidth of 10GHz; a dotted line indicates a pump beamwidth of 50GHz; and a dashed line indicates a pump beamwidth of 100GHz. Figure 2 The four curves from bottom to top in (a) show that as the laser spectral width increases (from 10MHz-10GHz-50GHz-100GHz), P x (z) The region of flat peak increases, that is, the region of uniform electronic polarizability increases, while the maximum value remains unchanged. Figure 2 (b) shows that the four curves from bottom to top (where the third and fourth curves overlap) indicate that as the laser spectral width increases (from 10MHz to 10GHz to 50GHz to 100GHz), the signal intensity (au, Arbitrary Unit) increases with the increase of the detection spot radius r (mm), and the wider the spectral line, the greater the signal intensity.

[0023] Figure 3 include Figure 3 (a) and Figure 3(b). 3(a) shows the curve of the peak-to-peak value of the magnetic field response signal ΔSignal (signal fluctuation parameter) as a function of the detection spot radius r when the pump power fluctuates by ±0.05%. Figure 3 (b) is the curve showing the change of the peak value of the magnetic field response signal ΔSignal with the radius r of the detection light spot when the pump light frequency fluctuates by ±1 GHz. Figure 3 A middle circle indicates a pump beam width of 10MHz; a diamond indicates a pump beam width of 10GHz; a lower triangle indicates a pump beam width of 50GHz; and a square indicates a pump beam width of 100GHz. Figure 3 The output signal fluctuation of medium-width linewidth lasers increases slowly with the increase of the detection spot radius, which reduces the noise that couples the power fluctuation of the light source into the signal or the noise that couples the frequency fluctuation of the light source into the signal by an order of magnitude.

[0024] Figure 4 include Figure 4 (a) and Figure 4 (b) Figure 4 (a) is the output laser linewidth Δυ after the laser passes through the electro-optic phase modulator. b The root mean square (RMS) phase fluctuation φ applied to the electro-optic phase modulation rms / π variation curve. Figure 4 (a) The dotted line indicates a cutoff bandwidth of 1 GHz applied to the electro-optic phase modulator; the solid line indicates a cutoff bandwidth of 3 GHz applied to the electro-optic phase modulator. Figure 4 (b) is the power spectral density distribution curve of the pump laser before and after electro-optic phase modulation. Figure 4 In (b), the dotted line represents the area before modulation; the solid line represents the area after modulation. From Figure 4 As can be seen, by reasonably selecting the power spectral density and noise bandwidth added to the electro-optic phase modulator, the output laser spectral width can be continuously adjusted from 5 GHz to 60 GHz.

[0025] The reference numerals in the attached diagram are listed below: 1-Gaussian white noise source; 2-First multi-channel line connection switch; 3-Low-pass filter bank; 4-Second multi-channel line connection switch; 5-Wideband RF power amplifier module; 6-Adjustable power attenuator; 7-Pump laser; 8-Electro-optic phase modulator; 9-First polarizer; 10-First quarter-wave plate; 11-Magnetic shielding barrel; 12-Triaxial magnetic compensation coil; 13-Non-magnetic electric heating system; 14-Alkali metal gas chamber; 15-Detection laser; 16-Second polarizer; 17-Photoelastic modulator; 18-Second quarter-wave plate; 19-Analyzer; 20-Photodetector; 21-Modulation controller; 22-Lock-in amplifier; 23-Function generator; 24-Host computer; xyz-Cartesian coordinate system (x-axis, y-axis, z-axis). Detailed Implementation

[0026] The following is in conjunction with the attached diagram ( Figures 1-4 The invention will be described in the following sections and examples.

