Spin-exchange Relaxation-free Magnetometer Research Articles

Enhancing the sensitivity of spin-exchange relaxation-free magnetometers using phase-modulated pump light with external Gaussian noise

An optical pumping scheme is proposed for reducing the gradient of electron spin polarization and suppressing light source noise in a spin-exchange relaxation-free magnetometer. This is achieved by modulating only the phase of a narrow-linewidth pump light field with external Gaussian noise. Compared to the absence of phase modulation, the uniformity of electron spin polarization was improved by over 40%, and the light-frequency noise suppression ratio of the magnetometer was enhanced by 4.3 times. Additionally, the response of the magnetometer was increased by 54%, resulting in a sensitivity of 0.34 fT/Hz1/2 at 30 Hz. The applicability of this scheme can extend to other optical pumping experiments involving large atom ensembles requiring uniform electron spin polarization distribution, which is beneficial for developing ultra-high sensitivity and high stability magnetometers essential for magneto-cardiography and magneto-encephalography research applications.

Metasurface-integrated elliptically polarized laser-pumped SERF magnetometers

The emergence of biomagnetism imaging has led to the development of ultrasensitive and compact spin-exchange relaxation-free (SERF) atomic magnetometers that promise high-resolution magnetocardiography (MCG) and magnetoencephalography (MEG). However, conventional optical components are not compatible with nanofabrication processes that enable the integration of atomic magnetometers on chips, especially for elliptically polarized laser-pumped SERF magnetometers with bulky optical systems. In this study, an elliptical-polarization pumping beam (at 795 nm) is achieved through a single-piece metasurface, which results in an SERF magnetometer with a high sensitivity reaching 10.61 fT/Hz1/2 by utilizing a 87Rb vapor cell with a 3 mm inner diameter. To achieve the optimum theoretical polarization, our design combines a computer-assisted optimization algorithm with an emerging metasurface design process. The metasurface is fabricated with 550 nm thick silicon-rich silicon nitride on a 2 × 2 cm2 SiO2 substrate and features a 22.17° ellipticity angle (a deviation from the target polarization of less than 2%) and more than 80% transmittance. This study provides a feasible approach for on-chip polarization control of future all-integrated atomic magnetometers, which will further pave the way for high-resolution biomagnetism imaging and portable atomic sensing applications.

A polarization-improved dual-beam spin-exchange relaxation-free magnetometer with reflection-assisted pumping

The highly sensitive SERF magnetometers hold exciting prospects for magnetocardiography and magnetoencephalography, and the accuracy of medical measurements depends largely on the performance of the magnetometers. A double-pass configuration is used to obtain uniform polarization and constructed by adding a mirror to retro-reflect the pump light. The transmission model of the reflected light which takes the effect of forward light into account is proposed, and the improved polarization is experimentally verified. The essential operation parameters of the magnetometer are optimized. The theoretical framework is established outlining the relationship between pump power, probe frequency, and signal response, and the optimal values for these parameters are experimentally determined. With the appropriate selection of probe power and operating temperature, the double-pass configuration achieves a sensitivity of 5.81 fT/Hz1/2, a bandwidth of 110 Hz, and represents a 17% improvement in stability compared to the single-pass configuration. The optimized magnetometer exhibits reduced susceptibility to laser conditions and enhanced stability, attributed to the reflection-assisted polarization improvement and better uniformity. This configuration efficiently utilizes the pumping laser, and its miniaturization offers significant benefits for magnetoencephalography and other biomagnetic measurements.

Magnetic noise modeling and calculation of Fe-based nanocrystalline in the SERF magnetometer considering temperature effects

The Fe-based nanocrystalline has been widely used in the innermost magnetic shielding of the spin-exchange relaxation-free (SERF) magnetometer due to its high permeability and low magnetic noise, which are significant factors limiting the sensitivity and accuracy of the measurements. However, the operating temperature of the inner shielding layer is around 80°C, and temperature has a significant impact on the magnetic noise of Fe-based nanocrystalline. To precisely analyze the magnetic noise of the Fe-based nanocrystalline in the SERF magnetometer, we first numerically simulate the 1K107 Fe-based nanocrystalline magnetic shielding barrel using the finite element method (FEM). Then, a variable-temperature dynamic Jiles-Atherton (VTDJA) model is proposed. The losses of 1K107 under various temperature are separated. The magnetic noise of 1K107 is computed using the Fluctuation-Dissipation theory. Finally, the magnetic noises of 1K107 at 25℃ and 80℃ are measured by setting up an experimental test system. Results show that the simulated deviation of the magnetic noise at 80°C and 25°C is 32 %, and the actual measured deviation is 48 %. The errors between simulated and measured magnetic noises at 25°C and 80°C are 5.29 % and 6.54 %, respectively. This study provides a novel method for the performance optimization of the SERF magnetometer.

