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Photonics

Design of a Fiber Alkali Vapor Cell for Atomic Magnetometer for Magnetoencephalography Applications

Xuejing Liu, Yanhui Hu, Yang Li, Xudong Wu, Min Chang, Xuedian Zhang

Published Oct 10, 2022• 3 citations
Publisher

Abstract

Spin exchange relaxation free (SERF) atomic magnetometer (AM), based on the Larmor precession of alkali atoms, is considered a promising candidate for magnetoencephalography (MEG) systems with the advantages of high sensitivity and no need for cryogenic devices. The footprint of the sensor header contains alkali vapor cell and bulk optical elements determining the spatial resolution of the MEG system. Optical fiber could separate the vapor cell far from other parts of the sensor header to improve the spatial resolution. However, coupling between glass cell and fibers limits the coupling loss of the light. Here, we describe the design of a fiber-based alkali vapor cell that could alleviate these issues. A pair of fiber cables combining a polarization maintaining fiber (PMF) and hollow-core photonic crystal fibers (HC-PCFs) are enclosed in a vacuum-sealed T-shape glass tube filled with alkali atoms. The fiber cell ensures a flexible integration with most fiber systems. The fiber structure, with an air gap between HC-PCFs, provides a large interaction volume between light and atoms. The vapor of the alkali atoms diffuses into the air core of the HC-PCF from the glass tube by heating. The alkali atoms still contained in SERF regime are within the wall relaxation rates of 12,764 s−1 in the coating fiber cell. The insertion loss due to fiber coupling is analyzed. The coupling efficiency could be 91%, with the fiber structure consisting of a 40 μm diameter HC-PCF and a 1 mm air gap. The limit sensitivity under this condition is simulated at 14.7 fT/Hz1/2. The fabrication technique and the light insertion loss are discussed. The fiber alkali vapor cell is of compact size and has flexible integration with the fiber atomic spin precession detection system.