[0027] Figure 1 This is a schematic diagram of the structure of the device for improving the uniformity and stability of electronic polarization in the SERF magnetic field measurement device according to the present invention. Figure 2 (a) is the electronic polarizability P of the alkali metal atoms in the gas cell. x (z) Distribution curve along the pumping direction; Figure 2 (b) is the curve of the magnetic field response signal Signal as a function of the radius r of the detection spot. Figure 3 (a) is the curve showing the change of the peak value of the magnetic field response signal ΔSignal with the radius r of the detection light spot when the pump power fluctuates by ±0.1%; Figure 3 (b) is the curve showing the change of the peak value of the magnetic field response signal ΔSignal with the radius r of the detection light spot when the pump light frequency fluctuates by ±1 GHz. Figure 4 (a) is the output laser linewidth Δυ when the laser passes through the electro-optic phase modulator. b With φ rms / π variation curve; Figure 4 (b) shows the power spectral density distribution of the pump laser before and after electro-optic phase modulation. (Reference) Figures 1 to 4 As shown, a device for improving the electronic polarization uniformity and stability of the SERF magnetic field measurement apparatus includes a pump laser 7 for generating a narrow-linewidth light source, an electro-optic phase modulator 8 for inducing optical field fluctuations, a power- and bandwidth-adjustable Gaussian noise source applied to the electro-optic phase modulator 8, an alkali metal gas cell 14, and a detection and magnetic field control system. The narrow-linewidth laser generated by the pump laser 7, after passing through the electro-optic phase modulator 8, outputs a broadband laser as the pump laser to polarize the alkali metal gas cell 14. The spectral linewidth of the broadband laser is adjusted by controlling the driving power and bandwidth applied to the electro-optic phase modulator 8 to broaden the narrow-linewidth laser spectral linewidth to the order of gas cell pressure broadening. The power- and bandwidth-adjustable Gaussian noise source includes a Gaussian white noise source 1, a low-pass filter bank 3, a broadband RF power amplifier module 5, and / or an adjustable power attenuator 6. The center frequency of the broadband laser is the same as the resonance peak frequency of the alkali metal atoms in the alkali metal gas cell 14.

[0028] The output signal of the Gaussian white noise source 1 is connected to the low-pass filter bank 3 via the first channel line connection switch 2. The output signal of the low-pass filter bank 3 is connected to the broadband RF power amplifier module 5 via the second multi-channel line connection switch 4. After passing through the adjustable power attenuator 6, it is input to the electrical port of the electro-optic phase modulator 8 to apply a driving effect. The laser output from the pump laser 7 passes sequentially through the optical port of the electro-optic phase modulator 8, the first polarizer 9, and the first quarter-wave plate 10 before being used as pump light to polarize the alkali metal gas cell 14. The alkali metal gas cell 14 is surrounded by atoms arranged from the inside out. The system includes a non-magnetic electric heating system 13, a triaxial magnetic compensation coil 12, and a magnetic shielding barrel 11. The detection light output from the detection laser 15 passes sequentially through a second polarizer 16, a photoelastic modulator 17, a second quarter-wave plate 18, an alkali metal gas chamber 14, and an analyzer 19 before being incident on a photodetector 20. The photodetector converts the optical signal into an electrical signal, which is then transmitted to a lock-in amplifier 22 via a coaxial cable. The lock-in amplifier 22 is connected to a modulation controller 21 and a host computer 24. The modulation controller 21 is connected to the photoelastic modulator 17, and the host computer 24 is connected to the triaxial magnetic compensation coil 12 via a function generator 23. The photoelastic modulator 17 is driven by the modulation controller 21. The electrical signal and the reference signal of the modulation controller 21 are demodulated by the lock-in amplifier 22 to obtain the DC component, first harmonic component, and second harmonic component of the detection electrical signal, which are then fed back to the host computer 24.

[0029] The low-pass filter bank 3 includes discrete filters with different cutoff frequencies, allowing coarse adjustment of the bandwidth of the output signal of the Gaussian white noise source 1, thereby achieving coarse adjustment of the laser linewidth; the adjustable power attenuator 6 can achieve power attenuation from 0 to 90 dB in 0.5 dB intervals, thereby achieving fine adjustment of the laser linewidth; the output signal of the adjustable power attenuator 6 is input to the electrical port of the electro-optic phase modulator 8.

[0030] The alkali metal chamber 14 contains a drop of potassium atom (or rubidium or cesium atom) and is filled with helium to slow down atomic diffusion and nitrogen as a quenching gas. The nitrogen pressure is not less than 1 atm and the helium pressure is approximately 50 Torr.

[0031] A method for improving the uniformity and stability of electronic polarization in a SERF magnetic field measuring device, utilizing the aforementioned apparatus for improving the uniformity and stability of electronic polarization in a SERF magnetic field measuring device, includes the following steps:

[0032] Step 1: Build the optical path according to the above instructions and connect the circuit part. Set the temperature of the alkali metal gas chamber 14 to 160-200℃, turn off the Gaussian noise source 1, and adjust the optical path and the triaxial magnetic compensation coil to make the device work in SERF state.

[0033] Step 2: Connect the first filter in the low-pass filter bank 3 through the multi-channel line connection switch 2, turn on the Gaussian noise source 1, and set the power attenuation to 90dB through the adjustable power attenuator 6.

