Jian-cheng Fang’s research while affiliated with Beihang University and other places

A validated method based on Haidinger fringes has been proposed to measure the optical constants for thin-walled glass cells. The method can significantly enhance measurement accuracy by utilizing the reflected spectrum to determine the optical constants of the sample. The reflected light off the inner and the outer surfaces of the cell combined together and formed the interferential pattern (Haidinger fringes) detected by the photodiodes. Taking into account that the sample in our experiment is a sealed cell, we deliberately set the incident angle to a non-zero value. First, the measurement principle has been analyzed, and subsequently, the relevant experimental platform has been established. To reduce random errors, we recalibrated the relationship between the frequency of the laser and the operating temperature. Furthermore, the incident angle has been accurately estimated using the method of non-linear least squares. Finally, the weighted average, considering correlated uncertainties for all measurements, is 1.873 ± 0.005 016 mm, which showcases clear advantages over conventional mechanical methods that risk potential damages.

Despite encouraging advances in early diagnosis and treatment, cardiovascular diseases (CVDs) remained a leading cause of morbidity and mortality worldwide. Increasing evidence has shown that the electromagnetic field (EMF) influences many biological processes, which has attracted much attention for its potential therapeutic and diagnostic modalities in multiple diseases, such as musculoskeletal disorders and neurodegenerative diseases. Nonionizing EMF has been studied as a therapeutic or diagnostic tool in CVDs. In this review, we summarize the current literature ranging from in vitro to clinical studies focusing on the therapeutic potential (external EMF) and diagnostic potential (internal EMF generated from the heart) of EMF in CVDs. First, we provided an overview of the therapeutic potential of EMF and associated mechanisms in the context of CVDs, including cardiac arrhythmia, myocardial ischemia, atherosclerosis, and hypertension. Furthermore, we investigated the diagnostic and predictive value of magnetocardiography in CVDs. Finally, we discussed the critical steps necessary to translate this promising approach into clinical practice.

The Doppler difference velocity between the measured solar spectrum and the measured asteroid spectrum is the measurement of the celestial Doppler difference navigation. Due to the absorption effect of the asteroid on the measured asteroid spectrum, the Doppler difference velocity is inevitably affected. In order to solve this problem, considering the fact that the asteroid reflectance is the ratio of the approximation of the asteroid spectrum to that of solar spectrum, we utilize the Gaussian genetic algorithm (GGA) and empirical mode decomposition (EMD) to extract the approximation of celestial spectra and propose an inversion method of celestial spectrum reflectance template based on Gaussian genetic algorithm-empirical mode decomposition (GGA-EMD). Firstly, the celestial spectrum is decomposed by EMD into the intrinsic mode functions (IMF). A certain combination of IMFs can embody the spectrum reflectance. And then, we utilize the undetermined spectrum reflectance function and the measured solar spectrum to produce the virtual asteroid spectrum and adopt the Taylor method to estimate the Doppler difference velocity between the measured asteroid spectrum and the virtual one. Finally, to converge fast, we utilize the Gaussian function to optimize the mutation and crossover operations of genetic algorithm(GA) and propose the Gaussian genetic algorithm. Using the Doppler difference velocity errors as the object of the fitness function, the GGA optimizes the combination of IMFs, which is used to construct an optimized celestial spectrum reflectance template. The experiment results show that the inversion method of celestial spectrum reflectance template based on GGA-EMD constructs the spectrum reflectance template effectively and improves the accuracy of the Doppler difference velocity.

We present an approach that allows detecting all three components of the residual magnetic field inside shielding, based on the electron paramagnetic resonance (EPR) of spin-polarized K atoms. The residual field experienced by spin-polarized K atoms dominates Larmor precession frequency, the smaller the frequency is, the more benefits it has to realize spin-exchange relaxation-free (SERF) regime under a definite spin-exchange rate. The measurements are accomplished based on depopulation optical pumping and optically-detected peak of the K EPR spectrum. The EPR-based cross modulation method is employed to determine all three components of the residual field independently by modulating the frequency of transverse field and the strength of longitudinal field. The weighted averages of all measurements with the corresponding uncertainties are (9.60 ± 0.06) nT, (2.25 ± 0.03) nT, and (2.83 ± 0.17) nT along with the negative directions of their related axes, respectively.

We have studied the polarization of the 85Rb and the fundamental sensitivity of two types of hybrid optical pumping spin exchange relaxation free (SERF) co-magnetometers based on 39K-85Rb-21Ne and 133Cs-85Rb-21Ne with taking the wall relaxation rate into account,which has a big effect on the polarization and fundamental sensitivity,but it is often neglected in the experiments,respectively.The polarization of the 85Rb varies with the thirteen systemic parameters: the number density of buffer gas 21Ne and quench gas N2,frequency rate of pump beam,cell temperature,the photon number flux,mole fraction of 85Rb,cell effective radius,pump beam spot radius,power density of pump beam,input rotation velocity in the x and y direction,external magnetic field in the y and z direction,respectively.The number density of the 21Ne,photon number flux,pump beam spot radius and frequency rate will affect the polarization of the 85Rb greatly by affecting the the pumping rate of pump beam,a magnetic field produced by 85Rb atom spins and experienced by the 21Ne spins.We obtain a general formula on the fundamental sensitivity of the hybrid optical pumping SERF co-magnetometers due to shot-noise,where the fundamental sensitivity varies with the twelve systemic parameters: the number density of buffer gas and quench gas,frequency rate of pump beam,photon number flux,mole fraction of 85Rb,cell effective radius,pump beam spot radius,power density of pump beam,external magnetic field,measurement volume,cell temperature and measurement time.The highest fundamental sensitivity $7.5878\times10^{-11}$ $rad/s/Hz^{1/2}$ of 133Cs-85Rb-21Ne co-magnetometer is higher than the ultimate theoretical sensitivity $2\times10^{-10}$ $rad/s/Hz^{1/2}$ of K-21Ne co-magnetometer.The fundamental sensitivity limit of the hybrid optical pumping SERF co-magnetometers is superior to $3.7334\times10^{-12}$ $rad/s/Hz^{1/2}$.

