Abstract The effect of pumping laser intensity uniformity (PLIU) on the operation of a hybrid optically pumped comagnetometer operating in the spin‐exchange relaxation‐free regime (SERF) regime is investigated in this paper. First, an analytical steady‐state output model for the comagnetometer with two alkali‐metal atoms and one noble‐gas atom is presented. By varying the diameter of the pumping laser beam to control the PLIU, 3D distribution models of the electron spin and nuclear spin polarization under different PLIU conditions are obtained. In the experiment, the effects of PLIU on multi‐parameter, low‐frequency magnetic‐noise suppression capability, and long‐term stability of the SERF comagnetometer are studied. The results indicate that within a certain range, increasing the diameter of the pumping laser beam improves the polarization uniformity of the atomic ensemble and reduces the light shift of the comagnetometer. As a result, both the low‐frequency magnetic noise suppression capability and the long‐term stability of the system increase. However, further reduction of the pumping laser diameter leads to a reversal of the system performance metrics, suggesting the presence of a tipping point. The research presented in this article is critical for advancing the efficient polarization study and hyperpolarization of SERF comagnetometers.
Abstract The effect of pumping laser intensity uniformity (PLIU) on the operation of a hybrid optically pumped comagnetometer operating in the spin‐exchange relaxation‐free regime (SERF) regime is investigated in this paper. First, an analytical steady‐state output model for the comagnetometer with two alkali‐metal atoms and one noble‐gas atom is presented. By varying the diameter of the pumping laser beam to control the PLIU, 3D distribution models of the electron spin and nuclear spin polarization under different PLIU conditions are obtained. In the experiment, the effects of PLIU on multi‐parameter, low‐frequency magnetic‐noise suppression capability, and long‐term stability of the SERF comagnetometer are studied. The results indicate that within a certain range, increasing the diameter of the pumping laser beam improves the polarization uniformity of the atomic ensemble and reduces the light shift of the comagnetometer. As a result, both the low‐frequency magnetic noise suppression capability and the long‐term stability of the system increase. However, further reduction of the pumping laser diameter leads to a reversal of the system performance metrics, suggesting the presence of a tipping point. The research presented in this article is critical for advancing the efficient polarization study and hyperpolarization of SERF comagnetometers.
We investigate the effects of pump laser power density on the hybrid optically pumped comagnetometer operated in the spin-exchange relaxation-free (SERF) regime. The analytic steady-state output model for the comagnetometer considering two alkali metal species and one nuclear species is presented for the first time. And the effects of pump laser power density on the rotation sensitivity, suppression of low-frequency magnetic noise and long-term stability of the comagnetometer are studied experimentally. The results indicate that when the product of pumping rate and density ratio of pumped atom to probed atom is equal to the spin relaxation rate of the probed atom, the maximum response and highest sensitivity of the comagnetometer are achieved. However, the suppression of low-frequency magnetic noise and long-term stability improve with the increasing of pump laser power density due to the increasing of nuclear spin polarization. Our focus is to optimize the performance of the comagnetometer for rotation sensing, but the theory and method presented here are relevant to all applications of the hybrid optical pumping technique.
Abstract Atomic spin comagnetometers based on the spin-exchange relaxation-free (SERF) effect offer significant potential for inertial sensing and support the development of long-endurance, high-precision navigation technologies. However, limited nuclear spin polarization constrains the long-term stability of these systems. Here, a flat-top optical pumping configuration is introduced for SERF comagnetometers, expanding the uniform-intensity region of the beam to approximately 62 % , which enhances atomic polarization by around 19 % and improves magnetic noise suppression by 32 % . Experimental results demonstrate notable improvements in both sensitivity and long-term stability, highlighting the effectiveness of flat-top beams in overcoming atomic polarization uniformity limitations and advancing SERF co-magnetometer performance.
Ultrasensitive spin exchange relaxation-free (SERF) comagnetometers have been widely applied in various new physics experiments and show great potential for inertial navigation. However, the spatially nonuniform spin polarization of alkali metal (AM) and noble gas (NG) atoms results in an effective magnetic field gradient on the order of 100 nT/cm which is much larger than that is produced by the coils and remanence on the magnitude of 1 nT/cm, thereby reducing the suppression ability of the magnetic noise and limiting the scale factor and noise floor of the comagnetometers. In this study, we establish a model of the effective magnetic field gradient caused by uneven spin polarization and analyze the influencing factors. Our results indicate that optimizing the density ratio of hybrid AM atoms can suppress the effective magnetic field gradient. Nevertheless, the available pump light power imposes a limitation on the density ratio. To further address this issue, we propose a balanced light-assisted hybrid spin exchange optical pumping (HSEOP) method. Our experimental results confirm the effectiveness of the mentioned approach, with 67% suppression of the nuclear transverse relaxation rate and a nearly threefold increase in the scale factor. Furthermore, the optimal density ratio in experimental applications can be reduced from 200 to below 80, thereby overcoming the limitation of laser power in SERF comagnetometers and providing the possibility for wider applications. Finally, the suppression factor of magnetic noise is enhanced by 89% by optimizing the density ratio and the balanced light-assisted HSEOP method. This research sheds light on the fundamental physics experiments and potential for inertial navigation.