[0034] Step 3: Use three-dimensional in-situ magnetic compensation technology to compensate for the residual magnetism of the system. Use function generator 23 to add a calibration magnetic field on the sensitive axis Y, adjust the pump optical power to make the first harmonic component of the lock-in amplifier 22 output the maximum, that is, the magnetic field response signal is the strongest, and record the pump optical power and the first harmonic signal strength at this time.

[0035] Step 4: Adjust the adjustable power attenuator 6 in 0.5 dB intervals within the range of 90-0 dB, and repeat step 3 for each power attenuation value;

[0036] Step 5: Connect the remaining filters in the low-pass filter bank 3 in sequence through the multi-channel line connection switch, and repeat step 4 for each filter.

[0037] Step 6: The electronic polarization uniformity and stability corresponding to the maximum output magnetic field response signal are optimal.

[0038] The principle of the device and method for improving the uniformity and stability of electronic polarization in a SERF atomic magnetometer is as follows:

[0039] When a beam of circularly polarized light passes through the gas cell, the pump rate R op (z) Along the propagation direction of the circularly polarized beam, z follows the following variation law:

[0040]

[0041] Where, n A For alkali metal density, σ(v) pump The pump light frequency is v. pump The absorption cross section, P(z) is the alkali metal polarizability, and the first focus of the pump light and gas cell is set as the zero point of the z-axis.

[0042] For broadband lasers, assuming their spectral distribution is Gaussian, each frequency component I(0,λ) in the laser... i The distribution is shown in the following formula:

[0043]

[0044] Where I0 is the total incident light intensity on the surface of the air chamber, e is the natural constant, c is the speed of light, λ is the wavelength, and λ i Let λi be the i-th wavelength contained in the broadband laser, λ0 be the center wavelength of the broadband laser, and Δν be the wavelength of the spectral laser. b The full width at half maximum (FWHM) of the spectral line. When broadband circularly polarized light passes through a gas cell, the i-th wavelength λ in the broadband laser... i pumping rate Rop (z,λ i The change of z along the propagation direction follows the following formula:

[0045]

[0046] Where, σ(λ) i ) represents the i-th wavelength λ in a broadband laser. i The absorption cross section, The i-th wavelength λ on the surface of the gas chamber i The corresponding pumping rate, where h is Planck's constant, v D1 R is the center frequency of the alkali metal D1 line. rel It is the relaxation rate.

[0047] Without the application of an external magnetic field, the initial state of an atom's spin under the influence of pump light and relaxation effects can be expressed as:

[0048]

[0049] When the magnetic field in the x and z directions is selected as B x =B z =0, y-direction magnetic field B y =10pT, the polarizability projection P along the detection light direction x x (z) varies with pump light transmission distance as follows Figure 2 As shown. By Figure 2 (a) It can be seen that as the laser spectral width increases, P x (z) The region of flat peak increases, that is, the region of uniform electronic polarizability increases, while the maximum value remains unchanged.

[0050] The P x (z) is:

[0051]

[0052] Wherein, the magnetic field parameter β = γ e B / (∫R op (z,λ i )dλ+R rel ), and β=[β x ,β y ,β z ], βx, βy, and βz are the x-axis, y-axis, and z-axis components of the magnetic field parameter β, respectively, and the magnetic field B = [B x B y B z ], γ e It is the electron gyromagnetic ratio.

[0053] Figure 2(b) The variation curve of the detection light probe signal intensity (Signal) with the detection spot radius was simulated. As the laser spectral width increases, the signal intensity increases with the increase of the detection light spot radius, and the wider the spectral line, the greater the signal intensity.

[0054] The signal strength Signal can be expressed as:

[0055]

[0056] Where L is the length of the air chamber, r is the radius of the detection light spot, and r e f is the electron radius. D1 v is the strength of the alkali metal D1 linear oscillator. pro To detect the light frequency, Γ D1 I0 is the pressure broadening value of the gas chamber, I0 is the intensity of the detection light, and σ(v) is the pressure broadening value of the gas chamber. pro -ν D1 ( ) is the cross-section for detecting light absorption.

