Nuclear magnetic resonance (NMR) oscillator is an optical atomic sensor which is developed to measure inertial angular velocity with navigation grade accuracy. It utilizes semiconductor laser, miniaturized magnetic shield and atom vapor cell to realize nuclear magnetic resonance, optical pump and detection in a very small volume (about 10cc to 100cc). NMR oscillator is much more compact than other classic gyros such as fiber optic gyro (FOG), so it is a potential substitution of FOG in the next generation compact navigation grade IMU. To achieve this goal, it is essential to analyse the short-term noise and long-term drift performance of the NMR oscillator. It is also necessary to decompose and track the random error source for further improvements of the device. We propose using the Allan Variance (AVAR) method to both analyse the drift performance and characterize the short-term noise. We also built an NMR oscillator prototype to test and verify the method’s accuracy. Theory analysis and experimental results are compared with the classic FOG for deeper comprehension of the differences between these two inertial sensors. We hope this is helpful for those who design the IMU system.
Nuclear magnetic resonance gyroscopes(NMRG) have received widespread attention to the fields of national defense and civil applications due to their potential for miniaturization and low cost. The relaxation time is an important parameter to measure the performance of the alkali vapor cell of the NMR gyroscope, the most commonly used industry standard measurement method of longitudinal relaxation time(T1) is the measurement method using π pulse plus 1/2π pulse, so the measurement accuracy of the T1 is affected by the accuracy of the pulse duration. Usually the maximum value of the signal amplitude correspond to the time of 1 /2πpulse duration, but it cannot eliminate the influence of the random error of the system, the use of multiple cycles to automatically fit the pulse duration improves the accuracy of the pulse duration, saves a lot of measurement time, eliminates human error, and ultimately improves the T1 measurement accuracy.
Atomics magnetometers achieve remarkable accuracy, applying to production and scientific research. However, their size and bulk components make it difficult to achieve higher accuracy. We investigate the influence of the skew angle of the pump beam on the optical pumping rate in an atomic magnetometer. An analysis based on the Bloch equation is proposed to decrease evaluate optical errors in the process of production and assembly. When the incident angle is non-zero, the pumping rate has a projection in the direction of a static magnetic field. By establishing the pumping rate equation, the pumping rate of each position in the vapor cell in the direction of static magnetic field at different pump light skew angles is calculated in our study. The sensitivity was measured experimentally to demonstrate the simulation results. The results indicate that the optical pumping rate decreases as the amplitude of skew angle and propagation distance increasing which can be evaluated by one-dimensional distribution while the decay rate increases with the rise of the angle. The simulation values of the rubidium pumping rate, obtained with an incident angle of 0.5° , in the center of the vapor cell are reduced by 46%. The sensitivity decreases with the increasing skew angle similar to the attenuation trend of the optical pumping rate but not the same. Our work provides a reference for evaluating the optical error of atomic magnetometer which is useful for miniaturization design.
Measuring the transverse relaxation time of noble gas nuclei with the Free-induction-decay (FID) signal is one of the key technical challenges and prerequisites for the nuclear magnetic resonance gyroscope (NMRG), which is a quantum sensor to form an inertial navigation system. In this paper, a novel method to estimate the parameter of the freeinduction-decay signal based on the adaptive internal model (AIM) controller is proposed and it is verified by the free induction decay data collected from the gyro prototype. In the proposed methods, the AIM con troller can provide estimates of the FID signal, such as the amplitude, the frequency, and the relaxation time. The feasibility of the scheme is proven by theoretical analysis and the reliability of the method is verified by the test results.
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