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Photonics

Laser Heating Method for an Alkali Metal Atomic Cell with Heat Transfer Enhancement

Yang Li, Guoqing Zhou, Shencheng Tian, Xuejing Liu, Xiangmei Dong, Xiumin Gao

Published Jun 1, 2023• 15 citations
Publisher

Abstract

Alkali metal atomic cells are crucial components of atomic instruments, such as atomic magnetometers, atomic gyroscopes, and atomic clocks. A highly uniform and stable heating structure can ensure the stability of the alkali metal atom density. The vapor cell of an atomic magnetometer that uses laser heating has no magnetic field interference and ease of miniaturization, making it superior to hot air heating and AC electric heating. However, the current laser heating structure suffers from low heating efficiency and uneven temperature distribution inside the vapor cell. In this paper, we designed a non-magnetic heating structure based on the laser heating principle. We studied the temperature distribution of the heating structure using the finite element method (FEM) and analyzed the conversion and transfer of laser energy. We found that the heat conduction between the vapor cell and the heating chips (colored filters) is poor, resulting in uneven temperature distribution and low heating efficiency in the vapor cell. Therefore, the addition of graphite film to the four surfaces of the vapor cell was an important improvement. This addition helped to balance the temperature distribution and improve the conduction efficiency of the heating structure. It was measured that the power of the heating laser remained unchanged. After the addition of the graphite film, the temperature difference coefficient (CVT) used to evaluate the internal temperature uniformity of the vapor cell was reduced from 0.1308 to 0.0426. This research paper is crucial for improving the heating efficiency of the non-magnetic heating structure and the temperature uniformity of the vapor cell.

