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[The Measuring Method of Atomic Polarization of Alkali Metal Vapor Based on Optical Rotation and the Analysis of the Influence Factors].

Huining Shang, Wei Quan, Yao Chen, Yang Li, Hong Li

Published Feb 1, 2016• 2 citations
OpenAlex

Abstract

High sensitivity measurements of inertia and magnetic field could be achieved by utilizing a category of devices, which manipulate the atomic spins in the spin-exchange-relaxation-free regime. The alkali cell which contains the alkali metal vapor is used to sense magnetic field and inertia. The atomic number density of alkali vapor and the polarization of alkali metal vapor are two of the most important parameters of the cell. They play an important role in the research on atomic spins in the spin-exchange-relaxation-free regime. Besides, optical polarization plays an important role in quantum computing and atomic physics. We propose a measurement of alkali vapor polarization and alkali number density by detecting the optical rotation in one system. This method simplifies existing experimental equipment and processes. A constant bias magnetic field is applied and the Faraday rotation angle is detected by a bunch of the probe beam to deduce alkali-metal density. Then the magnetic field is closed and a bunch of the pump laser is utilized to polarize alkali-metal. Again, the probe beam is utilized to obtain the polarization of alkali metal. The alkali density obtained at first is used to deduce the polarization. This paper applies a numerical method to analyze the Faraday rotation and the polarization rotation. According to the numerical method, the optimal wavelength for the experiment is given. Finally, the fluctuation of magnetic field and wavelength on signal analysis are analyzed.

