CN117347927A - Atomic magnetic gradiometer with symmetrical structure - Google Patents

Abstract

The invention relates to an atomic magnetic gradiometer with a symmetrical structure. The crosstalk of optical signals and electromagnetic signals does not exist between the two sensitive units of the symmetrical structure atomic magnetic gradiometer; the optical, thermal, magnetic and mechanical aspects have high consistency, the common mode of noise such as light fluctuation, temperature fluctuation, structure residual stress release and the like is increased, then the common mode noise is eliminated through gradient measurement, and the sensitivity is improved; the periphery of the sensitive air chamber in the symmetrical structure is not provided with an additional photoelectric device and a lead, so that the distance between the sensitive air chamber and a magnetic source to be detected can be reduced, and the signal to be detected is improved. The distance is mainly determined by the thickness of the air chamber and the thickness of the reflector, and the thickness of the reflector can be further reduced by a method of plating a reflecting film on the inner wall of the air chamber and the like. The symmetrical structure also comprises two on-line monitoring ports which can be used for debugging and real-time monitoring, and compensating and early warning signals according to monitoring output. The symmetrical structure employs single beam measurement, based on which scalar or vector measurements can be made.

Description

Atomic magnetic gradiometer with symmetrical structure

Technical Field

The invention relates to an atomic magnetic gradiometer with a symmetrical structure, belonging to the technical field of magnetic field precision measurement.

Background

The magnetic field gradiometer is an important sensitive device in the fields of biological magnetic measurement, geophysical research, seismic magnetic signal monitoring, magnetic anomaly target detection, nondestructive detection and the like. By means of magnetic field gradient measurement, magnetic source information detection can be achieved in a geomagnetic environment, and basic information is provided for further inversion of magnetic source positions, distribution, amplitude and the like.

There are many ways to measure magnetic fields at present, including SQUID magnetometers based on superconducting quantum interference effects, atomic magnetometers based on spin, magnetometers based on hall effect, magneto-resistive effect and magnetic induction effect, etc. Among these measurement methods, SQUID magnetometers and atomic magnetometers are currently recognized as high sensitivity magnetometers.

Atomic magnetometers come in a wide variety of sensitive ways including single axis parametric modulation SERF magnetometers, single axis discrete modulation SERF magnetometers, mx magnetometers, bell-Bloom magnetometers, free-running decay magnetometers, RF magnetometers, and the like.

When an atomic magnetometer is used for magnetic field gradient measurement, the measurement results can be subtracted to obtain a magnetic field differential signal by arranging probes at two different positions. Or the measurement result of one probe is utilized to compensate the magnetic field near the other probe, so that the other probe directly outputs a magnetic field gradient signal. Alternatively, more complex methods may be used, such as the "differential magnetic field gradient measurement method" of the applicant's patent at 2023, 8, 23 (application number 202311064644.2).

For atomic magnetic gradiometers, three important problems remain to be solved:

firstly, in the atomic gradient measurement, background magnetic field noise is natural common mode noise, and heating and exciting magnetic noise, optical noise, thermal noise and mechanical noise are required to realize common mode through special structural design so as to eliminate in the gradient measurement process, thereby realizing magnetic field sensitivity superior to that of a single probe.

And secondly, cross interference among different sensitive units. The optical signals and the magnetic field excitation signals between the two units are mutually and crossly coupled, so that the optical signals and the magnetic field excitation signals interfere with each other, and measurement uncertainty and noise are increased;

and thirdly, how to reduce the distance between the sensitive unit and the magnetic source to be measured. Because the periphery of the sensitive air chamber is often provided with an additional photoelectric component such as a 45-degree reflecting mirror, a photoelectric detector and the like and leads, a larger distance must be kept between the sensitive air chamber and a magnetic source to be measured, and particularly, when the periphery of the magnetic source is provided with an uneven contour, such as the measurement of heart magnetism at the lower part of the chest, the situation is more obvious.