Response optimization of a three-axis sensitive SERF magnetometer for closed-loop operation

Most triaxial-vectorial magnetic field measurements with spin-exchange relaxation free (SERF) atomic magnetometer (AM) are based on the quasi-steady-state solution of the Bloch equation. However, the responding speed of these methods is greatly limited because the frequency of the modulation signal should be slow enough to ensure the validity of the quasi-steady-state solution. In this work, a new model to describe the response of the three-axis sensitive SERF AM with high modulation frequency is presented and verified. The response of alkali-atomic spin to high-frequency modulation field is further investigated by solving the Bloch equation in a modulation-frequency-dependence manner. This solution is well verified by our experiments and can offer a reference for selection of modulation frequencies. The result shows a potential to achieve a SERF AM operating in a geomagnetic field without heavy aluminum shielding when the modulation frequencies are selected properly.

Analysis and suppression of the three-axis crosstalk effect in the modulated SERF atomic magnetometer

In this paper, the effects of modulation fields frequency configuration, modulation fields phase configuration and three-axis direct-current (DC) magnetic fields on the three-axis crosstalk are analyzed in the modulated spin-exchange relaxation-free (SERF) atomic magnetometer. Based on three-axis modulation fields configuration, a method is proposed to suppress the three-axis crosstalk, and the results show that the three-axis crosstalk is lower than 2%. This paper also demonstrates an in-situ magnetic compensation method, and the magnetic compensation accuracy of this method is on the order of pT. Additionally, the three-axis magnetic field closed-loop mode is showed to suppress the effect of the DC magnetic field on the three-axis crosstalk. The three-axis crosstalk is finally reduced by nearly two times compared with that of before crosstalk suppression, and the three-axis sensitivities are better than 30 fT/Hz1/2. The reduction of crosstalk is beneficial to realize the high-precision measurement of the three-axis SERF magnetometer.

Measurement and analysis of polarization gradient relaxation in the atomic comagnetometer

In the spin-exchange relaxation-free (SERF) comagnetometer, the transverse nuclear spin relaxation associated with the electron spin polarization gradient restricts the low-frequency magnetic-field suppression ability and self-compensation performance. A three-dimensional model and a measurement method of polarization gradient relaxation rate (PGRR) in the SERF comagnetometer are proposed in this paper. A plateau of PGRR is discovered and well explained by the model. The proposed measurement method confirms that the PGRR accounts for 90%∼98% of the transverse nuclear spin relaxation in the SERF comagnetometer. In addition, the model guides the selection of system parameters to manipulate the polarization distribution, thereby the PGRR is reduced by 14.88 times from 0.02166 1/s to 0.00145 1/s. This research supports improving the long-term stability of the SERF comagnetometer, and the proposed model and method can be extended to other atomic sensors, such as the SERF magnetometer and nuclear magnetic resonance gyroscope.

Nonzero-Order Resonances in Single-Beam Spin-Exchange Relaxation-Free Magnetometers

Zero-field optically pumped magnetometers operating in the spin-exchange relaxation-free (SERF) regime have been extensively studied, and usually depend on zeroth-order parametric resonance to measure the magnetic field. However, the studies conducted on this topic lack thorough analyses and in-depth discussion of nonzero-order magnetic resonances in single-beam SERF magnetometers. In this paper, we analyzed the nonzero-order resonance, especially the first-order resonance, based on a single-beam SERF magnetometer, and discussed its various applications. A comprehensive theoretical analysis and experiments were conducted with respect to multiple functions, including nonzero finite magnetic field measurements, spin polarization measurement, and in situ coil constant calibration. The results showed that first-order resonance can be utilized for nonzerofinite magnetic field measurements, and the spin polarization of alkali-metal atoms can be determined by measuring the slowing-down factor using the resonance condition. Furthermore, acquiring the first-order resonance point at an equivalent zero pump light power through fitting offers an approach for quick and precise in situ coil constant calibration. This study contributes to the applications of SERF magnetometers in nonzero finite magnetic fields.

