This paper presents a single-beam atomic magnetic field measurement gradiometer, which is a highly sensitive magnetic field gradient measuring instrument based on the atom spin-exchange relaxation-free (SERF) effect. The reflective detection optical path structure is adopted. The spin precession signals of an atom under incident and reflected light are different. There is also a difference in the corresponding magnetic field distribution. The final measurement of magnetic field gradients is conducted based on the different magnetic field distributions. The single-beam high-sensitive magnetic field gradiometers based on the atomic SERF effect are more sensitive than conventional two-probe magnetic field gradiometers or two-beam magnetic field gradiometers. The gradiometers are not affected by a difference in the detected optical power in the single-beam detection light measurements. The reflector uses an angular cone prism for two-dimensional magnetic field gradient measurements and is simple to construct. The single-beam highly sensitive magnetic field gradient measurement instrument based on the atomic SERF effect has a reflective detection optical path structure. It uses a quarter-wave plate to achieve the initial signal phase elimination of both incident and reflected signal and an angular cone prism as a reflector to achieve two-dimensional measurement.
This paper presents a single-beam atomic magnetic field measurement gradiometer, which is a highly sensitive magnetic field gradient measuring instrument based on the atom spin-exchange relaxation-free (SERF) effect. The reflective detection optical path structure is adopted. The spin precession signals of an atom under incident and reflected light are different. There is also a difference in the corresponding magnetic field distribution. The final measurement of magnetic field gradients is conducted based on the different magnetic field distributions. The single-beam high-sensitive magnetic field gradiometers based on the atomic SERF effect are more sensitive than conventional two-probe magnetic field gradiometers or two-beam magnetic field gradiometers. The gradiometers are not affected by a difference in the detected optical power in the single-beam detection light measurements. The reflector uses an angular cone prism for two-dimensional magnetic field gradient measurements and is simple to construct. The single-beam highly sensitive magnetic field gradient measurement instrument based on the atomic SERF effect has a reflective detection optical path structure. It uses a quarter-wave plate to achieve the initial signal phase elimination of both incident and reflected signal and an angular cone prism as a reflector to achieve two-dimensional measurement.
The single-beam comagnetometer working in the spin-exchange relaxation-free (SERF) state is being developed into a miniaturized atomic sensor with extremely high precision in rotation measurement. In this paper, we propose a reflective configuration for the single-beam SERF comagnetometer. The laser light simultaneously used for optical pumping and signal extraction is designed to pass through the atomic ensemble twice. In the optical system, we propose a structure composed of a polarizing beam splitter and a quarter-wave plate. With this, the reflected light beam can be separated entirely from the forward propagating one and realize a complete light collection with a photodiode, making the least light power loss. In our reflective scheme, the length of interaction between light and atoms is extended, and because the power of the DC light component is attenuated, the photodiode can work in a more sensitive range and has a better photoelectric conversion coefficient. Compared with the single-pass scheme, our reflective configuration has a stronger output signal and performs better signal-to-noise ratio and rotation sensitivity. Our work has an important impact on developing miniaturized atomic sensors for rotation measurement in the future.
A new design of compact zero-field atomic gradiometer was proposed and integrated using alkali vapor cell and customized optical components. This gradiometer used two parallel elliptically polarized lights, whose output intensity was measured to perform gradient magnetic field detection, respectively. To improve the gradiometer’s sensitivity of the magnetic field detection, the gradiometer was operated in the spin-exchange relaxation-free (SERF) regime. For the sensitivity study, the gradiometer was placed in a person-sized four-layer μ-metal magnetic shielding. With the magnetic shield closed up, the sensitivity of each channel was near 34 fT/ √ Hz, and the corresponding gradient sensitivity could reach 14 fT/√ Hz/cm on a 1cm baseline. When the cover of magnetic shielding was removed, the magnetometer sensitivity of single channel was about 90 fT/√ Hz, and the corresponding gradient sensitivity could reach 34 fT/√ Hz/cm. The experimental results implied that in a poor magnetic shielding environment, the performance of magnetometer was limited due to the fluctuations of the environmental magnetic field, while the gradiometer could work well.
