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Optics Express

Real-time measurement of pump beam polarization through the transmitted light intensity in a polarized alkali metal vapor cell

Lihong Duan, Lele Ma, Zeyi Hu, Ze Yu Cai, Hang Gao

Published May 30, 2024• 2 citations

Abstract

A laser beam with left-/right-handed circular polarization is generally used to create the oriented atomic spins for precision measurements in a spin-exchange relaxation-free (SERF) co-magnetometer. The fluctuation of laser polarization interferes with the spin polarization of alkali metal atoms, leading to the system performance degradation. Here, we report a method for real-time polarization state measurement by using the transmitted light intensity of the pump beam passing through the vapor cell. Based on the principle of circular dichroism, the optical absorption model of polarized alkali metal atoms is established. The simulation results of the transmittance of the pump laser with different polarization states through the alkali metal vapor cell are given and verified by experiments. The experimental results show that the circularly polarized beam has the weakest absorption, while the linearly polarized laser beam is absorbed the strongest. The achieved measurement accuracy stands at an impressive 98.83 %. This work provides a simple and easy-to-use way to measure the polarization state of the laser beam used in the vapor cell devices, particularly the microfabricated prototypes with limited space.

DOI: 10.1364/oe.527556

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In the optical pumping systems based on the pump-probe arrangement, the spin polarization of the atoms is generally monitored utilizing the probe laser beam, in which way an extra perturbation must be introduced and thus affects the normal operation of the sensors. By investigating the absorption rate of the circularly polarized pump laser, here we demonstrate the feasibility of extracting the electron-spin polarization from the transmitted pump laser intensity. We experimentally validate the method in a spin-exchange relaxation free (SERF) magnetometer and the results are in excellent agreement with the theory. The scheme operates in a silent mode and features a real-time observation. We also study the corresponding magnetic field response of the SERF magnetometer and a term arising from the diffusion effects has been added to the original model to explain the discrepancy of the response.

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.

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.

In the optically pumped systems, the spin polarization of the atoms is generally derived by employing additional modulations, in which way an extra perturbation will be introduced and thus affect the normal operation of the sensors. By investigating the absorption principle of the circularly polarized pump laser, here, we demonstrate the feasibility of extracting the electron-spin polarization from the transmitted intensity of the pump beam. This scheme can detect real-time polarization without disturbing the normal operation of the sensor, and the experimental result agrees well with the theory. This research is of value to the research and development of optically pumped systems, especially the magnetometer, and the comagnetometer operated in the spin-exchange relaxation free regime.

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\text{ }\text{fT}/\sqrt{\text{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.

We have developed a simple heuristic method for determining the polarization of an optically pumped alkali-metal vapor. A linearly polarized probe beam traverses a vapor cell perpendicular to the pump-beam propagation direction, and the transmitted beam intensity is monitored for orthogonal linear polarizations. As the probe beam is scanned in frequency across the

{D}_{1}

transition, its linear-polarization-dependent transmission can be used as a measure of the atomic orientation of the vapor. We analyze these transmission differences and their dependence on the alkali-metal number density in the vapor.