Sheng Zou

In situ magnetic compensation for potassium spin-exchange relaxation-free magnetometer considering probe beam pumping effect

A novel method to compensate the residual magnetic field for an atomic magnetometer consisting of two perpendicular beams of polarizations was demonstrated in this paper. The method can realize magnetic compensation in the case where the pumping rate of the probe beam cannot be ignored. In the experiment, the probe beam is always linearly polarized, whereas, the probe beam contains a residual circular component due to the imperfection of the polarizer, which leads to the pumping effect of the probe beam. A simulation of the probe beams optical rotation and pumping rate was demonstrated. At the optimized points, the wavelength of the probe beam was optimized to achieve the largest optical rotation. Although, there is a small circular component in the linearly polarized probe beam, the pumping rate of the probe beam was non-negligible at the optimized wavelength which if ignored would lead to inaccuracies in the magnetic field compensation. Therefore, the dynamic equation of spin evolution was solved by considering the pumping effect of the probe beam. Based on the quasi-static solution, a novel magnetic compensation method was proposed, which contains two main steps: (1) the non-pumping compensation and (2) the sequence compensation with a very specific sequence. After these two main steps, a three-axis in situ magnetic compensation was achieved. The compensation method was suitable to design closed-loop spin-exchange relaxation-free magnetometer. By a combination of the magnetic compensation and the optimization, the magnetic field sensitivity was approximately 4 fT/Hz(1/2), which was mainly dominated by the noise of the magnetic shield.

Optimizations of spin-exchange relaxation-free magnetometer based on potassium and rubidium hybrid optical pumping

The hybrid optical pumping atomic magnetometers have not realized its theoretical sensitivity, the optimization is critical for optimal performance. The optimizations proposed in this paper are suitable for hybrid optical pumping atomic magnetometer, which contains two alkali species. To optimize the parameters, the dynamic equations of spin evolution with two alkali species were solved, whose steady-state solution is used to optimize the parameters. The demand of the power of the pump beam is large for hybrid optical pumping. Moreover, the sensitivity of the hybrid optical pumping magnetometer increases with the increase of the power density of the pump beam. The density ratio between the two alkali species is especially important for hybrid optical pumping magnetometer. A simple expression for optimizing the density ratio is proposed in this paper, which can help to determine the mole faction of the alkali atoms in fabricating the hybrid cell before the cell is sealed. The spin-exchange rate between the two alkali species is proportional to the saturated density of the alkali vapor, which is highly dependent on the temperature of the cell. Consequently, the sensitivity of the hybrid optical pumping magnetometer is dependent on the temperature of the cell. We proposed the thermal optimization of the hybrid cell for a hybrid optical pumping magnetometer, which can improve the sensitivity especially when the power of the pump beam is low. With these optimizations, a sensitivity of approximately 5 fT/Hz(1/2) is achieved with gradiometer arrangement.

Low frequency magnetic field suppression in an atomic spin co-magnetometer with a large electron magnetic field

In a K-Rb-21Ne co-magnetometer, the Rb electron magnetic field which is experienced by the nuclear spin is about 100 times larger than that of the K in a K-3He co-magnetometer. The large electron magnetic field which is neglected in the K-3He co-magnetometer coupled Bloch equations model is considered here in the K-Rb-21Ne co-magnetometer to study the low frequency magnetic field suppression effect. Theoretical analysis and experimental results shows that in the K-Rb-21Ne spin co-magnetometer, not only the nuclear spin but also the large electron spin magnetic field compensate the external magnetic field noise. By comparison, only the 3He nuclear spins mainly compensate the external magnetic field noise in a K-3He co-magnetometer. With this study, in addition to just increasing the magnetic field of the nuclear spins, we can suppress the magnetic field noise by increasing the density of the electron spin. We also studied how the magnetic field suppression effect relates to the scale factor of the K-Rb-21Ne co-magnetometer and we compared the scale factor with that of the K-3He co-magnetometer. Lastly, we show the sensitivity of our co-magnetometer. The magnetic field noise, the air density fluctuation noise and pumping power optimization are studied to improve the sensitivity of the co-magnetometer.