The ultra-sensitivity magnetometer plays an important role in fundamental physics and applications, including magnetoencephalography (MEG) and magnetocardiography (MCG). The spin-exchange relaxation-free (SERF) atomic magnetometer (AM) has great sensitivity but is limited by the relatively complicated dual-beam structure for application, leading to the development of the compact AM based on one single laser beam. Compared with the single-beam optically pumped magnetometer (OPM) detecting the absorption of the circularly polarized beam, the SERF AM based on an elliptically polarized single beam detecting the optical rotation angle of polarization has a better response, lower laser power, and no need for signal modulation or demodulation. Here, we would like to report an elliptically-polarized single-beam SERF AM enhanced by reflecting the probe beam, increasing the light-atom interaction. Moreover, unlike the previous reflecting AM using the beam splitter, we used a D-shape mirror to extract the reflected beam, avoiding power losses of both incident and reflected beam and destruction of polarization. The sensitivity of our device in the reflecting scheme reaches 5 fT/Hz1/2 at 9–60 Hz, satisfying the need to measure the human MEG/MCG or search for the fifth force. Our research paves the way for developing the compact SERF AM for multiple future applications.
Magnetic field gradients interfere with the coherence of the atomic ensemble and degrade the performance of the spin-exchange relaxation-free (SERF) atomic magnetometer. In this paper, the influence of magnetic field gradients is incorporated into the response model of single-beam atomic magnetometer, and the theoretical models of transverse relaxation rate, spin polarization and scale factor are established. Based on this model,we find that magnetic field gradients in different directions can cause varying degrees of degradation in the performance parameters of the magnetometer. The magnetic linewidths under different light power and magnetic field gradients are measured using the designed dedicated magnetic field gradients coils, and the spin polarization and magnetic field gradients relaxation rate are obtained through fitting. An unevenness and deviation of the curve are also observed in the experiments, indicating the presence of the magnetic field gradient of approximately 2 nT/cm in the magnetic shields. Furthermore, in a magnetic field gradient environment of 20 nT/cm, the mean of the long-term sensitivity decreased by 3.5 times and the standard deviation increased by 11 times. It shows that the magnetic field gradients will not only affect the signal-to-noise ratio, but also make the magnetometer more vulnerable to external interference when working. The sensitivity and stability of magnetometer will be greatly reduced. The research in this article provides a theoretical and experimental basis for eliminating the influence of magnetic field gradients and improving the accuracy of magnetic field measurements.
Spin-exchange relaxation-free (SERF) atomic magnetometers usually adopt Faraday modulation to achieve better performance but suffer from laser intensity noise, thermal noise generated by the Faraday modulator, and nonmagnetic technical noise. These noises limit the sensitivity of the SERF magnetometer. We demonstrate a SERF magnetic gradiometer with dual-beam and closed-loop Faraday modulation. Operating in the SERF regime, the gradiometer utilizes an additional Faraday modulator rotation feedback to suppress probe laser intensity noise and thermal noise associated with the Faraday modulator, and it simultaneously uses dual-beam difference to cancel common-mode nonmagnetic technical noises. A gradient sensitivity of 14 fT/Hz1/2 per 1 cm gradiometer base length was achieved using Cs atoms.
Ultra-sensitive multi-channel optically pumped atomic magnetometers based on the spin-exchange relaxation-free (SERF) effect are powerful tools for applications in the field of magnetic imaging. To simultaneously achieve ultra-high spatial resolution and ultra-high magnetic field sensitivity, we proposed a high-resolution multi-channel SERF atomic magnetometer for two-dimensional magnetic field measurements based on a digital micro-mirror device (DMD) as the spatial light modulator for a single vapor cell. Under the optimal experimental conditions obtained via spatial and temporal modulation of the probe light, we first demonstrated that the average sensitivity of the proposed 25-channel magnetometer was approximately 25fT/Hz 1/2 with a spatial resolution of 216µm. Then, we measured the magnetic field distribution generated by a gradient coil and compared the experimentally obtained distributions with those calculated via finite element simulation. The obtained g value of 99.2% indicated good agreement between our experimental results and the theoretical calculations, thereby confirming that our proposed multi-channel SERF magnetometer was effective at measuring magnetic field distributions with an ultra-high spatial resolution.