DOI
10.3390/photonics9100749

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Spin exchange relaxation free (SERF) atomic magnetometer (AM), based on the Larmor precession of alkali atoms, is considered a promising candidate for magnetoencephalography (MEG) systems with the advantages of high sensitivity and no need for cryogenic devices. The footprint of the sensor header contains alkali vapor cell and bulk optical elements determining the spatial resolution of the MEG system. Optical fiber could separate the vapor cell far from other parts of the sensor header to improve the spatial resolution. However, coupling between glass cell and fibers limits the coupling loss of the light. Here, we describe the design of a fiber-based alkali vapor cell that could alleviate these issues. A pair of fiber cables combining a polarization maintaining fiber (PMF) and hollow-core photonic crystal fibers (HC-PCFs) are enclosed in a vacuum-sealed T-shape glass tube filled with alkali atoms. The fiber cell ensures a flexible integration with most fiber systems. The fiber structure, with an air gap between HC-PCFs, provides a large interaction volume between light and atoms. The vapor of the alkali atoms diffuses into the air core of the HC-PCF from the glass tube by heating. The alkali atoms still contained in SERF regime are within the wall relaxation rates of 12,764 s−1 in the coating fiber cell. The insertion loss due to fiber coupling is analyzed. The coupling efficiency could be 91%, with the fiber structure consisting of a 40 μm diameter HC-PCF and a 1 mm air gap. The limit sensitivity under this condition is simulated at 14.7 fT/Hz1/2. The fabrication technique and the light insertion loss are discussed. The fiber alkali vapor cell is of compact size and has flexible integration with the fiber atomic spin precession detection system.
The designs of microfabricated alkali vapor cells play an important role when the atomic magnetometers are developing toward on-chip integration. This paper presents microfabricated spherical alkali vapor cells with improved sensitivity for chip-scale atomic magnetometers. A microfabricated spherical rubidium vapor cell and a traditional microfabricated planar rubidium vapor cell are designed for comparison, which are of the same volume and interconnected by a microchannel to acquire equal inner buffer gas pressure. In chip-scale spin-exchange relaxation-free (SERF) atomic magnetometers, the rubidium polarization lifetimes of three pairs of spherical and planar vapor cells at different buffer gas pressures of 0.35, 0.73, and 2.00 amg are measured, respectively. The sensitivity of SERF atomic magnetometers using the vapor cells at a 0.35-amg buffer gas pressure is also measured. The results show that the transverse polarization lifetime of the spherical vapor cell is 2.46, 1.44, and 1.05 times that of the planar one at 0.35-, 0.73-, and 2.00-amg buffer gas pressures, respectively. Combining the improved polarization lifetime and increased optical path length, chip-scale SERF atomic magnetometer using the microspherical vapor cell has the sensitivity of 60 fT/Hz1/2from 5 to 15 Hz, while that based on the planar one is demonstrated with the sensitivity of 400 fT/Hz1/2from 5 to 15 Hz.
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.
The electronic spin polarization of alkali-metal-vapor atoms is a pivotal parameter for atomic magnetometers. Herein, a novel method is presented for determining the spin polarization with a miniaturized single-beam spin-exchange-relaxation-free (SERF) magnetometer on the basis of zero-field cross-over resonance. Two separate laser beams are utilized to heat the cell and interrogate the vapor atoms, respectively. Spin polarization can be extracted by measuring the resonance response signal of the magnetometer to the transverse magnetic field under different irradiances. Results of these experiments are consistent well with the theoretical predictions with the maximum deviation less than 4%. The proposed method has the integrated advantages of possessing a simple configuration and in-situ measurement. Furthermore, combined with a homemade optical differential detection system with a factor of approximately three of the power noise suppression, the developed single-beam SERF atomic magnetometer with a measuring sensitivity of 32 fT/Hz 1/2 has been achieved. This demonstrated approach can help guide the development of chip-scale atomic magnetometers for bio-magnetic field imaging applications.
Abstract The chip-scale hybrid optical pumping spin-exchange relaxation-free (SERF) atomic magnetometer with a single-beam arrangement has prominent applications in biomagnetic measurements because of its outstanding features, including ultrahigh sensitivity, an enhanced signal-to-noise ratio, homogeneous spin polarization and a much simpler optical configuration than other devices. In this work, a miniaturized single-beam hybrid optical pumping SERF atomic magnetometer based on a microfabricated atomic vapor cell is demonstrated. Although the optically thin Cs atoms are spin-polarized, the dense Rb atoms determine the experimental results. The enhanced signal strength and narrowed resonance linewidth are experimentally proven, which shows the superiority of the proposed magnetometer scheme. By using a differential detection scheme, we effectively suppress optical noise with an approximate five-fold improvement. Moreover, the cell temperature markedly affects the performance of the magnetometer. We systematically investigate the effects of temperature on the magnetometer parameters. The theoretical basis for these effects is explained in detail. The developed miniaturized magnetometer has an optimal magnetic sensitivity of 20 fT/Hz 1/2 . The presented work provides a foundation for the chip-scale integration of ultrahighly sensitive quantum magnetometers that can be used for forward-looking magnetocardiography (MCG) and magnetoencephalography (MEG) applications.
With the continuous improvement of weak magnetic detection technology, there has been a growing trend towards more compact and integrated magnetometer. The SERF magnetometer and the Mz magnetometer have many common features in operation. Therefore, we design a new kind of miniaturized atomic sensors to integrate the SERF magnetometer and the Mz magnetometer based on micro-fabricated alkali vapor cell. The sensor structure consists of alkali vapor cell, three-axis magnetic field coils, heating module and optical module. This small size sensor has the potential to achieve a portable multi-function device.
Wafer-level integration of micro heater on an alkali vapor cell for chip-scale SERF magnetometers is presented. In order to obtain a suitable resistivity, polysilicon and aluminum are mixed as the target material. Magnetron sputtering is adopted to fabricate the heater on the surface of the alkali vapor cell. Various voltages are applied to the heater, of which the temperatures are measured by thermal sensitive sensors. Spin exchange relaxation free (SERF) atomic magnetometer with the heater integrated on the alkali vapor cell is characterized. Results show that the micro heater is successfully integrated on the alkali vapor cell. With the integrated heater, the rubidium vapor cell is heated up to 120°C by a 445kHz AC current. The sensitivity of the SERF magnetometer is 6.3pT/Hz1/2. Results also indicate that it provides an effective and low-cost solution for the miniaturization of atomic magnetometers.