Spin exchange broadening of magnetic resonance lines in a high-sensitivity rotating K-Rb- 21 Ne co-magnetometer

Atomic co-magnetometers can be utilized for high-precision angular velocity sensing or fundamental physics tests. The sensitivity of a co-magnetometer determines the angle random walk of an angular velocity sensor and the detection limit for a fundamental physics test. A high-sensitivity K-Rb-21Ne co-magnetometer, which is utilized for angular velocity sensing, is presented in this paper. A new type of spin relaxation of Rb atom spins, which can broaden the zero-field magnetic resonance lines of the co-magnetometer, is discovered. Further studies show that the spin relaxation of Rb atoms is caused by a high Rb electron magnetization field. With this discovery, the total relaxation rate of Rb atoms is optimized to improve the sensitivity of the co-magnetometer. Moreover, its sensitivity is optimized by suppressing various noises. Especially, to suppress laser-related noises, the co-magnetometer is designed such that the sensitive axis of the co-magnetometer can be fixed to the direction in which the projection input of the earth’s rotation is 0. This is called a rotating co-magnetometer. A magnetic field sensitivity of 1.0 fT/Hz−1/2@5 Hz, which is equal to an angular velocity sensitivity of 2.1 × 10−8 rad s−1 Hz−1/2@5 Hz, is demonstrated using a spherical vapour cell with a diameter of 14 mm.

A Novel Calibration Method Research of the Scale Factor for the All-optical Atomic Spin Inertial Measurement Device

School of Instrument Science and Opto-Electronics Engineering, Beihang University, Beijing 100191, China(Received March 23, 2015 : revised June 9, 2015 : accepted July 6, 2015)A novel method to measure the scale factor for the all-optical atomic spin inertial measurement device (ASIMD) is demonstrated in this paper. The method can realize the calibration of the scale factor by a self-consistent method with small errors in the quiescent state. At first, the matured IMU (inertial measurement unit) device was fixed on an optical platform together with the ASIMD, and it has been used to calibrate the scale factor for the ASIMD. The results show that there were some errors causing the inaccuracy of the experiment. By the comparative analysis of theory and experiment, the ASIMD was unable to keep pace with the IMU. Considering the characteristics of the ASIMD, the mismatch between the driven frequency of the optical platform and the bandwidth of the ASIMD was the major reason. An all-optical atomic spin magnetometer was set up at first. The sensitivity of the magnetometer is ultra-high, and it can be used to detect the magnetization of spin-polarized noble gas. The gyromagnetic ratio of the noble gas is a physical constant, and it has already been measured accurately. So a novel calibration method for scale factor based on the gyromagnetic ratio has been presented. The relevant theoretical analysis and experiments have been implemented. The results showed that the scale factor of the device was 7.272 V/°/s by multi-group experiments with the maximum error value 0.49%.Keywords : Scale factor, All-optical, Atomic spin, Inertial measurement deviceOCIS codes : (230.1150) All-optical devices; (230.5160) Photo detectors; (280.4788) Optical sensing and sensors; (020.1335) Atom optics

On-site monitoring of atomic density number for an all-optical atomic magnetometer based on atomic spin exchange relaxation

We present a method for monitoring the atomic density number on site based on atomic spin exchange relaxation. When the spin polarization P ≪ 1, the atomic density numbers could be estimated by measuring magnetic resonance linewidth in an applied DC magnetic field by using an all-optical atomic magnetometer. The density measurement results showed that the experimental results the theoretical predictions had a good consistency in the investigated temperature range from 413 K to 463 K, while, the experimental results were approximately 1.5 ∼ 2 times less than the theoretical predictions estimated from the saturated vapor pressure curve. These deviations were mainly induced by the radiative heat transfer efficiency, which inevitably leaded to a lower temperature in cell than the setting temperature.

