CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from Chinese Patent Application No. 202110756202.9, filed on Jul. 5, 2021. The content of the aforementioned applications, including any intervening amendments thereto, is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to detection of weak magnetic fields, and more particularly to a device and method for measuring a scalar magnetic field based on pulsed optical pumping.

BACKGROUND

High-sensitivity weak magnetic detection has been widely used in biomedicine, space exploration, geophysics and ocean exploration. For example, some organs of the human body or other organisms, such as heart and brain, can produce extremely weak magnetic fields in which a plenty of information is contained, and provides an important reference for medical researches. All-optical atomic magnetometer and associated detection techniques thereof attract a lot of attention due to their desirable sensitivity. At present, the high-sensitivity weak magnetic field detection based on all-optics mainly includes spin-exchange-relaxation-free (SERF), optical pumping (OP) and non-linear magneto-optical rotation (NMOR) and the combination and improvement thereof. The basic principle of these techniques is to measure an oscillation signal or spin evolution signal of a polarized atom in a magnetic field using a narrow line-width laser, by analyzing characteristics of the signals, magnitude and variation of magnetic field are obtained.

The SERF is a vector weak magnetic detection method. Under a near-zero magnetic field (extremely weak background magnetic field), the spin of highly polarized and high-density atoms remain coherent for a long time and their decaying relaxation is magnetic field dependent. The SERF magnetometer is more suitable for precision measurements in near-zero magnetic fields, with high sensitivity, but requires magnetic shield. For larger background magnetic fields, such as the earth's magnetic field, the SERF will not work properly unless active compensation is used. In order to achieve high atomic density, high temperature above 150° C. is required. In addition, in order to improve sensitivity and suppress noise during measuring, a lock-in amplifier (LIA) is often required. An excessive sampling frequency will also directly attenuate the sensitivity, and thus the actual sampling frequency or bandwidth is often set to 10-100 Hz, seldom reaching the kHz level.

Optical pumping magnetometer (OPM) simultaneously enables the single-axial measurement of a scalar magnetic field and the multi-axial measurement of a vector magnetic field. Based on the Zeeman splitting of atomic energy levels under the action of a magnetic field, a radio-frequency (RF) field is used to match an energy difference between splitting two energy levels, causing atomic resonance and absorption, and a magnitude of the magnetic field is obtained from an absorption signal and a frequency of the RF field. The OPM can allow the high-sensitivity detection, and can be operated in a strong magnetic field such as geomagnetic field; however, the LIA is also necessary to improve sensitivity and reduce noise. In view of this, the sampling frequency is also not high, commonly 10-100 Hz.

Pulsed optical pumping-based NMOR is mainly a scalar detection method. Atoms polarized by pump beam in the magnetic field will produce precession (Larmor precession), by measuring the frequency of the precession, a magnitude of the magnetic field is obtained. On one hand, the beneficial effects of the NMOR are as follows. (1) The NMOR has an ability to operate in large background magnetic field such as the geomagnetic field. (2) compared to the SERF, where the same atomic medium is used, the NMOR can achieve better performance below 100° C., without a high temperature as the SERF. (3) The NMOR uses a direct frequency measurement with natural reference, thus a long-term operation is calibration-free. (4) The NMOR is not need the LIA, having a simpler structure. (5) The NMOR has high sensitivity, and can be optimized to reach a level of the SERF. One the other hand, the shortcomings of the NMOR are as follows. (1) The NMOR uses a pulsed pump-probe mode, in which a pump beam is a periodic pulse t1, the longer the pulse time, the higher the pumping efficiency, and then there is a certain duration of RF pulse t2. Thus, the sampling will consume a certain amount of time, which will affect the sampling frequency fBof the magnetic field measurement. (2) A repetition period T0for each sampling point is related to a signal-to-noise ratio of an output signal and a decaying relaxation time T2of atomic spins after polarization. If the repetition period is shortened to increase the sampling frequency fB, the signal-to-noise ratio and the sensitivity may be reduced. (3) A high sampling frequency fBwill lead to an increase in the data size and significantly increase the processing difficulty. In general, it is a challenge to increase the sampling frequency while keeping high sensitivity.

Nevertheless, some important applications such as space exploration and biomagnetic field detection require a higher magnetic field sampling frequency fBto obtain more information. Therefore, a method and device for measure magnetic field that can effectively increase the sampling frequency and have high sensitivity are needed.

SUMMARY

In order to overcome the above-described problems, the present disclosure provides a device and method for measuring a scalar magnetic field based on pulsed optical pumping, which have high sampling frequency as well as excellent detection sensitivity.

The technical solutions of the present disclosure are described as follows.

In a first aspect, the disclosure provides a device for measuring a scalar magnetic field based on pulsed optical pumping, comprising:

a first laser configured to emit a probe beam; and

a second laser configured to emit a pump beam;

wherein an output end of the first laser is connected to an input end of a first isolator; an output end of the first isolator is connected to an input end of a first λ/2 waveplate; an output end of the first λ/2 waveplate is connected to an input end of a first polarizing beam splitter configured to split a beam into two beams; a first output end of the first polarizing beam splitter is connected to an input end of a first fiber coupler; an output end of the first fiber coupler is connected to an input end of a first spectrum analyzer configured to perform dynamic detection; a beam output from a second output end of the first polarizing beam splitter is configured to be transformed by a reflector and then enter an atomic sensor probe; an emergent light from the atomic sensor probe is configured to be input to a second λ/2 waveplate; an output end of the second λ/2 waveplate is connected to an input end of a second polarizing beam splitter configured to split a beam into two beams; a first output end of the second polarizing beam splitter is connected to an input end of a first photoelectric converter; a second output end of the second polarizing beam splitter is connected to an input end of a second photoelectric converter; and a differential output end of the first photoelectric converter and a differential output end of the second photoelectric converter are both connected to a data acquisition system configured to acquire, store, process and display a detection signal;