Disclosure of Invention

The invention solves the technical problems that: common mode magnetic, optical, thermal and mechanical noise, elimination of cross interference of optical signals and electromagnetic signals between two sensitive units, reduction of distance between sensitive air chamber and magnetic source to be measured, which are important problems to be solved in current atomic magnetic gradiometers.

The invention aims at the problems, and designs an atomic magnetic gradiometer with a symmetrical structure, which not only realizes common mode of multiple physical fields, but also isolates cross coupling among sensitive probes, eliminates the addition of photoelectric devices and leads on the periphery of the sensitive probes, reduces the shortest distance between the sensitive probes and a magnetic source in measurement, and improves the magnetic signal to be measured.

The technical solution of the invention is as follows: aiming at three important problems to be solved in an atomic magnetic gradiometer, namely common-mode light, magnetism, heat and mechanical noise, eliminating cross interference of light signals and radio frequency signals between two sensitive units and reducing the distance between a sensitive air chamber and a magnetic source to be tested, the invention designs a highly symmetrical and mutually independent test structure, and the structure and a corresponding test signal connection mode are specifically shown in the attached figure 1. Firstly, a laser emits monochromatic coherent laser corresponding to atomic frequency, and then an acousto-optic modulator and a reflecting mirror are used for controlling the light intensity of the left and right light beams to be equal through a linear polaroid and a half wave plate to form a symmetrical light path. In order to isolate the cross coupling caused by the reflected light of the atomic gas chamber in the two light paths, a half wave plate, a polarization beam splitter prism and a quarter wave plate are added in front of the atomic gas chamber, so that the reflected light of the front and rear surfaces of the atomic gas chamber can not return along the original path, and the wave plate, the polarization beam splitter prism and the quarter wave plate can reduce reflection through coating an antireflection film. And a reflecting mirror is added behind the atomic gas chamber to enable the reflected light to return to the original path, and the reflected light is refracted by a polarization beam splitter prism to carry out photoelectric detectors. The structure simultaneously eliminates the additional photoelectric components and leads around the atomic gas chamber, reduces the distance between the sensitive gas chamber and the magnetic source to be measured, and improves the measurement signal under the condition that the magnetic source is unchanged. The symmetrical structure has two on-line monitoring ports at the same time, can be used for debugging and real-time monitoring of measurement states, and compensates and early warns signals according to monitoring output. The magnetic field sensitive modes which can be adopted by the symmetrical structure of the atomic magnetic gradiometer provided by the invention comprise a single-axis parametric modulation SERF magnetic measurement mode, a single-axis discrete modulation SERF magnetic measurement mode, an Mx mode, an Mz mode, a Bell-Bloom mode, a free damping oscillation mode, an RF radio frequency magnetic field detection mode, a self-excited oscillation mode and the like, and when the Mx, the Mz and the Bell-Bloom modes are adopted, the resonance frequencies of the two air chambers can be locked on the same frequency so as to eliminate the cross interference of electromagnetic signals.

Compared with the prior art, the invention has the advantages that:

(1) Compared with the conventional PBS beam splitting structure, the structure provided by the invention has the advantages that the light intensity of the two sensitive heads is equal, and the mechanical, thermal and electric signals are highly symmetrical.

(2) The structure provided by the invention isolates the reflected light of the front and rear surfaces of the air chamber, and eliminates the cross coupling of optical signals and magnetic field information carried by the optical signals between two sensitive air chambers.

(3) The invention adopts the method of locking the resonance frequency of the two air chambers on the same frequency, thereby eliminating the cross interference of electromagnetic signals. In addition, compared with a feedback method that a common mode magnetic field is applied by a coil, the method ensures the complete consistency of feedback links of two sensitive units.

(4) Compared with the conventional method for placing the photoelectric detector at the rear end or the side surface of the air chamber, the structure provided by the invention eliminates the photoelectric components and leads at the periphery of the air chamber, is beneficial to the sensitive unit as a probe to approach the magnetic source to be detected, and reduces the distance between the sensitive unit and the magnetic source to be detected.

