Binquan Zhou

Effects of temperature on Rb and 129Xe spin polarization in a nuclear magnetic resonance gyroscope with low pump power

We propose an average Rb polarization model to analyze the influence of temperature on the spin polarization of Rb and 129Xe in a Nuclear Magnetic Resonance Gyroscope (NMRG) with low pump power. This model is essentially based on summing the Rb spin polarization along the direction of the pump beam and dividing the result by the cell length. We experimentally study the spin polarization of Rb and 129Xe atoms as a function of the cell temperature at low values of the pump power. The experimental results and the values calculated with the average Rb polarization model are in good agreement for both Rb and 129Xe. The spin polarization of Rb atoms decreases with increasing cell temperature, with a decreasing trend which is rapid at temperatures below 110 °C, and slower at temperatures above 110 °C. The experimental values of the 129Xe polarization, obtained with a pump power of 1 mW, first increase to a maximum P  129Xe−ave = 0.66 % at 118 °C, and then decreases as the temperature increases. Increasing the po...

A method for calibrating coil constants by using the free induction decay of noble gases

We propose a precise method to calibrate the coil constants of spin-precession gyroscopes and optical atomic magnetometers. This method is based on measuring the initial amplitude of Free Induction Decay (FID) of noble gases, from which the π/2 pulse duration can be calculated, since it is inversely proportional to the amplitude of the π/2 pulse. Therefore, the coil constants can be calibrated by measuring the π/2 pulse duration. Compared with the method based on the Larmor precession frequency of atoms, our method can avoid the effect of the pump and probe powers. We experimentally validated the method in a Nuclear Magnetic Resonance Gyroscope (NMRG), and the experimental results show that the coil constants are 436.63±0.04 nT/mA and 428.94±0.02 nT/mA in the x and y directions, respectively.

A method for measuring the spin polarization of 129Xe by using an atomic magnetometer

We propose a method for the precise determination of nuclear spin polarization, based on the atomic magnetometers, which employs the effective magnetic field produced by the spin polarization of 129Xe nuclei. This effective magnetic field can be estimated by measuring the initial induced voltage of the Free Induction Decay (FID) signal of the 129Xe nuclei, which is based on the calibration coefficient between the transverse magnetic field and the output voltage signal of the atomic magnetometer, by using an off-resonant transverse driven magnetic field. Compared with the method based on measuring the longitudinal relaxation time of the 129Xe nuclei and the spin polarization of alkali-metal atoms, our method can directly measure the nuclear spin polarization, without being affected by inaccuracies in the measurement of the spin polarization of alkali-metal atoms.

Laser intensity stabilization with a liquid crystal variable retarder for a nuclear magnetic resonance gyroscope prototype

The laser intensity stability counts for the performance of Nuclear Magnetic Resonance Gyroscope (NMRG). We switch to attenuate the fluctuation of laser intensity with the aid of an opto-electric modulator and feedback control. The Liquid Crystal Variable Retarder (LCVR) has a sharp edge over its counterparts such as AOM and EOM benefiting from its compact size, low operation voltage and large clear aperture. In this paper, we demonstrate a LCVR based laser intensity stabilization system designed for a NMRG prototype. The setup mainly compromises of two crossed linear polarizers, a LCVR, a polarized beam splitter, a photo detector and a digital servo control unit. The intensity of a small portion of laser split by the PBS is detected by the photodiode and then fed into the servo control unit. It compares the current laser intensity with the setpoint value, generates a proper control signal under the supervision of the built-in algorithm and drives the LCVR to change the incident laser polarization state, and hence the output laser intensity. In addition, we derive the formula of the relative output laser intensity with voltage, which helps to design the control algorithm. Finally, the long-term stability of the system reaches 0.038% in a 4-hour continuous measurement.

Effects of the pulse-driven magnetic field detuning on the calibration of coil constants while using noble gases

In the calibration of coil constants using the Free Induction Decay (FID) signal of noble gases, we analyse the effects of the pulse-driven magnetic field detuning on the calibration results. This method is based on the inverse relation between the π/2 pulse duration and its amplitude. We confirmed that obtaining a precise frequency is a prerequisite for ensuring the accuracy of research using the initial amplitude of the FID signal. In this paper, the spin dynamics of noble gases and its time-domain solution under the driving pulse have been discussed with regard to different detuning ranges. Experimental results are in good agreement with our theoretical predictions, which indicate the correctness of our theoretical deduction. Therefore, the frequency of the pulse-driven magnetic field is an important factor to the calibration of coil constants, it should be determined with a high degree of accuracy.

Novel nested saddle coils used in miniature atomic sensors

A novel uniform magnetic field coil structure with two pairs of saddle coils nested is proposed in order to minish the aspect ratio without losing much field uniformity, which is significant in innovative miniature atomic instruments and sensors. Optimal configuration parameters are obtained from the Taylor expansion of the magnetic field. The remainder terms are used to estimate the field uniformity and optimize the configuration while minishing computation time. Compared with traditional saddle coils, the nested saddle coils have unique advantages in miniature applications where the aspect ratios are strictly limited. The optimized nested saddle coils were manufactured by Flexible Printed Circuit (FPC) technology and the magnetic field uniformities were verified experimentally using a flux-gate magnetometer. Furthermore, the example application of a miniature nuclear magnetic resonance (NMR) gyro demonstrates the practical use of the nested coils.