Parameter optimisation of miniaturised SERF magnetometer below relaxation rate saturation region

The miniaturised spin-exchange relaxation-free (SERF) magnetometer has promising applications in biomagnetic measurements owing to its ultrasensitive magnetic field measurement capacity. In this paper, we propose a single-beam parameter-optimised SERF magnetometer that operates below the relaxation rate saturation region. Based on the relaxation rate model under zero-field resonance, the relationship between the magnetometer optimal working point and multiphysical parameters, including the optical power density, cell temperature, and nitrogen density, was studied. The experimental results verified the accuracy of the law deduced from the model. By adjusting the parameters according to our experimental results, a sensitivity of 17 fT/Hz1/2 (@31.5Hz) and bandwidth of 110 Hz were obtained optimally in a 5.5 cm3 magnetometer. The superiority of the optimised parameters in the miniaturised SERF magnetometer is verified. We found that the relaxation rate saturation region results from light absorption saturation, which leads to a performance decline of the SERF magnetometer. This relaxation rate saturation phenomenon widely exists in miniaturised SERF magnetometers, and the study of this phenomenon is helpful for further applications in arrayed integration of biomagnetic measurement.

Structure optimization of non-magnetic electric heating film for spin exchange relaxation free magnetometer

Spin exchange relaxation free (SERF) atomic magnetometer is one of the most sensitive magnetometers. High temperature and a non-magnetic environment are two key conditions to achieve ultra-high sensitivity. In this paper, a non-magnetic heating film with magnetic field self-suppression is optimized by the genetic algorithm method. Four structural parameters of the non-magnetic heating film including the wire spacing, wire width, wire thickness, and layer distance are optimized to minimize the magnetic field in the heated space. The simulation result based on the finite element analysis method shows that the magnetic field and the temperature field produced by a pair of heating films at the vapour cell are uniform. Finally, the magnetic field experiment proved that when the current is 10 ​mA, the magnetic field is 3.0520 ​nT. The temperature control experiment indicates that the temperature could be stabilized at 180 ​± ​0.2 ​°C. This study is significant for electric heating with a lower magnetic field and contributes to further improving the performance of atomic sensors.

Simultaneous in-situ compensation method of residual magnetic fields for the dual-beam SERF atomic magnetometer

Near-zero magnetic field condition is essential to keep the spin-exchange relaxation-free (SERF) regime for the atomic magnetometer. We analyzed the output response variations of the dual-beam SERF magnetometer under different DC residual magnetic fields. Considering the problem that the gradient coil may introduce an additional DC magnetic field when compensating the gradient field, we propose a novel method of simultaneous in-situ compensation of gradient and DC residual magnetic field with high precision. The sensitivity and effectiveness of the compensation method for residual magnetic fields under 1 nT were verified by simulation and experiment. The precision of the gradient residual magnetic field compensation is better than 2 pT/mm. We built a dual-beam SERF atomic magnetometer platform to validate the proposed method. The magnetic field measurement sensitivity is 7 fT/Hz1/2 after residual magnetic fields compensation by the method mentioned in this paper, which is 4 times higher than the sensitivity of 36 fT/Hz1/2 without residual magnetic fields compensation.

Optimal buffer gas pressure in dual-beam spin-exchange relaxation-free magnetometers

Optically pumped magnetometers (OPMs) operated in the spin-exchange relaxation-free (SERF) regime have emerged as prevailing biomagnetism measurement devices. The buffer gas in vapor cells can suppress wall collision relaxation but introduce unexpecting destruction relaxation, so as to be crucial to determine the magnetometer performance. In this study, we demonstrated the optimal buffer gas pressure of a dual-beam high sensitivity SERF magnetometer theoretically and experimentally. An optimization model was proposed revealing that the maximum response signal was dependent on the combined effects of the spin-relaxation rate and the pressure broadened linewidth of optical absorption. Six 3 mm × 3 mm × 3 mm Rb-N 2 vapor cells with different gas pressures were fabricated and constructed in a dual-beam SERF magnetometer to verify the optimal gas pressure model. The experimental results coincided well with the theoretical analysis, and a sensitivity of 4 fT/Hz 1/2 was achieved. • An optimal gas pressure exists to optimize the dual-beam SERF magnetometer performance. • The optimal pressures of the buffer and quench gases by establishing a calculation model for the Rb-N 2 magnetometer. • The comprehensive effects of nitrogen pressure on spin relaxation and pressure broadening are experimentally verified.