Drawings

FIG. 1 is a diagram showing the light path and signal of an atomic magnetic gradiometer with symmetrical structure according to the invention

The meaning of the label in the figure is: the device comprises a laser (1), an acousto-optic modulator (2), a reflector (3), a polarizer (4), a half-wave plate (5), a polarization beam splitter prism (6), a half-wave plate (7), a polarization beam splitter prism (8), a photoelectric detector (9), a quarter-wave plate (10), an air chamber (11), a triaxial coil (12), a reflector (13), a photoelectric detector (14), a quarter-wave plate (15), a reflector (16), a half-wave plate (17), a polarization beam splitter prism (18), a photoelectric detector (19), a quarter-wave plate (20), an air chamber (21), a triaxial coil (22), a reflector (23), a photoelectric detector (24), a phase-locked amplifier (25), a signal generator (26), a phase-locked amplifier (27), a PID controller (28), a display (29) and a host (30).

Detailed Description

The invention is further described below with reference to the drawings and specific examples.

As shown in fig. 1, an atomic magnetic gradiometer with symmetrical structure comprises a laser (1) whose light source can be an external cavity semiconductor laser, DFB, DBR or VCSEL laser, or from an optical fiber. In this example, a VCSEL laser produced by Vixar was used, and its output wavelength was chosen to be 894nm. The output of the laser (1) is modulated monochromatic laser, the modulation mode can be amplitude modulation or frequency modulation, in the embodiment, an acousto-optic modulator (2) is adopted to modulate the laser amplitude, and the output is amplitude modulated square wave light signals.

The square wave light signal after amplitude modulation of the acousto-optic modulator (2) passes through the polarizer (4) and a half wave plate (5) to be output as linearly polarized light with an adjustable polarization plane, and the linearly polarized light is used for adjusting the light intensity ratio of the left light path and the right light path, wherein the light intensity ratio is 1:1 in the embodiment. Since the laser (1) output is typically elliptical polarized light approaching linear polarization, the polarizer (4) may be omitted in this embodiment from the point of adjusting the symmetry of the light intensity.

The laser then passes through a polarizing beam splitter prism (6), in this example using PBS052 produced by Thorlab, where some of the light is reflected into the left light path and some of the light is transmitted. The transmitted light firstly passes through a quarter wave plate (15) to be circularly polarized light, and after being reflected by a reflecting mirror (16), the transmitted light passes through the quarter wave plate (15) again in the opposite direction to be changed into s light, and after being reflected by a polarization beam splitter prism (6), the s light enters a right light path.

Then, the left and right light paths are completely symmetrical, and the s light is changed into p light through the half wave plates (7) and (17), and GCL-060812 which is produced by large constant photoelectricity is adopted in the embodiment. And then passing through polarization splitting prisms (8) and (18), in this embodiment PBS052 produced by Thorlab is adopted, and the transmitted light passes through quarter wave plates (10) and (20), in this embodiment GCL-060802 produced by large constant photoelectric is adopted, and becomes circularly polarized light. On the reflection path, photoelectric detectors (9) and (19) are arranged, in this embodiment, FDS010 produced by Thorlab is adopted, and the FDS010 is used for adjusting the half wave plate (5) and monitoring the working state of the gradiometer in real time in the process of assembly and debugging, and compensating and early warning signals are carried out according to monitoring output. After the subsequent treatment, the transmitted light enters the photodetectors (14) and (24) again through the polarization beam splitting prisms (8) and (18) for detecting magnetic field signals. The magnetic field signal detection method may adopt a uniaxial parametric modulation SERF magnetic measurement method, a uniaxial discrete modulation SERF magnetic measurement method, an Mx method, an Mz method, a Bell-Bloom method, a free-fall oscillation and self-oscillation method, etc., and in this embodiment, the Bell-Bloom method through resonance of an optical modulation frequency and a larmor precession frequency of a magnetic field is to be selected.

