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Sensors

Measuring Transverse Relaxation with a Single-Beam 894 nm VCSEL for Cs-Xe NMR Gyroscope Miniaturization

Qingyang Zhao, Ruochen Zhang, Hua Liu

Published Sep 1, 2024• 5 citations
Publisher

Abstract

The spin-exchange-pumped nuclear magnetic resonance gyroscope (NMRG) is a pivotal tool in quantum navigation. The transverse relaxation of atoms critically impacts the NMRG’s performance parameters and is essential for judging normal operation. Conventional methods for measuring transverse relaxation typically use dual beams, which involves complex optical path and frequency stabilization systems, thereby complicating miniaturization and integration. This paper proposes a method to construct a 133Cs parametric resonance magnetometer using a single-beam vertical-cavity surface-emitting laser (VCSEL) to measure the transverse relaxation of 129Xe and 131Xe. Based on this method, the volume of the gyroscope probe is significantly reduced to 50 cm3. Experimental results demonstrate that the constructed Cs-Xe NMRG can achieve a transverse relaxation time (T2) of 8.1 s under static conditions. Within the cell temperature range of 70 °C to 110 °C, T2 decreases with increasing temperature, while the signal amplitude inversely increases. The research lays the foundation for continuous measurement operations of miniaturized NMRGs.

DOI
10.3390/s24175692

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A new generation of small low-power atomic sensors, including clocks, magnetometers, and gyroscopes, is being developed based on recently available MEMS and VCSEL technologies.These sensors rely on spectroscopic interrogation of alkali atoms, typically rubidium or cesium, contained in small vapor cells.The relevant spectroscopic wavelengths (in vacuum) are 894.6 nm (D1) and 852.3 nm (D2) for cesium, and 795.0 nm (D1) and 780.2 nm (D2) for rubidium.The D1 wavelengths are either preferred or required, depending on the application, and vertical-cavity surface-emitting lasers (VCSELs) are preferred optical sources because of their low power consumption and circular output beam.This paper describes the required VCSEL characteristics for atomic clocks and magnetometers.The fundamental VCSEL requirement is single-frequency output with tunability to the particular spectroscopic line of interest.Singlepolarization and single-transverse-mode operation are implicit requirements.VCSEL amplitude noise and frequency noise are also important because they contribute significantly to the sensor signal-to-noise ratio.Additional desired VCSEL attributes are low cost, low power consumption, and several years of continuous operating lifetime.This paper also describes the 894-nm VCSELs that we have developed for cesium-based atomic sensors.In particular, we discuss VCSEL noise measurements and accelerated lifetime testing.Finally, we report the performance of prototype atomic clocks employing VCSELs.