an output end of the second laser is connected to an input end of a second isolator; an output end of the second isolator is connected to an input end of an acousto-optic modulator configured to control beam intensity; an output end of the acousto-optic modulator is connected to an input end of a third λ/2 waveplate; an output end of the third λ/2 waveplate is connected to an input end of a third polarizing beam splitter configured to split a beam into two beams; a first output end of the third polarizing beam splitter is connected to an input end of a second fiber coupler; an output end of the second fiber coupler is connected to an input end of a second spectrum analyzer configured to perform dynamic detection; a second output end of the third polarizing beam splitter is connected to a λ/4 waveplate configured to transform a beam into a circularly polarized beam; and an output beam of the λ/4 waveplate is configured to be expanded by a beam expander and then enter the atomic sensor probe; and

a first planar reflector is arranged at one side of the atomic sensor probe, and a second planar reflector is arranged at the other side of the atomic sensor probe; the first planar reflector and the second planar reflector both face toward the atomic sensor probe; and the first plane reflector and the second planar reflector are arranged parallel to a pump beam input to the atomic sensor probe.

In some embodiments, the atomic sensor probe comprises a vapor cell; a thermal insulation chamber is provided outside the vapor cell; a heater configured to heat the vapor cell is provided inside the thermal insulation chamber; the heater is connected to a temperature controller configured for temperature stabilization; a three-dimensional (3D) magnetic field coil is provided outside the vapor cell; and a precision current source of the 3D magnetic field coil is electrically connected to a controller configured to control magnetic field intensity.

In some embodiments, the vapor cell is transparent and filled with saturated rubidium vapor.

In some embodiments, the first laser is electrically connected to a controller; and the controller is configured to control a temperature and current of the first laser.

In some embodiments, the second laser is electrically connected to a controller; and the controller is configured to control a temperature and current of the second laser.

In some embodiments, the acousto-optic modulator is electrically connected to a controller configured to control the acousto-optic modulator.

In a second aspect, the disclosure provides a method for measuring a scalar magnetic field using the device of claim1, comprising:

(a) adjusting positions of the first planar reflector and the second planar reflector; placing the atomic sensor probe into a magnetic shield; heating the atomic sensor probe to a preset temperature to generate a static magnetic field along a z-direction of a 3D magnetic field coil; and superimposing an alternating magnetic field on the static magnetic field;

(b) from an initial moment t=0, periodically turning off and on, by the acousto-optic modulator in combination with a controller, the second laser to periodically emit the pump beam, wherein a turn-on duration of the second laser is t1, and a duration of each on-off cycle is T0;

(c) from the initial moment t=0, loading a radio frequency (RF) pulse in a y-direction of the 3D magnetic field coil, wherein a duration of the RF pulse is t2; the RF pulse is in a sinusoidal waveform; a frequency of the RF pulse is the same as an atomic spin precession frequency; and an amplitude and the duration of the RF pulse are optimizable according to an optical measurement signal;

(d) from the initial moment t=0, continuously turning on the first laser to emit the probe beam to pass through the atomic sensor probe to generate a signal; and receiving, by the first photoelectric converter and the second photoelectric converter, the signal;

(e) from the initial moment t=0, acquiring and analyzing, by the data acquisition system, the signal received by the first photoelectric converter and the second photoelectric converter, wherein an effective data acquiring duration of the signal is T0−t2;

(f) under a continuous mode or a gap mode, from the initial moment t=0, acquiring an ambient magnetic field data; and

(g) subjecting the ambient magnetic field data to spectral analysis and fitting to obtain a measurement sensitivity and a magnetic field change frequency.

In some embodiments, under the continuous mode, the ambient magnetic field data is periodically and continuously collected; a sampling frequency fsof an original data point is determined by the data acquisition system; in each pulse period which is the same as each on-off cycle T0, the signal is analyzed to obtain an ambient magnetic field data point Bncontaining a time stamp, such that a sampling frequency of ambient magnetic field data points B1, B2, B3. . . Bnis 1/T0.

In some embodiments, under the gap mode, the ambient magnetic field data is collected through steps of: performing periodic and continuous sampling N times followed by an interval without sampling; then performing periodic and continuous sampling N times again followed by an interval without sampling; and repeating such processes; a sampling frequency fsof an original data point is determined by the data acquisition system; in each pulse period T0, the signal is analyzed to obtain an ambient magnetic field data point Bncontaining a time stamp; every N ambient magnetic field data points are grouped into one group, and a sampling frequency of each group is 1/T0; an interval is provided between adjacent two groups; and data measurement is not performed during the interval.

The present disclosure has the following beneficial effects.

(1) The pump beam and the probe beam provided herein are capable of multiple optical path foldback by a combination of parallel plane reflectors and atomic sensor probe technology, effectively increasing an action distance and angle of Faraday rotation light, having a stable structure, improving a sensitivity of magnetic field measurement and easier to build and adjust.

(2) The continuous mode or gap mode is performed. Other interrupt operations can be performed during the measurement process, and a sampling can still be efficiently performed when a data volume is large, increasing the sampling frequency fBof the magnetic field measurement.

(3) The measurement method provided herein enables the high-sensitivity measurement of a weak magnetic field, and has an improved sampling frequency.