The assembly of the devices (1) - (24) can be carried out in two ways, one is that the housing supporting structure is machined or 3D printed, and then the devices are fixed on the housing, and the adjustment and positioning of the optical axis angle are carried out in the fixing process. The other is a modularized mode, the optical glue is used for bonding the optical axes (4) - (10), (14) - (19) and 20 together to be used as an independent module, and the optical axis is adjusted and positioned by a special tool in the bonding process. And then mechanically fixing with other devices through the supporting structure. The second mode is to be adopted in this embodiment.

After passing through the quarter wave plates (10) and (20), the laser enters the two air chambers (11) and (21) with the same air inflation atmosphere and pressure respectively. The gas chambers (11) and (21) may be film plating low-pressure gas chambers or buffer gas high-pressure gas chambers, the alkali metal atoms contained therein may be potassium, rubidium, cesium or the like, and the quenching gas and buffer gas contained therein may be nitrogen, helium, neon or the like. In this example, a cesium atom chamber was used and 760torr nitrogen was charged as the quenching gas and buffer gas.

The rear ends of the air chambers (11) and (21) are provided with reflecting mirrors (13) and (23), and a vertical reflecting film plating high reflecting mirror is adopted in the embodiment.

The air cells (11) and (21) can be heated by using a demagnetizing heating film and a high-frequency heating current, in this embodiment, a polyimide substrate demagnetizing heating film, the heating current frequency being 20kHz.

Triaxial coils (12) and (22) with identical dimensional parameters are respectively arranged around the air chambers (11) and (21), and can be Helmholtz coils, saddle coils, plane coils or other types of coil structures. In the embodiment, the Helmholtz coils processed by the flexible substrate are adopted, and the circuit connection mode of the two coils is reverse connection, so that the opposite directions of magnetic fields generated by the two coils after current is introduced are ensured, and the compensation magnetic field gradient is generated.

The outputs of the photodetectors (14) and (24) are connected to phase-locked amplifiers (25) and (27), and the reference signals of the phase-locked amplifiers (25) and (27) and the signal driving the acousto-optic modulator (2) adopt the same signal source (26). The two phase-locked amplifier parameter settings are identical and the sum of the phase difference detection results or the sum of the out-of-phase demodulation results is input to a PID controller (28), and the output of the PID controller (28) is used for controlling the output frequency of the signal source (26). The difference between the phase difference detection results of the two lock-in amplifiers or the difference between the out-of-phase demodulation results controls the current on the two triaxial coils (12) and (22) to compensate the magnetic field gradient and also serves as a measurement output of the magnetic field gradient.

If the vector magnetic field measurement mode is adopted, the gradient compensation current can be applied to the coils in the corresponding directions. In this embodiment, the magnetic field scalar is measured, so that it is necessary to intermittently measure the magnetic field direction and then apply the gradient feedback current through a fixed ratio. For example, the included angles between the magnetic field direction and the coordinate axes formed by the directions of the triaxial coils (12) and (22) are alpha, beta and gamma respectively, and the proportion of the applied current on the corresponding three coils is cos alpha, cos beta and cos gamma. Or the external large coil is used for canceling the magnetic fields in two directions in advance, and the rest unidirectional magnetic field and magnetic field gradient are measured and controlled in a closed loop.

The specific implementation steps are as follows:

first, the assembly of the structure and the optical axis calibration are performed according to the foregoing description.

After that, the air chamber was heated, and the heating temperature in this example was set to 75 ℃.

The laser (1), the acousto-optic modulator (2) and the signal generator (26) are started to output amplitude modulated laser.

The photodetectors (9) and (19) are turned on, the outputs thereof are compared with a set threshold value, and the operating state is detected.

Photodetectors (14) and (24), lock-in amplifiers (25) and (27), and PID controller (28) are turned on. The outputs of the photodetectors (14) and (24) are fed to phase-locked amplifiers (25) and (27) as input signals, and the output of the signal generator (26) is fed to the phase-locked amplifiers (25) and (27) as reference signals. The sum of the phase difference detection results of the two lock-in amplifiers (25) and (27) is input as a feedback signal to a PID controller (28), and the output of the PID controller (28) is used to control the output frequency of the signal generator (26).