Spin exchange relaxation free (SERF) comagnetometers, which use coupled alkali metal and noble gas spins, are promising for precision measurements due to their potential for miniaturization and high sensitivity. In these systems, alkali atoms are optically pumped with circularly polarized light, and their polarization is transferred to noble gas nuclei via spin-exchange collisions. However, spatial inhomogeneities in alkali polarization induced by factors such as optical absorption, gas diffusion, and temperature gradients generate effective magnetic field gradients for the noble gas spins. These gradients accelerate spin relaxation and degrade overall sensor performance. This work introduces a pump beam reflection compensation scheme to improve alkali polarization uniformity in a SERF comagnetometer. By retro reflecting the pump beam back along its original optical path with reversed helicity, alkali atoms are repolarized from the opposite direction. This counter propagating beam compensates polarization gradients through reverse optical pumping, thereby homogenizing the effective field experienced by the noble gas spins. The modified pumping configuration enhances spatial uniformity while maintaining a simple and compact sensor architecture. A revised polarization rate model incorporating the retro reflected beam is presented. Simulations comparing single beam, 90% reflection, and total reflection configurations confirm a marked reduction in alkali polarization gradients for reflection based designs. Experimentally, the transverse relaxation time of the noble gas spins is extended by 77.66%, indicating significantly suppressed gradient induced decoherence. The proposed method supports the development of miniaturized, high performance SERF comagnetometers.
As a novel quantum sensor utilizing atomic spin coupling mechanisms, spin-exchange relaxation-free (SERF) comagnetometers demonstrate promising applications in precision metrology such as magnetic field and rotation sensing. However, the ultra-high precision measurement capability exhibits vulnerability to multiple disturbance factors, with pump light intensity fluctuations emerging as a critical instability source that compromises operational reliability during prolonged measurements. In this paper, a model-driven paradigm for pump light intensity error suppression in SERF comagnetometers is proposed. The nonlinear dynamics are rigorously linearized via Taylor expansion within their valid range to construct a state-space model, ultimately deriving the pump light intensity frequency response model. The proposed methodology undergoes experimental validation on a K-Rb-21Ne comagnetometry platform, confirming the predictive capability for stability degradation mechanisms. Through theoretical analysis and experimental verification, this study reveals that magneto-optical non-orthogonality, which introduces a differential component in the light intensity transfer function, creates an approximate proportional relationship between the output signal and pump light intensity within the operating bandwidth. Light intensity noise is decomposed, and light intensity errors are quantitatively evaluated via the frequency response model. To address this critical issue affecting inertial measurement accuracy, a closed-loop control-based suppression method with optimized feedback gain is proposed. By dynamically adjusting the closed-loop feedback gain through real-time intensity monitoring, the intensity instability associated with optical pumping is suppressed from 0.56% to 0.14%, concomitant with a 2.8-fold improvement in the long-term stability of the SERF comagnetometer.
The light shift generated by the circularly polarized light leads to an error in the spin-exchange relaxation-free (SERF) comagnetometer, since it turns the fluctuations of the pumping light frequency and intensity into a drift in the SERF comagnetometer. Moreover, the light shift is a source of cross-talk effect between the two sensitive axes. Firstly, the relationship between the vapor cell temperature and the total light shift felt by Rb atoms in the K-Rb hybrid pumping SERF comagnetometer is formulated and simulated. It is found that the total light shift is the result of the collision mixing of K and Rb ac Stark shifts and the increase of cell temperature makes the light shift of K felt by Rb dominate the total light shift. Furthermore, different from the method of adjusting the frequency of laser, a new method that suppresses the total light shift in hybrid pumping by adjusting the cell temperature is proposed and experimentally verified. The total light shift at 165.7°C is -0.00152 nT experimentally, which is 3 orders of magnitude smaller than that at 185.0°C, indicating that it can be suppressed effectively by adjusting the cell temperature. The method does not require any extra hardware or sacrifice the pumping efficiency and can be applied to suppress the error induced by the pumping light and the cross-talk effect between the axes. In addition, the relationship between total relaxation rate and cell temperature is briefly discussed.