Dynamics of Rb and 21Ne spin ensembles interacting by spin exchange with a high Rb magnetic field

We report on the dynamics of spin-polarized Rb and 21Ne ensembles in which the 21Ne spin ensemble in a glass vapor cell experiences a high magnetic field produced by a Rb electron-spin ensemble. The coupled spin ensembles are modeled and the response of the transverse-step magnetic field excitation is solved and studied experimentally. Moreover, we analyze the frequency response of the ensembles to a transverse-oscillating magnetic field. We demonstrate the strong transverse damping and large frequency shift of the 21Ne spin ensemble as the precession frequencies of 21Ne spin and Rb spin match and the magnetic resonance spectroscopies of the two ensembles merge into one. We also demonstrate the operation of the spin ensembles as a self-compensating co-magnetometer that is insensitive to low-frequency magnetic fields that would be useful for rotation rate sensing. For such sensing applications, a large Rb density is achieved to polarize the 21Ne spins. This density leads to a high Rb electron spin magnetic field and we demonstrate its effect on the dynamics of the co-magnetometer.

Measurement Sensitivity Improvement of All-Optical Atomic Spin Magnetometer by Suppressing Noises

Quantum manipulation technology and photoelectric detection technology have jointly facilitated the rapid development of ultra-sensitive atomic spin magnetometers. To improve the output signal and sensitivity of the spin-exchange-relaxation-free (SERF) atomic spin magnetometer, the noises influencing on the output signal and the sensitivity were analyzed, and the corresponding noise suppression methods were presented. The magnetic field noises, including the residual magnetic field noise and the light shift noise, were reduced to approximately zero by employing the magnetic field compensation method and by adjusting the frequency of the pump beam, respectively. With respect to the operation temperature, the simulation results showed that the temperature of the potassium atomic spin magnetometer realizing the spin-exchange relaxation-free regime was 180 °C. Moreover, the fluctuation noises of the frequency and the power were suppressed by using the frequency and the power stable systems. The experimental power stability results showed that the light intensity stability was enhanced 10%. Contrast experiments on the sensitivity were carried out to demonstrate the validity of the suppression methods. Finally, a sensitivity of 13 fT/Hz1/2 was successfully achieved by suppressing noises and optimizing parameters.

Ultra-sensitive atomic magnetometer for studying magnetization fields produced by hyperpolarized helium-3

An ingenious approach to acquire the absolute magnetization fields produced by polarized atoms has been presented in this paper. The method was based on detection of spin precession signal of the hyperpolarized helium-3 with ultra-sensitive atomic magnetometer of potassium by referring to time-domain analysis. At first, dynamic responses of the mixed spin ensembles in the presence of variant external magnetic fields have been analyzed by referring to the Bloch equation. Subsequently, the relevant equipment was established to achieve the functions of hyperpolarizing helium-3 and detecting the precession of spin-polarized noble gas. By analyzing the transient response of the magnetometer in time domain, we obtained the relevant damping ratio and natural frequency. When the value of damping ratio reached the maximum value of 0.0917, the combined atomic magnetometer was in equilibrium. We draw a conclusion from the steady response: the magnetization fields of the polarized electrons and the hyperpolarized nuclei were corresponding 16.12 nT and 90.74 nT. Under this situation, the nuclear magnetization field could offset disturbing magnetic fields perpendicular to the orientation of the electronic polarization, and it preserved the electronic spin staying in a stable axis. Therefore, the combined magnetometer was particularly attractive for inertial measurements.

Ingenious Method for Measuring the Non-Orthogonal Angle of the Saddle-Shaped Coils of an SERF Atomic Magnetometer System

The spin exchange relaxation free (SERF) atomic magnetometer has been a research hotspot due to its ultra-high sensitivity. The SERF atomic magnetometers sensitivity is subject to the accuracy of its compensating magnetic field and the calibration signal. These two processes need a tri-axial coil system to generate the required magnetic fields, which are directly affected by the non-orthogonality of the tri-axial coil system, especially the non-orthogonality between the x- and y-axes coils. In order to solve this problem, an ingenious and effective method for measuring the non-orthogonal angle between the x- and y-axes in a magnetic coil system has been demonstrated in this paper for the first time. This measurement method was only based on the dynamics of the SERF atomic magnetometer and did not need any additional calibration equipment. A large dc magnetic field By was applied to the y-axis, such that the magnetic field Byx projected from By on to the x-axis could be measured. By then solving the relevant trigonometric function, we could obtain the non-orthogonal angle. Using this method, we experimentally obtained the non-orthogonal angle of about 84.08° between the x- and y-axes saddle-shaped coils, with a variance of about 0.14°. Finally, the correctness of this method for calculating the non-orthogonal angle was analyzed.