•Jul 26, 2016•0 citations
The spin-exchange-pumped nuclear magnetic resonance gyroscope (NMRG) is a pivotal tool in quantum navigation. The transverse relaxation of atoms critically impacts the NMRG’s performance parameters and is essential for judging normal operation. Conventional methods for measuring transverse relaxation typically use dual beams, which involves complex optical path and frequency stabilization systems, thereby complicating miniaturization and integration. This paper proposes a method to construct a 133Cs parametric resonance magnetometer using a single-beam vertical-cavity surface-emitting laser (VCSEL) to measure the transverse relaxation of 129Xe and 131Xe. Based on this method, the volume of the gyroscope probe is significantly reduced to 50 cm3. Experimental results demonstrate that the constructed Cs-Xe NMRG can achieve a transverse relaxation time (T2) of 8.1 s under static conditions. Within the cell temperature range of 70 °C to 110 °C, T2 decreases with increasing temperature, while the signal amplitude inversely increases. The research lays the foundation for continuous measurement operations of miniaturized NMRGs.
In order to investigate the influence of pump beam truncation on the transverse relaxation time of 129Xe nuclear spins in a nuclear magnetic resonance gyroscope, we measured the transverse relaxation rate as a function of pump laser power and vapor cell temperature for two ovens with different optical access sizes. The experimental results conform qualitatively to a theoretical model based on magnetic field gradient induced by polarization gradient of Rb atoms. It is found that the non-uniform power distribution in the beam cross-section led to remarkable relaxation, especially for the 129Xe nuclear spins in a large-size vapor cell. To reduce the polarization gradient, the spatial distribution of the pump laser power in the cell should be as homogenous as possible. These results are of significance to the design of a high precision nuclear magnetic resonance gyroscope.
The transverse relaxation time (T2) is an important indicator to determine the fundamental sensitivity limit of alkali-metal atomic magnetometers. We propose a method based on the principle of longitudinal field modulation that obtains T2 by scanning the transverse static magnetic field. The previous technique of extracting T2 from the linewidth of the modulation frequency and the traditional magnetic-resonance-broadening-fitting method are also described. The T2 measurements of Cesium (Cs) atoms are carried out through these three methods, whose operating environments are applicable to different atomic magnetometers, respectively. The method that we propose can be used for obtaining the T2 of Cs atoms as well as detecting the transverse static magnetic field and is customized for the study of the Cs–Xenon ensemble for the construction of nuclear magnetic resonance gyroscopes. Moreover, the relationship between the limit sensitivities and cell temperatures is further studied in the experiment.
The spin polarization of 87Rb and 129Xe atomic ensemble has great impact on the signal intensity of a nuclear magnetic resonance gyroscope. The coupled spin polarization modelling process for 87Rb and 129Xe atoms ensemble is presented in detail. Three dimensional spatial distribution of the spin polarization field is obtained by numerical solution and is in good agreement with the experiment results. Influencing factors including temperature, power and detuned frequency of the pumping laser, average collision times of anti-relaxation coatings and partial pressure of gas composition are investigated. The work presents an indirect method to estimate the average collision times given the Xe polarization with different temperatures. It is found the average collision times of the 129Xe atoms with the inner surface of a bare glass cell of 3×3×3mm3 can reach up to about 2.7×105 times. It is also found that the pumping frequency corresponding to the maximum polarization of the Xe atoms polarized by spin exchange of Rb atoms is not consistent with the D1 line of the 87Rb atom when the temperature of the cell is above a certain temperature, but is detuned at a certain frequency. Quantitative analysis of the effect of anti-relaxation coatings on the spin polarization suggests that the appropriate average number of collisions of the Rb atoms with the inner surface of the cell is about 1500 times.
We report on the fabrication and characterization of MEMS atomic vapor cells suitable for applications in miniaturized quantum sensors such as atomic gyroscopes. Our MEMS cells are filled with natural abundance Rb alkali atoms and enriched noble Xe atoms and are operated in the regime of spin exchange optical pumping. The transverse relaxation time T2* of the nuclear spin in 129Xe atoms directly defines the angular random walk parameter of an atomic gyroscope. Using a field switch technique, we measure the dephasing time T2* of the 129Xe isotope as a function of temperature. Our results showing a decrease of T2* from about 1 to 0.4 seconds with an increasing temperature in the range from 80 to 150 °C are in good agreement with a simple theoretical model taking into account the most important decoherence mechanisms. We show that the observed decoherence behavior can be mostly explained trough collisions of the Xe atoms with the walls. Further characterization steps in order to gain more insight in the decoherence physics involved in our MEMS cells are discussed.
We theoretically investigate several parameters for the nuclear magnetic resonance gyroscope based on 133 Cs– 129 Xe/ 131 Xe. For a cell containing a mixture of 133 Cs at saturated pressure, we investigate the optimal quenching gas (N 2 ) pressure and the corresponding pump laser intensity to achieve 30% 133 Cs polarization at the center of the cell when the static magnetic field B 0 is 5 μ T with different 129 Xe/ 131 Xe pressure. The effective field produced by spin-exchange polarized 129 Xe or 131 Xe sensed by 133 Cs can also be discussed in different 129 Xe/ 131 Xe pressure conditions. Furthermore, the relationship between the detected signal and the probe laser frequency is researched. We obtain the optimum probe laser detuning from the D2 ( 6 2 S 1 / 2 → 6 2 P 3 / 2 ) resonance with different 129 Xe/ 131 Xe pressure owing to the pressure broadening.
A new generation of small low-power atomic sensors, including clocks, magnetometers, and gyroscopes, is being developed based on recently available MEMS and VCSEL technologies.These sensors rely on spectroscopic interrogation of alkali atoms, typically rubidium or cesium, contained in small vapor cells.The relevant spectroscopic wavelengths (in vacuum) are 894.6 nm (D1) and 852.3 nm (D2) for cesium, and 795.0 nm (D1) and 780.2 nm (D2) for rubidium.The D1 wavelengths are either preferred or required, depending on the application, and vertical-cavity surface-emitting lasers (VCSELs) are preferred optical sources because of their low power consumption and circular output beam.This paper describes the required VCSEL characteristics for atomic clocks and magnetometers.The fundamental VCSEL requirement is single-frequency output with tunability to the particular spectroscopic line of interest.Singlepolarization and single-transverse-mode operation are implicit requirements.VCSEL amplitude noise and frequency noise are also important because they contribute significantly to the sensor signal-to-noise ratio.Additional desired VCSEL attributes are low cost, low power consumption, and several years of continuous operating lifetime.This paper also describes the 894-nm VCSELs that we have developed for cesium-based atomic sensors.In particular, we discuss VCSEL noise measurements and accelerated lifetime testing.Finally, we report the performance of prototype atomic clocks employing VCSELs.