The difference between the phase difference detection results of the two phase-locked amplifiers (25) and (27) is the open-loop output of the magnetic field gradient.

The difference between the phase difference detection results of the two phase-locked amplifiers (25) and (27) is fed back to the triaxial coils (12) and (22) through the PID controller (28), so that closed loop measurement of the magnetic field gradient can be realized.

The above examples are provided for the purpose of describing the present invention only and are not intended to limit the scope of the present invention. The scope of the invention is defined by the appended claims. Various equivalents and modifications that do not depart from the spirit and principles of the invention are intended to be included within the scope of the invention.

Claims (11)

1. An atomic magnetic gradiometer with symmetrical structure is characterized in that: the laser is emitted from the laser (1) and passes through the acousto-optic modulator (2) to realize amplitude modulation of the laser, and the acousto-optic modulator (2) can be removed by modulating the light intensity or frequency in other modes or adopting a non-modulation detection method. And then the laser passes through the reflector (3) to change the direction of the light path, so that the light enters the symmetrical atomic magnetic gradiometer probe consisting of the components (3) to (23). The laser (1) can be separated from the probe, and can be introduced into the atomic gradiometer probe by means of space propagation or optical fiber propagation, or can be integrated in the probe. The laser after the reflector (3) passes through the polarizer (4) and the half wave plate (5), so that the angle between the polarization direction and the horizontal plane is about 45 degrees, the laser passes through the polarization splitting prism (6), s light is reflected, p light is transmitted, the transmitted light is changed into circularly polarized light after passing through the quarter wave plate (15), the circularly polarized light is reflected by the reflector (16) and returns along the original path, the p light is changed into s light after passing through the polarization splitting prism (6) again through the quarter wave plate (16), and the light intensity of the two beams of light is identical by adjusting the half wave plate (5) in opposite directions to the light reflected for the first time. The optical axes of the half wave plates (7) and (17) can be rotated in the adjustment process to maximize the light intensity of the detectors (9) and (19), and then the half wave plate (5) is adjusted to equalize the outputs of the photodetectors (9) and (19). Thereafter, the optical axes of the half wave plates (7) and (17) are rotated by 45 °, respectively. The right light beam is changed into p light after passing through a half wave plate (17), then passes through a polarization beam splitter prism (18), most photons are transmitted, few photons are reflected to carry out a photoelectric detector (19) for on-line monitoring, the transmitted light passes through a quarter wave plate (20) to be changed into circularly polarized light, then enters an atomic air chamber (21), passes through the atomic air chamber (21), returns along an original path through a reflecting mirror (23), is changed into s light after passing through the quarter wave plate (20) again, and then passes through the polarization beam splitter prism (18) to be reflected to enter the photoelectric detector (24) for magnetic field measurement. The left Bian Guanglu and right light paths are completely symmetrical, the light is changed into p light through a half wave plate (7), and then the p light passes through a polarization beam splitting prism (8), most of photons are transmitted, and few photons are reflected to carry out a photoelectric detector (9) for on-line monitoring. The transmitted light is changed into circularly polarized light through the quarter wave plate (10), then enters the atomic air chamber (11), returns along the original path through the reflecting mirror (13) after passing through the atomic air chamber (11), becomes s light after passing through the quarter wave plate (10) again, and then enters the photoelectric detector (14) after being reflected through the polarization splitting prism (8) for measuring a magnetic field. The atom air chambers (11) and (21) are surrounded by triaxial coils (12) and (22) for controlling the magnetic field inside the air chambers. The signals output by the photodetectors (14) and (24) are used for measuring fields, the measuring mode can be scalar or vector, the output can be directly differentiated to obtain gradient signals, and the measurement and control method or other signal acquisition methods described in the embodiment of the patent can also be adopted.

2. The atomic magnetic gradiometer of symmetrical construction according to claim 1, wherein: the two sensitive units are completely symmetrical in mechanical structure, so that errors caused by residual stress and vibration are common mode signals.