DOI
10.3390/photonics10060637

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Alkali metal atomic cells are crucial components of atomic instruments, such as atomic magnetometers, atomic gyroscopes, and atomic clocks. A highly uniform and stable heating structure can ensure the stability of the alkali metal atom density. The vapor cell of an atomic magnetometer that uses laser heating has no magnetic field interference and ease of miniaturization, making it superior to hot air heating and AC electric heating. However, the current laser heating structure suffers from low heating efficiency and uneven temperature distribution inside the vapor cell. In this paper, we designed a non-magnetic heating structure based on the laser heating principle. We studied the temperature distribution of the heating structure using the finite element method (FEM) and analyzed the conversion and transfer of laser energy. We found that the heat conduction between the vapor cell and the heating chips (colored filters) is poor, resulting in uneven temperature distribution and low heating efficiency in the vapor cell. Therefore, the addition of graphite film to the four surfaces of the vapor cell was an important improvement. This addition helped to balance the temperature distribution and improve the conduction efficiency of the heating structure. It was measured that the power of the heating laser remained unchanged. After the addition of the graphite film, the temperature difference coefficient (CVT) used to evaluate the internal temperature uniformity of the vapor cell was reduced from 0.1308 to 0.0426. This research paper is crucial for improving the heating efficiency of the non-magnetic heating structure and the temperature uniformity of the vapor cell.
Laser heating of alkali metal vapor cells of atomic magnetometers is applied owing to its none magnetic field interference and little energy consumption, compared to heating by hot air and heating by AC electrical current. The development of laser heating technology, including the attachment of optical filters for better heat absorption, has expanded its usage from small chip-scale cells to relatively large cells, and from heating of single magnetometer to heating of an array of magnetometers. This paper offers a review of the evolution of laser heating in atomic magnetometers, introduces the different configurations of laser heating in experiments and points out the next possible target of the application of laser heating.
Alkali-metal vapor cells are key elements of atomic sensors. The temperature uniformity in the vapor cell is important for the sensitivity of the atomic sensors especially highly-sensitive SERF atomic magnetometers. This study proposes a novel double-layer cylindrical oven to improve the heating performance and the temperature uniformity of the vapor cell. We use the finite element method to build the thermal simulation models of the conventional square oven and the novel double-layer cylindrical oven. The performances of two ovens are compared. Then we build the experimental system to carry out the measurement experiment of heating temperature of the conventional and novel ovens and the in-situ measurement experiment of temperature distribution of the vapor cell using different ovens. We also measure the change of output signal amplitude of the SERF atomic magnetometer with frequency, i.e. the frequency response R(f) using the conventional and novel ovens respectively to evaluate the performance of the SERF atomic magnetometer using different ovens. The experimental results demonstrate that the proposed oven structure can improve the temperature uniformity of the alkali-metal vapor cell by 63.63% and the amplitude of R(f) using the novel oven increases by 24.7% on average. The novel double-layer cylindrical oven improves the temperature uniformity in the alkali-metal vapor cell and increases amplitude of the output signal of the SERF atomic magnetometer. The study is of great significance for improving the sensitivity of the atomic sensors especially SERF atomic magnetometers.
The SERF atomic magnetometer typically operates in a high-temperature and zero-magnetic-field regime. Among common heating methods, laser heating is a promising method since it is magnetically clean and stable. In this work, we for the first time introduce an orthogonal laser heating configuration to improve the temperature uniformity of the atomic cell, thereby enhancing the sensitivity of the atomic magnetometer. What’s more, a theoretical model is provided to illustrate the relationship between temperature variation and sensitivity. The temperature distributions of different laser heating configurations are evaluated by the finite element method. Experimental measurements were conducted with a single-beam atomic magnetometer. The spatial distribution of the conversion coefficient within the atomic cell was constructed through a simulation method. And the results show that, compared with the coaxial heating configuration, the orthogonal heating configuration improves the magnetic field sensitivity by 11.3%. Experimental results show that the magnetic field sensitivity under coaxial heating configuration is 56 f T / H z , while that under orthogonal heating configuration is 48 f T / H z , representing a 14.3% improvement. Both the simulation and experiments demonstrate that the orthogonal heating configuration significantly enhances the temperature uniformity within the atomic cell, thereby improving the magnetic field sensitivity of the atomic magnetometer.
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.
We propose non-magnetic heating film based on indium tin oxide (ITO) and apply it in a miniaturized spin exchange relaxation free (SERF) gradient magnetometer. To achieve ultrahigh sensitivity, alkali metal atoms inside the vapor cell of the magnetometer require an elevated temperature field that is stable and uniform. ITO is a transparent and electrically conductive material which is suitable for such application and can be used as resistance wire to form the heating film. A film structure composed of an insulation layer sandwiched between two ITO layers with the same optimized pattern is used to configure the heating current to double back on itself to reduce the magnetic noise caused by it. Experiments show the proposed film can be heated to over 200 °C, which is sufficient for vaporizing alkali metal atoms such as potassium, rubidium, and cesium. The magnetic current coefficients of the heating films are below 0.2 nT/mA and with the high frequency heating current, low-frequency magnetic noise can be further reduced. We compare ITO-based heating films and conventional flexible printed circuit (FPC) heating films side by side in the same gradient magnetometer and with ITO films a more uniform temperature field and better intrinsic magnetic field sensitivity of 22 fT/cm/Hz 1/2^{1/2}1/2 is achieved. The proposed ITO-based non-magnetic heating film can be used not only in atomic magnetometers but also in other atomic devices such as chip scale gyroscopes and atomic clocks.
Heating the alkali metal vapor cell to obtain high atomic density is a prerequisite for realizing the high-precision measurement for spin-exchange relaxation-free (SERF) co-magnetometer. However, the thermo-magnetic and thermo-optical coupling noise caused by the temperature diffusion from the alkali vapor cell is one of the main factors restricting the co-magnetometer performance at present. Here the influence of heating on the SERF co-magnetometer was comprehensively studied, and the thermal error model of the system was established. A novel heat insulation structure for the co-magnetometer was designed, and the effectiveness was verified through monitoring of characteristic temperature points. Compared to the conventional oven support, the average maximum temperature difference within 12 h of the optical path and magnetic shielding systems (OPMSS) decreased by 52%, the bias instability of the co-magnetometer decreased by 24%, and the noise of the co-magnetometer at 1 Hz decreased by 33%. The experimental results demonstrate that this method reduces the temperature fluctuation of the OPMSS, effectively suppresses the thermal coupling noise, and thus improves the performance of the SERF co-magnetometer. Moreover, this method is also applicable to other quantum sensors based on alkali metal vapor cell.