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High sensitivity measurements of inertia and magnetic field could be achieved by utilizing a category of devices, which manipulate the atomic spins in the spin-exchange-relaxation-free regime. The alkali cell which contains the alkali metal vapor is used to sense magnetic field and inertia. The atomic number density of alkali vapor and the polarization of alkali metal vapor are two of the most important parameters of the cell. They play an important role in the research on atomic spins in the spin-exchange-relaxation-free regime. Besides, optical polarization plays an important role in quantum computing and atomic physics. We propose a measurement of alkali vapor polarization and alkali number density by detecting the optical rotation in one system. This method simplifies existing experimental equipment and processes. A constant bias magnetic field is applied and the Faraday rotation angle is detected by a bunch of the probe beam to deduce alkali-metal density. Then the magnetic field is closed and a bunch of the pump laser is utilized to polarize alkali-metal. Again, the probe beam is utilized to obtain the polarization of alkali metal. The alkali density obtained at first is used to deduce the polarization. This paper applies a numerical method to analyze the Faraday rotation and the polarization rotation. According to the numerical method, the optimal wavelength for the experiment is given. Finally, the fluctuation of magnetic field and wavelength on signal analysis are analyzed.
Abstract The actual vapor density characterizing the alkali metal spin-exchange rate remains a compelling issue for spin-exchange optical pumping. Based on the deduced relationship between the Faraday rotation angle and the rubidium vapor number density using the electrodynamics theory, we report a measurement of the number density for rubidium vapor sealed inside a cell based on a Faraday modulator. The measurement relies on the optical rotation angle due to rubidium vapor under a bias magnetic field (∼0.08 T) produced by a samarium–cobalt magnet. A Faraday modulator with a lock-in amplifier is used to accurately measure the tiny optical rotation angle in a temperature range of 387–468 K. In addition, a synchronization verification is performed by the photoelastic modulator (PEM). The recurring data showed that the two methods are consistent with each other. Compared with the PEM method, the Faraday modulator detection system does not need to adjust the optical axis difference of 45° in the PEM detection system, thereby reducing the complexity of the experiment and the error caused by the alignment of the optical axis, which showed that the Faraday modulator detection method more advantageous in measuring the alkali metal density.
Spin-exchange relaxation free alkali-metal magnetometers typically operate in the regime of high optical density, presenting challenges for simple and efficient optical pumping and detection. We describe a high-sensitivity Rb magnetometer using a single elliptically polarized off-resonant laser beam. Circular component of the light creates relatively uniform spin polarization while the linear component is used to measure optical rotation generated by the atoms. Modulation of the atomic spin direction with an oscillating magnetic field shifts the detected signal to high frequencies. Using a fiber-coupled distributed feedback laser, we achieve magnetic field sensitivity of 7 fT/Hz7\text{ }\text{fT}/\sqrt{\text{Hz}}7 fT/Hz​ with a miniature $5\ifmmode\times\else\texttimes\fi{}5\ifmmode\times\else\texttimes\fi{}5\text{ }\text{mm}\text{ }\text{Rb}$ vapor cell.
The atomic density of an alkali vapor cell is a critical factor in determining the performance of atomic magnetometers. This paper presents a novel method to detect the density of an alkali atomic ensemble under arbitrary polarization states. By establishing a comprehensive model that correlates the absorption spectrum with the incident light’s polarization, our approach enables real-time and flexible measurements across varying temperatures and polarization angles. Experimental validations are conducted by systematically rotating a quarter-wave plate and controlling the vapor cell temperature, thereby collecting transmission spectra at multiple polarizations. The results demonstrate that the proposed method simplifies density measurements while preserving high accuracy, laying a solid foundation for further research on Spin-Exchange Relaxation-Free (SERF) atomic magnetometers. This technique serves as a versatile and effective tool for in-situ monitoring of alkali metal vapor density in various sensor applications.
The number density of alkali atoms in vaper cell is an important parameter for atomic magnetometer. The vapor cell is usually heated to a high temperature for the number density exceed to 1018m−3to achieve the spin-exchange-relaxation-free (SERF) process. The optical rotation of multichannel atomic magnetometers is affected by the number density distribution. We present an effective method for monitoring and analyzing the spatial distribution homogeneity of the atomic number density in the alkali metal cell within the SERF magnetometer. The numerical results from researching the model of electric heating films and ovens show that the temperature gradient increases with an increase in the cell temperature. This causes an nonuniform number density distribution based on the saturated vapor pressure curve. We present a laser absorption spectroscopy method by using the high-resolution beam profiling camera to record the power of the beam before and after the cell. An optimized optical system of the atomic magnetometer has been established by measuring the the number density variation in the spatial distribution in mm scale. The number density variation coefficient, which is introduced to evaluate the distribution homogeneity, increases from 0.0257 to 0.0333 when the temperature of the oven wall rises from 140°C to 190 °C, showing the same trend as the simulation results. The optical rotation distribution is calculated according to the magnetometer output signal formula. This study is of great interest for optimizing the oven structure and improving the stability of atomic spin polarization and sensitivity of the multichannel magnetometer.
The alkali-atom density measurement method based on light absorption is highly suitable for a spin-exchange relaxationfree (SERF) atomic magnetometer because of its high-precision measurement and complete nonmagnetic interference.In this study, the optical rotation angle detection system based on polarization balance detection is utilized to realize the alkali-atom density real-time measurement without affecting magnetic field measurement.We discovered that there exists an optimal frequency detuning of the probe light, which offers the highest sensitivity in alkali-atom density measurement and the lowest susceptibility to temperature fluctuations in terms of the scale factor.In contrast to conventional light absorption measurements based on pump light, this method demonstrated a threefold improvement in alkali-atom density measurement sensitivity while remaining immune to ambient magnetic fields and incident light intensity fluctuations.Furthermore, we utilized this method to achieve closed-loop temperature control with an accuracy of 0.04°C.
This study presents a novel method for measuring the number density of K in K-Rb hybrid vapor cells using circularly polarized pump light on polarized alkali metal atoms. This proposed method eliminates the need for additional devices such as absorption spectroscopy, Faraday rotation, or resistance temperature detector technology. The modeling process involved considering wall loss, scattering loss, atomic absorption loss, and atomic saturation absorption, with experiments designed to identify the relevant parameters. The proposed method is real-time, highly stable, and a quantum nondemolition measurement that does not disrupt the spin-exchange relaxation-free (SERF) regime. Experimental results demonstrate the effectiveness of the proposed method, as the longitudinal electron spin polarization long-term stability increased by 204% and the transversal electron spin polarization long-term stability increased by 44.8%, as evaluated by the Allan variance.