[0057] As the pump laser spectral width increases, the distribution of unsaturated spin polarizability of alkali metal atoms at the center of the gas cell tends to become more uniform, which can significantly improve the intensity of the magnetic field response signal. To improve the sensitivity of the device, we must also consider the noise introduced by laser fluctuations. Figure 3 (a) illustrates the signal fluctuation ΔSignal when the laser power fluctuates by ±0.05%. Unlike narrow-linewidth lasers, the output signal fluctuation increases slowly with increasing detection spot radius. When the laser spectral width is greater than 10 GHz, the output signal fluctuation is better than ±0.01%, while the narrow-linewidth laser corresponds to a signal fluctuation greater than 0.1%. Therefore, under the same power fluctuation, using a broadband laser can reduce the noise coupled into the signal by at least one order of magnitude from the source power fluctuation. Similar to power fluctuation, a broadband laser can reduce the noise coupled into the signal by at least one order of magnitude from the source frequency fluctuation, such as... Figure 3 As shown in (b).

[0058] As can be seen from the above analysis, using broadband lasers can significantly suppress the polarization non-uniformity caused by the absorption of high-density alkali metal atoms in the SERF state, improve the magnetic field detection signal intensity, and reduce the noise caused by fluctuations in light source power and frequency, which is beneficial to improving the sensitivity of the device.

[0059] Since the full width at half maximum (FWHM) of high-performance narrow-linewidth laser spectra is typically within tens of MHz, it is difficult to directly meet the application requirements of this device and cannot give full play to the advantages of broadband lasers in pumping high-density alkali metal atoms.

[0060] The output optical field E(t) of the narrow-linewidth seed laser after passing through the high-speed phase modulator is:

[0061]

[0062] Where E0 is the amplitude of the laser field, and v0 is the center frequency of the laser. To match the laser linewidth Δυ laser The relevant phase noise, The phase modulation is applied by the electro-optic modulator, where j is the imaginary unit and t is time. According to the Wiener-Hinchin theorem, the power spectral density S of the output laser is... out (ν) is the intrinsic laser spectrum S laser (ν) and modulation spectrum S m Convolution of (ν):

[0063]

[0064] For Gaussian white noise phase modulation, the modulation spectrum S m (ν)=S white (ν),

[0065]

[0066] Where S0 is a coefficient of the Gaussian noise spectrum, related to the power of the RF noise signal modulated by white noise phase, and the parameter σ is related to the spectral width of the RF white noise signal power, and thus to the linewidth of the power spectrum of the optical field fluctuations caused by Gaussian white noise phase modulation. Assume the modulation bandwidth of the electro-optic modulator is Δν. EOM And σ≤Δν EOM / 2. Power spectral density S of the modulated laser output. out (ν)=S laser (ν)*S white (ν). Because convolution has a broadening effect, phase modulation can broaden the linewidth of the spectrum. The broadened power spectral density distribution S out The expression for (λ) is as follows:

[0067]

[0068] in, λ is the modulation carrier rejection ratio, λ0 is the center wavelength, I is the peak intensity, and S is the peak intensity. φ B and W represent the noise power spectral density and cutoff bandwidth applied to the electro-optic phase modulator, respectively, and the full width at half maximum (FWHM) Δυ is the full width at half maximum (FWHM) of the spectral line. b It can be approximated as

[0069]

[0070] Without modulation, the laser output spectrum is a typical narrow-linewidth Lorentz spectrum. When the RF noise signal power is low, the modulated laser output spectrum is a superposition of the narrow linewidth of the optical carrier and a wide Gaussian spectrum. As long as the RF noise signal power is strong enough, Gaussian white noise phase modulation can completely suppress the optical carrier, and the modulated laser output spectrum becomes a broadened Gaussian power spectrum. Furthermore, Gaussian white noise phase modulation does not generate carrier harmonics and does not require ultra-high bandwidth high-frequency electronic circuitry. Figure 4 (a) shows the trend of the output laser Gaussian linewidth as a function of the phase jitter applied to the phase modulator, where the horizontal axis represents the root mean square (rms) phase ripple applied to the phase modulator. As shown in the figure, by reasonably selecting the power spectral density and noise bandwidth added to the electro-optic phase modulator, the output laser spectral width can be continuously adjusted from 5 GHz to 60 GHz. Figure 4 (b) The laser power spectral density distribution before and after laser modulation was simulated.

