Chao Wang

The common-mode interference (CMI) caused by the high-temperature environment and the integration of multiple sensors affects the magnetic measurement accuracy of the magnetometer, and the noise performance of the photoelectric detection circuit (PDC) is also a key factor affecting the sensitivity of the magnetometer. In order to suppress CMI and improve the sensitivity of magnetic measurement, a differential structure PDC is proposed, and the equivalent noise model is established by analyzing the circuit noise source. By optimizing the parameters, the PDC with low noise performance and low CMI is designed and applied to the optically pumped magnetometer to realize magnetic measurement. The test results show that the differential structure PDC can effectively suppress the CMI, the signal-to-noise ratio can be increased by 1.4 times compared with the single-ended PDC, and the voltage noise is 57.65 nV/Hz ^1/2 , which is consistent with the theoretical analysis results of the noise model, and meets the expected requirements of low noise design of the PDC of low noise magnetometer. Finally, the self-made PDC is employed in the borehole optically pumped magnetometer for magnetic field measurement, which verifies the effectiveness of the circuit.

Helium optically pumped sensor is widely used in the application for the detection of weak magnetic fields, and the cell is the core component of that, which needs an external excitation signal to ignite and maintain its luminosity, and its luminosity affects the sensitivity performance of the sensor. To improve the performance of the cell and reduce the power consumption of the system, which is the largest power consumption component in the sensor, this study presents the design of a driver circuit for a helium cell based on a high-voltage pulse source and high-frequency excitation source and uses a T-type impedance matching circuit to realize the efficient transmission of energy. The experimental results demonstrate that the driver circuit can effectively light up the helium cell, in which the pulse voltage of the high-voltage excitation is more than 1.0 kV, the output power of the high-frequency excitation signal is in the range of 0–6 W, and it is easy to adjust the output power of the high-frequency excitation signal to optimize the sensitivity of the sensor with an the optimal power density of 1.1 W/cm2 and a sensitivity of 29.4 pT/Hz1/2 is obtained. The driver circuit method designed in this study is also suitable for other inert gases to generate metastable atoms.

Stability in the emission intensity of a spectral lamp is an essential factor determining the performance of a ⁴ He optically pumped sensor (He-OPS). However, in a He-OPS, the stability of the lamp drive signal source is greatly affected by temperature. This is a huge obstacle to its application in borehole magnetic surveys, which have harsh conditions such as a confined environment and high temperatures. Hence, a real-time control excitation circuit with proportional integral derivative (PID) feedback regulation to enhance the stability of the emission intensity is proposed in this paper. The instability of the luminous intensity was first analyzed theoretically and an extreme learning machine (ELM)–Hammerstein model was established. Then, instability was regulated in real-time using a programmable PID controller. Experimental results indicate that the designed excitation circuit can work at environment temperatures up to 75 °C and magnetic drift of up to 1.3 nT is suppressed. Therefore, the excitation circuit with PID feedback regulation is an effective and reliable way to improve measurement stability and has potential for utilization in extreme environment applications such as borehole magnetic field measurement to detect resources and monitor seismic activity.

In ⁴ He optically pumped magnetometers, the helium cell is an essential component, mainly affected by excitation parameters that influence the performance of the magnetometer. This study aims at analytically and experimentally exploring suitable parameters based on frequency values and power densities of excitation to achieve optimal performances. First, a plasma model is established and theoretically analyzed in this field. Then, a favorable frequency value of the high-frequency (HF) signal is calculated based on the impedance of the He cell measured at a certain HF frequency, and an experiment on sensitivity varying with power density absorbed by the cell is conducted. Experimental results indicate that the optimal value is 45 MHz, the desirable power density is 1.1 W/cm ² within the scope of 0.6 W/cm ² to 1.8 W/cm ² , and the sensitivity is improved prominently, thereby providing systematic sensitivity of 29.4 pT/Hz ^1/2 in the frequency from 1 Hz to 30 Hz. Theoretical analyses and experimental results demonstrate a prominent performance improvement of the magnetometer by optimizing the parameters of the excitation circuit. The proposed method reliably provides a potential direction for future research on the sensitivity enhancement of He cells.

Earthquake monitoring requires advanced instruments, such as a high-accuracy deep well magnetometer. However, many challenges are encountered in harsh environments, such as the measurement drift of magnetic field due to high temperatures and the influence of confined and narrow space. This study investigated a helium optically-pumped magnetometer (He-OPM) with 98 cm in length and 11 cm in diameter using radio frequency field modulation (RFM) technology and light stabilization to achieve better measurement performance. The experimental results indicate that RFM and light stabilization can enhance sensitivity and reduce measurement drift of magnetic field. The He-OPM has a bandwidth up to 0.3 Hz and the optimal sensitivity is 20 pT/Hz ^1/2 . The stability of the measured magnetic field was obtained at deep well temperatures. Furthermore, the proposed system suppressed measurement drift of magnetic field up to 0.8 nT. The He-OPM was tested in a deep well at the southwest China seismic belt about a month. The test results validated a satisfactory performance of the He-OPM for deep-well earthquake monitoring.

Aeromagnetic exploration is an important method of geophysical exploration. We study the compensation method of the towed bird system and establish the towed bird interference model. Due to the geomagnetic gradient changing greatly, the geomagnetic gradient is considered in the towed bird interference model. In this paper, we model the geomagnetic field gradient and analyze the influence of the towed bird system on the aeromagnetic compensation results. Finally, we apply the ridge regression method to solve the problem. We verify the feasibility of this compensation method through actual flight tests and further improve the data quality of the towed bird interference.

In this paper, a multiple optical parametric oscillator based on MgO:APLN is reported. The polarization structure of MgO:APLN and output coupler transmittance are optimized, with a high repetition rate of 200 kHz and 1064 nm laser pumping. The 1.57 μm and 3.84 μm cross period parameters light output can be achieved by a single poled crystal for the first time. The maximum average output powers of 2.4 W at 1.57 μm and 1.31 W at 3.84 μm are obtained, corresponding to optical-optical conversion efficiency of 11.54% and 6.25%, respectively. For the phenomenon of energy back conversion appearing in the multiple optical parametric oscillation process, the numerical evolution of multi-wave coupling process through the coupled wave equations is investigated, and the concept of “back conversion in energy transmission area” is induced, and indicates that the existence of back conversion in energy transmission area causes the weak gain parameters of optical parametric oscillator to be enhanced twice. The theoretical analyses are basically in agreement with the experimental results.