Intensity and frequency stabilization of a laser diode by simultaneously controlling its temperature and current

Nuclear magnetic resonance gyroscope (NMRG) detects the angular velocity of the vehicle utilizing the interaction between the laser beam and the alkali metal atoms along with the noble gas atoms in the alkali vapor cell. In order to reach high precision inertial measurement target, semiconductor laser in NMRG should have good intensity and frequency stability. Generally, laser intensity and frequency are stabilized separately. In this paper, a new method to stabilize laser intensity and frequency simultaneously with double-loop feedback control is presented. Laser intensity is stabilized to the setpoint value by feedback control of laser diode’s temperature. Laser frequency is stabilized to the Doppler absorption peak by feedback control of laser diode’s current. The feedback control of current is a quick loop, hence the laser frequency stabilize quickly. The feedback control of temperature is a slow loop, hence the laser intensity stabilize slowly. With the feedback control of current and temperature, the laser intensity and frequency are stabilized finally. Additionally, the dependence of laser intensity and frequency on laser diode’s current and temperature are analyzed, which contributes to choose suitable operating range for the laser diode. The advantage of our method is that the alkali vapor cell used for stabilizing laser frequency is the same one as the cell used for NMRG to operate, which helps to miniaturize the size of NMRG prototype. In an 8-hour continuous measurement, the long-term stability of laser intensity and frequency increased by two orders of magnitude and one order of magnitude respectively.

Study on VCSEL laser heating chip in nuclear magnetic resonance gyroscope

In recent years, atomic gyroscope has become an important direction of inertial navigation. Nuclear magnetic resonance gyroscope has a stronger advantage in the miniaturization of the size. In atomic gyroscope, the lasers are indispensable devices which has an important effect on the improvement of the gyroscope performance. The frequency stability of the VCSEL lasers requires high precision control of temperature. However, the heating current of the laser will definitely bring in the magnetic field, and the sensitive device, alkali vapor cell, is very sensitive to the magnetic field, so that the metal pattern of the heating chip should be designed ingeniously to eliminate the magnetic field introduced by the heating current. In this paper, a heating chip was fabricated by MEMS process, i.e. depositing platinum on semiconductor substrates. Platinum has long been considered as a good resistance material used for measuring temperature The VCSEL laser chip is fixed in the center of the heating chip. The thermometer resistor measures the temperature of the heating chip, which can be considered as the same temperature of the VCSEL laser chip, by turning the temperature signal into voltage signal. The FPGA chip is used as a micro controller, and combined with PID control algorithm constitute a closed loop control circuit. The voltage applied to the heating resistor wire is modified to achieve the temperature control of the VCSEL laser. In this way, the laser frequency can be controlled stably and easily. Ultimately, the temperature stability can be achieved better than 100mK.

A nuclear spin-based 129Xe and 131Xe using optical polarization

We present a design for a spin-exchange optical pumping system to produce large quantities of highly polarized 129Xe and 131Xe. Low xenon concentrations in the flowing gas mixture which allow the laser to maintain high Cs polarization. The large spin-exchange rate between Cs and Xe through the long-lived van der Waals molecules at low pressure, combined with a high flow rate, results in large production rates of hyperpolarized xenon. The fast rates make it possible to obtain large nuclear polarizations after several minutes of optical pumping with a laser.At high Xe pressures. According to the theory, the longitudinal spin-elaxation rate 1T1 of Xe in a high-pressure sample containing only Xe and Cs vapor has the simple form is the velocity averaged binary spin-exchange cross section, It is the relaxation rate due to wall collisions and perhaps magnetic field inhomogeneities. Our results complement earlier studies performed at 129Xe pressures of about 20 Torr and 131Xe pressures of about 20 Torr and N2 pressures of 600 Torr . This work is useful for predicting spin-exchange rates between polarized Cs atoms and Xe nuclei.

The optimal frequency and power of a probe beam for atomic sensor

Nuclear magnetic resonance gyroscope (NMRG) is the smallest atomic sensor in navigation level. Spin precession can be detected by measuring the optical rotation of the plane of an off-resonant linearly polarized probe beam. The optimal frequency and power of the probe beam counts for the performance of NMRG, which has been verified by our former experiments. The NMRG system would have higher sensitivity and lower consumption comparing to the circularly polarized probe beam. In this paper, we demonstrate the optimal frequency and power of an off-resonant linearly polarized probe beam by theoretical analysis and experimental verification. In theory, the off-resonant linearly polarized probe beam can be decomposed into two circularly polarized components of opposite helicity. Its plane of polarization will be rotated by an angle, due to the positive and negative helicity light experience different induces of refraction as it propagates through a birefringent medium. The off-resonant linearly polarized probe beam becomes partially absorbed by the alkali vapor as it propagates through the NMRG cell. The overall signal is determined by both optical signal and beam absorption. After optimizing the frequency and power of the probe, the magnetic field sensitivity was 2pT/Hz1/2.