Analysis and Suppression of the Cross-Axis Coupling Effect for Dual-Beam SERF Atomic Magnetometer

Spin-exchange relaxation-free (SERF) atomic magnetometers operated under a near-zero magnetic field are used for vector magnetic field measurements with high sensitivity. Previously, the cross-axis coupling error evoked by a nonzero background magnetic field has been verified to be adverse in modulated single-beam magnetometers. Here, in a dual-beam unmodulated SERF magnetometer, we propose a somewhat different solution model for the cross-axis coupling effect where the field of interest couples with the interference field. Considering two cases where the transverse or longitudinal background field exists, the cross-axis coupling effect dependence on multiple factors is investigated here based on the dynamic response under a background magnetic field within ±5 nT. The theoretical and experimental investigation suggests that it has an adverse impact on the output response amplitude and phase and tilts the sensitive axis by several degrees, causing a measurement error on the dual-beam magnetometer. To suppress this effect, the background magnetic field is compensated through the PI closed-loop control. The coupling effect is effectively suppressed by 1.5 times at the 10–40 Hz low-frequency band and the sensitivity reaches 2.4 fT/Hz1/2.

Single-beam integrated hybrid optical pumping spin exchange relaxation free magnetometer for biomedical applications

An ingenious approach to accomplish the high signal strengthen and relatively homogeneous spin polarization has been presented in a hybrid optical pumping spin-exchange-relaxation-free atomic magnetometer only utilizing single-beam configuration. We have experimentally demonstrated an approximately three-fold enhancement of the output signal at the optimal spin polarization by optically pumping the thin vapor due to the same spin evolution behavior of the two different kinds of vapor atoms. Eventually, a measuring sensitivity of 30 fT/Hz1/2 was achieved combined with the homemade differential detection system for attenuating large background offset and suppressing optical power noise. This scheme provides a prospect for the development of ultra-highly sensitive and chip-scale atomic magnetometer for the applications that desire both high signal-to-noise ratio and uniform spin polarization, such as magnetocardiography and magnetoencephalography.

Scanning a multi-channel spin-exchange relaxation-free atomic magnetometer with high spatial and time resolution.

The emerging multi-channel spin-exchange relaxation-free (SERF) atomic magnetometer is a promising candidate for non-intrusive biomagnetism imaging. In this study, we propose a scanning 9-channel SERF magnetometer based on an acousto-optic modulator (AOM). Using the diffraction light of the AOM as the probe laser (with a low laser power of 1.7 mW), 9 channels were rapidly scanned by altering the diffraction angle. The scanning imaging scheme provides a new, to the best of our knowledge, approach for multi-channel magnetic field measurement and realizes a single-channel sensitivity of about 3 fT/Hz1/2, a spatial resolution of 0.6 mm, and a time resolution of about 2.7 ms, which is well suited for real-time extremely weak magnetic field imaging.

A compact and closed-loop spin-exchange relaxation-free atomic magnetometer for wearable magnetoencephalography

Atomic magnetometers operated in the spin-exchange relaxation-free (SERF) regime are the promising sensor to replace superconducting quantum interference devices (SQUIDs) in the biomagnetism field. The SERF magnetometer with compact size and good performance is crucial to the new generation of wearable magnetoencephalography (MEG) system. In this paper, we developed a compact and closed-loop SERF magnetometer with the dimensions of 15.0×22.0×30.0 mm3 based on a single-beam configuration. The bandwidth of the magnetometer was extended to 675 Hz while the sensitivity was maintained at 22 fT/Hz1/2. A nearly 3-fold enhancement of the bandwidth was obtained in comparison with the open-loop control. The implementation of the closed-loop control also greatly improved the dynamic range, enabling the magnetometer to be robust against the disturbance of the ambient field. Moreover, the magnetometer was successfully applied for the detection of human α-rhythm and auditory evoked fields (AEFs), which demonstrated the potential to be extended to multi-channel MEG measurements for future neuroscience studies.

Fast extraction of the electron spin-relaxation rate in the SERF magnetometer from a transient response.