3. The atomic magnetic gradiometer of symmetrical construction according to claim 1, wherein: the two sensitive units are completely symmetrical on the light path, the light intensity of the two beams of reflected light can be finely adjusted through the half wave plate (5), the light intensity of the left and right light paths is ensured to be completely equal, and noise caused by light intensity and frequency fluctuation is a common mode signal.

4. The atomic magnetic gradiometer of symmetrical construction according to claim 1, wherein: after light reflected from the front and rear surfaces of the atomic gas chambers (11) and (21) passes through the quarter wave plates (10) and (20), the light is reflected into the photodetectors (14) and (24) through the polarization beam splitting prisms (8) and (18) and does not enter the opposite gas chambers, so that first isolation is realized, p light is converted into s light through the half wave plates (7) and (17), and second isolation is carried out through the polarization beam splitting prism (6). Ensuring that no optical signal and cross-coupling of the magnetic field information carried thereby is generated.

5. The atomic magnetic gradiometer of symmetrical construction according to claim 1, wherein: the sensitive air chambers (11) and (21) have no additional structures such as 45-degree reflecting mirrors, photoelectric detectors and the like and signal leads thereof in five directions. Therefore, when one of the sensitive air chambers (22) is used as a probe to approach the magnetic source to be measured, the technology reduces the shortest distance between the sensitive air chamber and the magnetic source. The distance is mainly determined by the thickness of the air chamber and the thickness of the reflector, wherein the reflector can be further eliminated by the process of plating the side wall of the air chamber with a reflecting film.

6. The atomic magnetic gradiometer of symmetrical construction according to claim 1, wherein: the symmetrical structure comprises two on-line monitoring ports (9) and (19), can be used for adjusting the symmetry of the optical path in the debugging process, monitoring the measurement state in real time, and compensating and early warning signals according to the monitoring output.

7. The atomic magnetic gradiometer of symmetrical construction according to claim 1, wherein: with single beam measurement, scalar gradient and vector gradient measurements can be made with different detection modes based on the symmetrical structure. The gradient output can be direct difference of the probe output, or can be that one probe carries out magnetic field measurement and feedback, the triaxial coils (12) and (22) are connected positively, and the magnetic fields around the two probes are compensated at the same time, so that the second probe directly outputs a gradient signal. Gradient signals may also be obtained using the methods detailed in the examples of this patent.

8. The atomic magnetic gradiometer of symmetrical construction according to claim 1, wherein: an Mx measurement mode may be adopted, where one or both of the three-axis coils (12) and (22) are used to generate a radio frequency excitation magnetic field, the coils are connected in forward direction, ensuring that the generated magnetic fields are in the same direction. Since the parameters of the coils are identical, the magnetic field is also the same. The driving frequency of the coil is controlled by the sum of demodulation signals of the two probes, so that the two radio frequency signals always keep the same frequency, and the cross interference of electromagnetic signals is eliminated. Meanwhile, compared with a feedback method of applying a common-mode magnetic field by adopting a coil, the method ensures the complete consistency of feedback links of two sensitive units.

9. The atomic magnetic gradiometer of symmetrical construction according to claim 1, wherein: the method can adopt a Bell-Bloom light synchronous modulation detection method, the sum of the output signals after demodulation of the two probes is used as a feedback signal for controlling the light modulation frequency, the frequency of the modulation signals on the two sensitive probes is ensured to be consistent all the time, and the cross interference of electromagnetic signals is eliminated. Meanwhile, compared with a feedback method of applying a common-mode magnetic field by adopting a coil, the method ensures the complete consistency of feedback links of two sensitive units.

10. The atomic magnetic gradiometer of symmetrical construction according to claim 1, wherein: the triaxial coils (12) and (22) may be connected in reverse for compensation of magnetic field gradients and closed-loop measurement of gradients.

11. The atomic magnetic gradiometer of symmetrical construction according to claim 1, wherein: the triaxial coils (12) and (22) may be connected in a forward direction for compensation and closed loop control of the background common mode magnetic field.