[0071] 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 device for improving the electronic polarization uniformity and stability of a SERF magnetic field measurement device, characterized in that, The application relates to a device for improving the electronic polarization uniformity and stability of a SERF magnetic field measuring device, and the device comprises a pump laser for generating a narrow linewidth light source, an electro-optical phase modulator for causing light field fluctuation, a power and bandwidth adjustable Gaussian noise source applied to the electro-optical phase modulator, an alkali metal cell, and a detection and magnetic field control system, wherein the narrow linewidth laser generated by the pump laser is output as a wide spectrum laser after passing through the electro-optical phase modulator, the wide spectrum laser is used to polarize the alkali metal cell, the spectral linewidth of the wide spectrum laser is adjusted by controlling the driving power and bandwidth applied to the electro-optical phase modulator so as to expand the spectral linewidth of the narrow linewidth laser to the pressure expansion order of the alkali metal cell, and the power and bandwidth adjustable Gaussian noise source comprises a Gaussian white noise source, a low-pass filter set, a wideband radio frequency power amplification module and / or an adjustable power attenuator. The output signal of the Gaussian white noise source is connected to the low-pass filter set through a first channel line connection switch, the output signal of the low-pass filter set is connected to the wideband radio frequency power amplification module through a second multi-channel line connection switch, and the signal after passing through the adjustable power attenuator is input to the electrical port of the electro-optical phase modulator to apply a driving effect; the laser output by the pump laser sequentially passes through the optical port of the electro-optical phase modulator, a first polarizer and a first 1 / 4 wave plate, and then is incident on the alkali metal cell to polarize alkali metal atoms; a non-magnetic electric heating system, a three-axis magnetic compensation coil and a magnetic shielding barrel are sequentially arranged around the alkali metal cell from inside to outside; detection laser output detection light sequentially passes through a second polarizer, an optical elastic modulator, a second 1 / 4 wave plate, the alkali metal cell and a detection polarizer, and then is incident on a photoelectric detector to convert the optical signal into an electrical signal, which is transmitted to a lock-in amplifier through a coaxial cable; the lock-in amplifier is connected to a modulation controller and an upper computer; the modulation controller is connected to the optical elastic modulator; and the upper computer is connected to the three-axis magnetic compensation coil through a function generator. The optical elastic modulator is driven by the modulation controller, the electrical signal and a reference signal of the modulation controller are demodulated by the lock-in amplifier to obtain a direct current component, a first-order frequency component and a second-order frequency component of the inspection electrical signal, and the components are fed back to the upper computer. The low-pass filter set comprises discrete filters with different cutoff frequencies, which allows the bandwidth of the Gaussian white noise source output signal to be coarsely adjusted, thereby realizing coarse adjustment of the laser linewidth; the adjustable power attenuator can realize power attenuation of 0 to 90 dB at intervals of 0.5 dB, thereby realizing fine adjustment of the laser linewidth; and the output signal of the adjustable power attenuator is input to the electrical port of the electro-optical phase modulator.

2. The device for improving the electronic polarization uniformity and stability of a SERF magnetic field measurement device according to claim 1, characterized in that The center frequency of the wide spectrum laser is the same as the resonance peak frequency of the alkali metal atoms in the alkali metal cell.

3. A method of improving the electronic polarization uniformity and stability of a SERF magnetic field measurement device, characterized in that, The application also discloses a device for improving the electronic polarization uniformity and stability of a SERF magnetic field measuring device.

4. The method for improving SERF magnetic field measurement device electronic polarization uniformity and stability of claim 3, wherein, The device comprises the following steps: Step 1, building an optical path and connecting a circuit part, setting the temperature of the alkali metal cell to be 160-200 DEG C, turning off the Gaussian white noise source, adjusting the optical path and the three-axis magnetic compensation coil, and making the device work in a SERF state; Step 2, connect the first filter in the low-pass filter set through the first multi-channel line connection switch, turn on the Gaussian white noise source, and set the power attenuation to 90 dB through the adjustable power attenuator 6; Step 3, compensate the system residual magnetism by using the three-dimensional in-situ magnetic compensation technology, add a calibration magnetic field to the sensitive axis Y using a function generator, adjust the pump light power, make the lock-in amplifier output a maximum first-order frequency component, that is, the magnetic field response signal is the strongest, and record the pump light power and the first-order frequency signal strength at this time; Step 4, adjust the adjustable power attenuator in the range of 90-0 dB at an interval of 0.5 dB, and repeat step 3 at each power attenuation value; Step 5, connect the remaining filters in the low-pass filter set in turn through the first multi-channel line connection switch, and repeat step 4 at each filter; Step 6, the electronic polarization uniformity and stability corresponding to the maximum output magnetic field response signal are the best.

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