The magnitude of the electron spin-relaxation rate Rrel of the atomic ensemble directly affects the sensitivity of the spin-exchange relaxation-free (SERF) atomic magnetometer (AM). The rapid and in-situ characterization of Rrel is of great importance. In this work, a fast extraction method of Rrel is proposed with a measurement period shorten to 0.5 s, merely detecting the transient response of SERF AM to a transverse DC excitation magnetic field after switching off the pump beam. In contrast to the conventional methods based on the measurement of the magnetic resonance linewidth, this method circumvents the involvement of optical pumping rate, and enables monitoring Rrel under arbitrary polarization, which is expected to improve the authenticity of Rrel measurement in a more convenient way.

Single beam Cs-Ne SERF atomic magnetometer with the laser power differential method.

We describe a single beam compact spin exchange relaxation free (SERF) magnetometer whose configuration is simple and compatible with the silicon-glass bonding micro-machining method. Due to the small size of the vapor cell utilized in a miniature atomic magnetometer, the wall relaxation could not be neglected. In this study we show that Ne buffer gas is more efficient than that of the other typically utilized gas species such as nitrogen and helium for wall relaxation reduction theoretically and experimentally. 3 Amagats (1 Amagat=2.69×1019/cm3) Ne gas is filled in the vapor cell and this is the first demonstration of a Cs-Ne SERF magnetometer. In order to reduce the laser amplitude noise and the large background detection offset, which is reported to be the main noise source of a single beam absorption SERF magnetometer, we developed a laser power differential method and a factor of approximately two improvement of the power noise suppression has been demonstrated. In order to reduce the power consumption of the magnetometer, the Cs based atomic magnetometer is studied. We did an optimization of the magnetometer and a sensitivity of 23fT/Hz1/2@100Hz has been achieved. This is the first demonstration of a single beam Cs based SERF magnetometer.

Transverse light-shift in a spin-exchange relaxation-free co-magnetometer: measurement, decoupling, and suppression.

The transverse light-shift can induce non-negligible polarization error in the output signal of spin-exchange relaxation-free (SERF) co-magnetometer. In this paper, a novel method for rapid measurement of transverse light-shift based on the error of steady-state response of co-magnetometer is proposed firstly, then the sources of transverse light-shift in a compact SERF co-magnetometer is modeled and analyzed from three aspects: the non-ideal linear polarization of probe laser, the circular dichroism of the atomic spin ensembles, and the stress-induced birefringence effect of the cell wall. Furthermore, the decoupling and suppression methods of transverse light-shift based on a degree of circular polarization (DOCP) regulation scheme is presented, to realize the decoupling measurement of the transverse light-shift introduced by the whole co-magnetometer cell, and cancel it out with the non-ideal linear polarization of the probe laser. Eventually, the DOCP regulation scheme suggested in this paper achieves more than a 67% suppression ratio in transverse light-shift, and the short- and long-term performance of SERF co-magnetometer are improved due to the reduction of the coupling effect between the probe laser power and transverse field. Moreover, the measurement, decoupling and suppression methods provided in this paper also have the potential to be applied to other atomic sensors, such as the SERF magnetometers and nuclear spin co-magnetometers.

An Integrated High-Sensitivity VCSEL-Based Spin-Exchange Relaxation-Free Magnetometer With Optical Rotation Detection

In the integrated design of spin-exchange relaxation-free (SERF) magnetometers, vertical cavity surface emitting laser (VCSEL) is a promising candidate as a light source. However, VCSEL often has high frequency and amplitude noise, thereby hindering the development of high-sensitivity SERF magnetometers. In the previous VCSEL-based SERF magnetometer with optical absorption detection, a laser control loop in magnetometer electronic system is used to address this issue, but it is a challenging task to design the loop due to the strong resonance absorption. In this paper, we propose a different approach by using optical rotation detection instead of the absorption detection to develop an integrated high-sensitivity VCSEL-based SERF magnetometer. It can be implemented easily, just by switching the circularly polarized light to the elliptically polarized light in magnetometer optical system. The elliptically polarized light used in here is not only to implement a single-beam configuration as in the previous fiber-coupled SERF magnetometer with a stable distributed feedback laser, but more importantly to accomplish the optical rotation detection and hence to strongly suppress the VCSEL noise. Thus the challenging electronic control problem can be avoided, and meanwhile a simplified optical arrangement is maintained. The difficulty of the integration of VCSEL-based SERF magnetometer can then be significantly reduced. Eventually, an integrated VCSEL-based SERF magnetometer is implemented and achieves a sensitivity of approximately 35 fT/Hz^1/2. Using the integrated magnetometer, magnetocardiography signals are recorded.