We reported an efficient approach to analyze spin precessions of different hyperpolarized noble gases with atomic magnetometers based on system identification. Spin evolution of the magnetometer can be robustly approximated as the second order system by referring to the Bloch equation under the negligible self-interaction and its transfer function model can be deduced. We measured the dynamic responses of different atomic pairs, such as cesium magnetometer with a partner of xenon-129, potassium magnetometer with a partner of helium-3, and hybrid of cesium and rubidium magnetometer with a partner of neon-21. By referring to the experimental results, corresponding systematic parameters for the transfer function models have been identified based on least-square method, whose outputs were consistent with experimental results. Based on the models, the system stabilities and their variations with different gains have been analyzed with control theory, both of which are described here. Applying control theory to analyze dynamic responses of spin ensembles plays an important role for the future applications.

In the Venus capture period, it is difficult for celestial autonomous navigation to satisfy the requirement of high precision. To improve autonomous navigation performance, a Direction, Distance and Velocity (DDV) measurements deeply integrated navigation method is proposed. The “deeply” integrated navigation reflects the fact that the direction and velocity measurements suppress the Doppler effects in the pulsar signals. In the pulsar observation period, the direction and velocity measurements are utilised to compensate for Doppler effects in the pulsar signals. By these means, the residual effects can be ignored. When the direction, distance or velocity measurements are obtained, they are fused to improve the navigation performance. Simulation results demonstrate that the DDV measurements deeply integrated navigation filter converges very well, and provides highly accurate position estimation without a high quality requirement on navigation sensors.

The hybrid optical pumping spin exchange relaxation free (SERF) atomic magnetometers can realize ultrahigh sensitivity measurement of magnetic field and inertia. We have studied the ⁸⁵Rb polarization of two types of hybrid optical pumping SERF magnetometers based on ³⁹K-⁸⁵Rb-⁴He and ¹³³Cs-⁸⁵Rb-⁴He respectively. Then we found that ⁸⁵Rb polarization varies with the number density of buffer gas ⁴He and quench gas N2, pumping rate of pump beam and cell temperature respectively, which will provide an experimental guide for the design of the magnetometer. We obtain a general formula on the fundamental sensitivity of the hybrid optical pumping SERF magnetometer due to shot-noise. The formula describes that the fundamental sensitivity of the magnetometer varies with the number density of buffer gas and quench gas, the pumping rate of pump beam, external magnetic field, cell effective radius, measurement volume, cell temperature and measurement time. We obtain a highest fundamental sensitivity of 1.5073 aT/Hz1/2 (1 aT = 10⁻¹⁸T) with ³⁹K-⁸⁵Rb-⁴He magnetometer between above two types of magnetometers when ⁸⁵Rb polarization is 0.1116. We estimate the fundamental sensitivity limit of the hybrid optical pumping SERF magnetometer to be superior to 1.8359 × 10⁻²aT/Hz1/2, which is higher than the shot-noise-limited sensitivity of 1 aT/Hz1/2 of K SERF atomic magnetometer.

In the traditional observability analysis methods, much attention is paid to the accuracy of the condition number. However, the authors find the fact that besides the accuracy, the structure of condition numbers is important, such as their range and sequence. A structured condition number method is proposed, which includes the double-reciprocal condition number and the compensation based on the continuous period. As the condition number is in non-linear relation with the navigation error, the mean of condition numbers does not reflect the navigation performance accurately. To solve this problem, the double-reciprocal condition number is proposed, where the impact of a large condition number is small. Considering the fact that the longer the continuous period is, the worse the navigation performance is, the authors develop the compensation method based on the continuous period. Amending the double-reciprocal condition number with the continuous period-based compensation, the authors propose the structured condition number which possesses the advantages of them, and apply it into the celestial direction measurement-based integrated navigation systems which have sharply variable observability degree. The simulation results demonstrate that the structured condition number can reflect the navigation performance and select a proper navigation star effectively.

In the traditional observability analysis methods, only current and a few historical measurement values are considered. For the linear systems, the satisfactory analysis results can be obtained. However, for the nonlinear systems, the analysis results are not completely in accordance with the navigation errors since the high-order nonlinearity of the navigation system is not embodied on the observability matrix. In order to solve this problem, a fractional differentiation-based observability analysis method is developed. Considering the fact that the fractional differentiation with respect to time is characterized by long-term memory effects, the fractional derivative of the measurement model with respect to time is considered. By this means, more historical measurement data can be exploited. And then the fractional differentiation-based observability matrix, which reflects the high-order nonlinearity of the navigation system, is constructed. Finally, as the condition number is not directly proportional to the positioning error, to evaluate the navigation performance, the exponent weighted condition number is developed, where small condition numbers have large weight. The simulation results demonstrate that the fractional differentiation-based observability analysis method is sensitive to the orbital elements in the nonlinear X-ray pulsar navigation system, and has small calculation load. In addition, compared to the traditional observability analysis methods, it is observed that its analysis result matches well with the X-ray pulsar navigation performance.