Application Cases

Experiment Name: Online Measurement Method for Transverse Relaxation Time of Nuclear Magnetic Resonance Gyroscope Embedded Magnetometer

Research Direction: Precision Measurement

Test Objective:
In a nuclear magnetic resonance gyroscope, variations in the probe light frequency lead to changes in the amplitude of the signal measured by the embedded magnetometer, which in turn causes bias drift in the gyroscope. Based on the structural characteristics of the nuclear magnetic resonance gyroscope, an integrated probe laser frequency stabilization method is proposed. This method involves applying amplitude modulation to the probe light and then demodulating the first harmonic of the light intensity signal received by the balanced detector to obtain a frequency stabilization error signal. The probe light frequency can be stabilized at a suitable reference point through PID feedback control. The principle of this method and the factors affecting frequency stabilization accuracy are analyzed in detail. Subsequently, the nuclear magnetic resonance gyroscope experimental device integrated with the probe light frequency stabilization system is introduced, and the feasibility of this frequency stabilization technique is experimentally verified. Under the current system conditions, a frequency stabilization accuracy better than 10 MHz can be achieved. This technique does not require an external frequency stabilization system, which is beneficial for the miniaturization of nuclear magnetic resonance gyroscopes.

Test Equipment: ATA-4012 high-voltage power amplifier, data acquisition card, magnetic control circuit board, lock-in amplifier.

Experimental Procedure:

Figure: Schematic Diagram of the NMRG Experimental System Setup

The nuclear magnetic resonance gyroscope system experimental device was constructed, with the schematic diagram and physical setup shown in the figures above and below, respectively.

Figure: Physical Setup of the NMRG Experimental System

The atomic vapor cell is surrounded by three-dimensional Helmholtz magnetic field coils and magnetic field gradient coils, as shown in the figure below. The black coil frames are made of acrylic material with insulating properties, and the coil wires are made of oxygen-free copper. The coil wires and the power supply are connected using shielded cables. The current flowing through the coils is driven by a data acquisition card and a dedicated magnetic control circuit board.

Figure: Three-Dimensional Magnetic Field Coils of the Experimental System (Placed Inside a Magnetic Shield Cylinder)

The pump laser is emitted by a higher-power semiconductor laser, producing quasi-monochromatic linearly polarized light. The laser frequency is tuned to the center frequency of the D1 line transition of the alkali metal atoms. After beam expansion and collimation, the pump light passes through a linear polarizer (with a preceding λ/2 waveplate used to adjust the laser power incident on the vapor cell) and a λ/4 waveplate, becoming left-handed circularly polarized light. It then propagates along the z-axis direction into the atomic vapor cell to optically pump the spins of the alkali metal atoms.

The probe laser is emitted by a lower-power semiconductor laser, also producing quasi-monochromatic linearly polarized light. The laser frequency is tuned appropriately away from the center frequency of the D1 line transition of the alkali metal atoms. After beam expansion and collimation, and passing through a linear polarizer, the probe light propagates along the x-axis direction into the atomic vapor cell. Upon exiting the atomic vapor cell, the probe light, which carries the optical rotation angle signal, passes through a λ/2 waveplate (used to balance the output of the balanced detector under initial conditions with no pump light or magnetic field applied). It then passes through a Wollaston prism, splitting into two linearly polarized beams with vertical and horizontal polarizations, respectively. The intensities I₁ and I₂ of these two split beams are detected by a balanced detector.

Next, the differential output signal from the balanced detector is input to a lock-in amplifier and demodulated at the reference frequency. Through the quadrature and in-phase demodulation outputs of the lock-in amplifier, the signals corresponding to the x-axis component Bₓ and the y-axis component Bᵧ of the transverse magnetic field can be obtained, respectively. The demodulated output signals from the lock-in amplifier are acquired by the data acquisition device and transmitted to a computer for data processing, signal demodulation, and spectrum analysis. After filtering, amplifying, and phase-shifting the acquired Xe atom precession signal, it is applied to the transverse magnetic field driving coils together with the residual magnetic field compensation control signal. This feedback loop maintains the system in a closed-loop magnetic resonance state.

Experimental Results:

Figure: Experimental Results of Magnetometer Absorption Signals at Different Atomic Vapor Cell Temperatures

Under the same experimental methods and conditions as the simulation, the experimental results of the absorption signals at different atomic vapor cell temperatures are represented by points of different colors and shapes in the figure above. The center frequencies of the three absorption curves show slight relative shifts. It is inferred that the reason for these shifts is that the polarization field of the ⁸⁷Rb atomic spins increases with temperature, causing a change in the z-axis magnetic field and consequently a frequency shift in the ⁸⁷Rb atomic magnetic resonance. However, this does not affect the measurement result of the relaxation time, as the transverse relaxation rate is determined solely by the linewidth of the absorption curve.

Figure: Experimental Measurement Results of Magnetometer Amplitude-Frequency Response Characteristics at Different Atomic Vapor Cell Temperatures

The lower the temperature, the larger the relaxation rate. It is speculated that this is because the pressure of the buffer gas also decreases as the temperature drops. The diffusion constant of alkali metal atoms in the buffer gas is inversely proportional to the buffer gas pressure. Consequently, the relaxation rate due to collisions between alkali metal atoms and the cell walls increases as the temperature decreases.

Figure: ATA-4012C High-Voltage Power Amplifier Specifications and Parameters